Guide To Instruments and Methods of Observation: Volume I - Measurement of Meteorological Variables - PDFCOFFEE.COM (2025)

Guide to Instruments and Methods of Observation Volume I –Measurement of Meteorological Variables

WEATHER CLIMATE WATER

2018 edition

WMO-No. 8

Guide to Instruments and Methods of Observation Volume I – Measurement of Meteorological Variables

2018 edition

WMO-No. 8

EDITORIAL NOTE METEOTERM, the WMO terminology database, may be consulted at http://public.wmo.int/en/ resources/meteoterm. Readers who copy hyperlinks by selecting them in the text should be aware that additional spaces may appear immediately following http://, https://, ftp://, mailto:, and after slashes (/), dashes (-), periods (.) and unbroken sequences of characters (letters and numbers). These spaces should be removed from the pasted URL. The correct URL is displayed when hovering over the link or when clicking on the link and then copying it from the browser.

WMO-No. 8 © World Meteorological Organization, 2018 The right of publication in print, electronic and any other form and in any language is reserved by WMO. Short extracts from WMO publications may be reproduced without authorization, provided that the complete source is clearly indicated. Editorial correspondence and requests to publish, reproduce or translate this publication in part or in whole should be addressed to: Chair, Publications Board World Meteorological Organization (WMO) 7 bis, avenue de la Paix P.O. Box 2300 CH-1211 Geneva 2, Switzerland

Tel.: +41 (0) 22 730 84 03 Fax: +41 (0) 22 730 81 17 Email: [emailprotected]

ISBN 978-92-63-10008-5 NOTE The designations employed in WMO publications and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of WMO concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or products does not imply that they are endorsed or recommended by WMO in preference to others of a similar nature which are not mentioned or advertised.

PUBLICATION REVISION TRACK RECORD

Date

Part/ chapter/ section

Purpose of amendment

Proposed by

Approved by

CONTENTS Page

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi CHAPTER 1. GENERAL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Meteorological observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.2 Representativeness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1.3 Metadata. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Meteorological observing systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 General requirements of a meteorological station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3.1 Automatic weather stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.2 Observers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.3 Siting and exposure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3.3.1 Site selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3.3.2 Coordinates of the station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.3.3 Operating equipment in extreme environments. . . . . . . . . . . . . . . . . 7 1.3.4 Changes of instrumentation and homogeneity. . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.5 Inspection and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.5.1 Inspection of stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.5.2 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 General requirements of instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4.1 Desirable characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4.2 Impact of the Minamata convention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4.3 Mechanically recording instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Measurement standards, traceability and units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5.1 Definitions of standards of measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5.2 Traceability assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.5.3 Symbols, units and constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.3.1 Symbols and units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.3.2 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6 Uncertainty of measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6.1 Meteorological measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6.1.2 Definitions of measurements and measurement errors . . . . . . . . . . . 15 1.6.1.3 Characteristics of instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.6.2 Sources and estimates of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.6.3 The measurement uncertainties of a single instrument. . . . . . . . . . . . . . . . . . . . . 18 1.6.3.1 The statistical distributions of observations . . . . . . . . . . . . . . . . . . . . . 18 1.6.3.2 Estimating the true value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1.6.3.3 Expressing the uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.6.3.4 Measurements of discrete values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.6.4 Accuracy requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.6.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.6.4.2 Required and achievable performance. . . . . . . . . . . . . . . . . . . . . . . . . 22 Annex 1.A. Operational measurement uncertainty requirements and instrument performance requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Annex 1.B. Strategy for traceability assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Annex 1.C. Regional Instrument Centres. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Annex 1.D. Siting classifications for surface observing stations on land. . . . . . . . . . . . . . . . . . . 43 Annex 1.E. Operating equipment in extreme environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Annex 1.F. Station exposure description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

vi

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

CHAPTER 2. MEASUREMENT OF TEMPERATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.1.1 Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 2.1.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 2.1.3.2 Measurement uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82 2.1.3.3 Response times. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 2.1.3.4 Recording the circumstances in which measurements are taken. . . . 82 2.1.4 Methods of measurement and observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.1.4.1 General measurement principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.1.4.2 General exposure requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.1.4.3 Sources of error – general comments . . . . . . . . . . . . . . . . . . . . . . . . . . 85 2.1.4.4 Maintenance – general comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 2.1.4.5 Implications of the Minamata Convention for temperature measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.2 Electrical thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.2.1 General description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.2.1.1 Metal resistance thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.2.1.2 Thermistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 2.2.1.3 Thermocouples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 2.2.2 Measurement procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 2.2.2.1 Electrical resistance thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 2.2.2.2 Thermocouples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 2.2.3 Exposure and siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 2.2.4 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.2.4.1 Electrical resistance thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.2.4.2 Thermocouples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.2.5 Comparison and calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.2.5.1 Electrical resistance thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.2.5.2 Thermocouples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.2.6 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.2.7 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 2.3 Liquid-in-glass thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 2.3.1 General description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 2.3.1.1 Ordinary (station) thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.3.1.2 Maximum thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.3.1.3 Minimum thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.3.1.4 Soil thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.3.2 Measurement procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 2.3.2.1 Reading ordinary thermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 2.3.2.2 Measuring grass minimum temperatures. . . . . . . . . . . . . . . . . . . . . . . 100 2.3.3 Thermometer siting and exposure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 2.3.4 Sources of error in liquid-in-glass thermometers. . . . . . . . . . . . . . . . . . . . . . . . . . 101 2.3.4.1 Elastic errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 2.3.4.2 Errors caused by the emergent stem. . . . . . . . . . . . . . . . . . . . . . . . . . . 101 2.3.4.3 Parallax and gross reading errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 2.3.4.4 Errors due to differential expansion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 2.3.4.5 Errors associated with spirit thermometers. . . . . . . . . . . . . . . . . . . . . . 102 2.3.5 Comparison and calibration in the field and laboratory. . . . . . . . . . . . . . . . . . . . 103 2.3.5.1 Laboratory calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 2.3.5.2 Field checks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 2.3.6 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.3.7 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.3.7.1 Breakage in the liquid column. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.3.7.2 Scale illegibility. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.3.8 Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

CONTENTS

vii Page

2.4 Mechanical thermographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.4.1 General description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.4.1.1 Bimetallic thermograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.4.1.2 Bourdon-tube thermograph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.4.2 Measurement procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.3 Exposure and siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.4 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.5 Comparison and calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.5.1 Laboratory calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.5.2 Field comparison. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 2.4.6 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 2.4.7 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 2.5 Radiation shields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 2.5.1 Louvred screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 2.5.2 Other artificially ventilated shields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 2.6 Traceabilitty assurance and calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Annex. Defining the fixed points of the International Temperature Scale of 1990. . . . . . . . . . . 111 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE. . . . . . . . . . . . . . . . . . . . . . . . . . . 115 3.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 3.1.1 Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 3.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 3.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 3.1.4 Methods of measurement and observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 3.1.4.1 General measurement principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 3.1.4.2 General exposure requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 3.1.4.3 Sources of error: general comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 3.1.4.4 Maintenance: general comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 3.1.4.5 Implications of the Minamata Convention for pressure measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 3.2 Electronic barometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 3.2.1 Integrated-circuit-based variable capacitive sensors. . . . . . . . . . . . . . . . . . . . . . . 119 3.2.2 Digital piezoresistive barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 3.2.3 Cylindrical resonator barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 3.2.4 Aneroid displacement transducers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 3.2.5 Exposure of electronic barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 3.2.6 Reading electronic barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 3.2.7 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 3.2.7.1 Drift between calibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 3.2.7.2 Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3.2.7.3 Electrical interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3.2.7.4 Nature of operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3.3 Aneroid barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3.3.1 Construction requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3.3.2 Achievable measurement uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.3 Exposure of aneroid barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.4 Reading aneroid barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.4.1 Accuracy of readings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.4.2 Reductions applied to barometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.5 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 3.3.5.1 Incomplete compensation for temperature. . . . . . . . . . . . . . . . . . . . . 123 3.3.5.2 Elasticity errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 3.4 Barographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 3.4.1 General requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 3.4.2 Construction of barographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 3.4.3 Exposure of barographs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

viii

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

3.4.4 3.4.5

Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Reading a barograph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 3.4.5.1 Accuracy of readings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 3.4.5.2 Corrections to be applied to barograph readings. . . . . . . . . . . . . . . . 126 3.4.5 Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 3.5 Barometric change and Pressure tendency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 3.5.1 Pressure tendency and pressure tendency characteristics . . . . . . . . . . . . . . . . . . 126 3.5.2 Measurement of a barometric change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 3.6 Traceability assurance and calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 3.6.1 General comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 3.6.2 Laboratory calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 3.6.2.1 General equipment set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 3.6.2.2 Laboratory standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 3.6.2.3 Method of calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 3.6.2.3.2 Calculation of reversibility (hysteresis). . . . . . . . . . . . . . . . . . . . . . . . . 132 3.6.3 Field inspections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 3.7 Adjustment of barometer readings to standard and other levels. . . . . . . . . . . . . . . . . . . . 133 3.7.1 Standard levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 3.7.2 General reduction formula. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 3.7.3 Reduction formula for low-level stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Annex 3.A. Methods of measurement with mercury barometers . . . . . . . . . . . . . . . . . . . . . . . . 135 Annex 3.B. Correction of mercury barometer readings to standard conditions. . . . . . . . . . . . . 144 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 CHAPTER 4. MEASUREMENT OF HUMIDITY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 4.1.4 Methods of measurement and observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 4.1.4.1 Overview of general measurement principles. . . . . . . . . . . . . . . . . . . 150 4.1.4.4 Maintenance: general comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 4.1.5 Implications of the Minamata Convention for humidity measurement . . . . . . . 155 4.2 Electrical capacitance hygrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.2.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.2.2 Electrical capacitance hygrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.2.3 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 4.2.4 Exposure and siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 4.2.5 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 4.2.6 Calibration and field inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 4.2.7 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.3 The psychrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.3.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.3.1.1 Psychrometric formulae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.3.1.2 The specification of a psychrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 4.3.1.3 The wet-bulb sleeve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 4.3.1.4 Operation of the wet bulb below freezing. . . . . . . . . . . . . . . . . . . . . . 160 4.3.1.5 General procedure for making observations . . . . . . . . . . . . . . . . . . . . 161 4.3.1.6 Use of electrical resistance thermometers. . . . . . . . . . . . . . . . . . . . . . . 161 4.3.1.7 Psychrometric formulae and tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 4.3.1.8 Sources of error in psychrometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 4.3.2 Assmann and other aspirated psychrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 4.3.2.1 Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 4.3.2.2 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 4.3.2.3 Exposure and siting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 4.3.2.4 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

CONTENTS

ix Page

4.3.2.5 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Screen psychrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 4.3.3.1 Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 4.3.3.2 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 4.3.3.3 Exposure and siting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 4.3.3.4 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 4.3.3.5 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 4.3.4 Sling or whirling psychrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 4.3.4.1 Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 4.3.4.2 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 The chilled-mirror dew-point hygrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 4.4.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 4.4.1.1 Theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 4.4.1.2 Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 4.4.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 4.4.2.1 Sensor assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 4.4.2.2 Optical detection assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 4.4.2.3 Thermal control assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 4.4.2.4 Temperature display system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 4.4.2.5 Instrument format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 4.4.2.6 Auxiliary systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 4.4.3 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 4.4.4 Exposure and siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 4.4.5 Calibration and field inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 4.4.5.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 4.4.5.2 Field inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Hygrometers using absorption of electromagnetic radiation. . . . . . . . . . . . . . . . . . . . . . . 173 The hair hygrograph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 4.6.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 4.6.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 4.6.3 Observation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 4.6.4 Exposure and siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.5 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.5.1 Changes in zero offset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.5.2 Errors due to contamination of the hair. . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.5.3 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.6 Calibration and field inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.6.7 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Traceability assurance and calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 4.7.1 Principles involved in the calibration of hygrometers. . . . . . . . . . . . . . . . . . . . . . 176 4.7.2 Primary standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 4.7.2.1 Gravimetric hygrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 4.7.2.2 Dynamic two-pressure standard humidity generator. . . . . . . . . . . . . 177 4.7.2.3 Dynamic two-temperature standard humidity generator . . . . . . . . . 178 4.7.3 Secondary standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 4.7.4 Working standards (and field reference instruments). . . . . . . . . . . . . . . . . . . . . . 178 4.7.5 Salt solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 4.7.6 Calibration methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 4.7.6.1 General comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 4.7.6.2 Laboratory calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 4.7.6.3 Field calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Time constants, Protective filters and Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.8.1 Time constants of humidity sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.8.2 Protective filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.8.3 Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 4.3.3

4.4

4.5 4.6

4.7

4.8

Annex 4.A. Definitions and specifications of water vapour in the atmosphere . . . . . . . . . . . . . 184 Annex 4.B. Formulae for the computation of measures of humidity. . . . . . . . . . . . . . . . . . . . . . 188

x

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

Annex 4.C. Instruments and methods in limited use, or no longer used. . . . . . . . . . . . . . . . . . . 190 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 CHAPTER 5. MEASUREMENT OF SURFACE WIND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 5.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 5.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 5.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.1.4 Methods of measurement and observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 5.2 Estimation of wind. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 5.2.1 Wind speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 5.2.2 Wind direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.2.3 Wind fluctuations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.3 Simple instrumental methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.3.1 Wind speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.3.2 Wind direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.4 Cup and propeller sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 5.5 Wind-direction vanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 5.6 Other wind sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 5.7 Sensors and sensor combinations for component resolution. . . . . . . . . . . . . . . . . . . . . . . 202 5.8 Data-processing methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 5.8.1 Averaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 5.8.2 Peak gusts and standard deviations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 5.8.3 Recommendations for the design of wind-measuring systems . . . . . . . . . . . . . . 205 5.9 Exposure of wind instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 5.9.1 General problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 5.9.2 Anemometers over land. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .206 5.9.3 Anemometers at sea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 5.9.4 Exposure correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 5.10 Calibration and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Annex. The effective roughness length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 CHAPTER 6. MEASUREMENT OF PRECIPITATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 6.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 6.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 6.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 6.1.3 Meteorological and hydrological requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 215 6.1.4 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 6.1.4.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 6.1.4.2 Reference gauges and intercomparisons . . . . . . . . . . . . . . . . . . . . . . . 216 6.1.4.3 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 6.2 Siting and exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 6.3 Non-recording precipitation gauges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 6.3.1 Ordinary gauges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 6.3.1.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 6.3.1.2 Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 6.3.1.3 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 6.3.2 Storage gauges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 6.4 Precipitation gauge errors and corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 6.5 Recording precipitation gauges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 6.5.1 Weighing-recording gauge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 6.5.1.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 6.5.1.2 Errors and corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 6.5.1.3 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

CONTENTS

xi Page

6.5.2

Tipping-bucket gauge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.5.2.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.5.2.2 Errors and corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 6.5.2.3 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 6.5.3 Float gauge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 6.5.4 Other raingauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 6.6 Measurement of dew, ice accumulation and fog precipitation. . . . . . . . . . . . . . . . . . . . . .230 6.6.1 Measurement of dew and leaf wetness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 6.6.2 Measurement of ice accumulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 6.6.2.1 Measurement methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 6.6.2.2 Ice on pavements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 6.6.3 Measurement of fog precipitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Annex 6.A. Standard reference raingauge pit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 Annex 6.B. Precipitation intercomparison sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 Annex 6.C. Standardized procedure for laboratory calibration of catchment type rainfall intensity gauges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Annex 6.D. Suggested correction procedures for precipitation measurements. . . . . . . . . . . . . 240 Annex 6.E. Procedure for field calibration of catchment type rainfall intensity gauges. . . . . . . 241 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 CHAPTER 7. MEASUREMENT OF RADIATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 7.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 7.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 7.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 7.1.2.1 Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 7.1.2.2 Standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 7.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 7.1.3.1 Data to be reported. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 7.1.3.2 Uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 7.1.3.3 Sampling and recording. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 7.1.3.4 Times of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 7.1.4 Measurement methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 7.2 Measurement of direct solar radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 7.2.1 Direct solar radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 7.2.1.1 Primary standard pyrheliometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 7.2.1.2 Secondary standard pyrheliometers . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 7.2.1.3 Field and network pyrheliometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 7.2.1.4 Calibration of pyrheliometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 7.2.2 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 7.3 Measurement of global and diffuse sky radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 7.3.1 Calibration of pyranometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 7.3.1.1 By reference to a standard pyrheliometer and a shaded reference pyranometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 7.3.1.2 By reference to a standard pyrheliometer. . . . . . . . . . . . . . . . . . . . . . . 259 7.3.1.3 Alternate calibration using a pyrheliometer. . . . . . . . . . . . . . . . . . . . . 260 7.3.1.4 By comparison with a reference pyranometer. . . . . . . . . . . . . . . . . . . 261 7.3.1.5 By comparison in the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 7.3.1.6 Routine checks on calibration factors . . . . . . . . . . . . . . . . . . . . . . . . . . 262 7.3.2 Performance of pyranometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 7.3.2.1 Sensor levelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 7.3.2.2 Change of sensitivity due to ambient temperature variation. . . . . . . 262 7.3.2.3 Variation of response with orientation. . . . . . . . . . . . . . . . . . . . . . . . . . 263 7.3.2.4 Variation of response with angle of incidence . . . . . . . . . . . . . . . . . . . 263 7.3.2.5 Uncertainties in hourly and daily totals. . . . . . . . . . . . . . . . . . . . . . . . . 263

xii

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

7.3.3

7.4

7.5

7.6

Installation and maintenance of pyranometers . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 7.3.3.1 Correction for obstructions to a free horizon . . . . . . . . . . . . . . . . . . . . 264 7.3.3.2 Installation of pyranometers for measuring global radiation. . . . . . . 265 7.3.3.3 Installation of pyranometers for measuring diffuse sky radiation . . . 265 7.3.3.4 Installation of pyranometers for measuring reflected radiation. . . . . 266 7.3.3.5 Maintenance of pyranometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 7.3.3.6 Installation and maintenance of pyranometers on special platforms.266 Measurement of total and long-wave radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 7.4.1 Instruments for the measurement of long-wave radiation. . . . . . . . . . . . . . . . . . 267 7.4.2 Instruments for the measurement of total radiation . . . . . . . . . . . . . . . . . . . . . . . 270 7.4.3 Calibration of pyrgeometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 7.4.4 Installation of pyrradiometers and pyrgeometers. . . . . . . . . . . . . . . . . . . . . . . . . 271 7.4.5 Recording and data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 Measurement of special radiation quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 7.5.1 Measurement of daylight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 7.5.1.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 7.5.1.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 7.5.1.3 Recording and data reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Measurement of Ultraviolet radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 7.6.1 Instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 7.6.1.1 Broadband sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 7.6.1.2 Narrowband sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 7.6.1.3 Spectroradiometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.6.2 Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

Annex 7.A. Nomenclature of radiometric and photometric quantities . . . . . . . . . . . . . . . . . . . . 283 Annex 7.B. Meteorological radiation quantities, symbols and definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Annex 7.C. Specifications for World, Regional and National Radiation Centres. . . . . . . . . . . . . 287 Annex 7.D. Useful formulae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Annex 7.E. Diffuse sky radiation – correction for a shading ring. . . . . . . . . . . . . . . . . . . . . . . . . . 294 Annex 7.F. Governance and traceability of atmospheric longwave irradiance. . . . . . . . . . . . . . 296 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 CHAPTER 8. MEASUREMENT OF SUNSHINE DURATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 8.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 8.1.1 Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 8.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 8.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 8.1.3.1 Application of sunshine duration data . . . . . . . . . . . . . . . . . . . . . . . . . 300 8.1.3.2 Correlations to other meteorological variables . . . . . . . . . . . . . . . . . . 300 8.1.3.3 Requirement of automated records. . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 8.1.4 Measurement methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 8.2 Instruments and sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 8.2.1 Pyrheliometric method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 8.2.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 8.2.1.2 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 8.2.2 Pyranometric method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 8.2.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 8.2.2.2 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 8.2.3 The Campbell-Stokes sunshine recorder (burn method) . . . . . . . . . . . . . . . . . . . 304 8.2.3.1 Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 8.2.3.2 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 8.2.3.3 Special versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 8.2.3.4 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

CONTENTS

xiii Page

8.2.4 8.2.5

Contrast-evaluating devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Contrast-evaluating and scanning devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 8.2.5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 8.2.5.2 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 8.3 Exposure of sunshine detectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 8.4 General sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 8.5 Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 8.5.1 Outdoor methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 8.5.1.1 Comparison of sunshine duration data. . . . . . . . . . . . . . . . . . . . . . . . . 308 8.5.1.2 Comparison of analogue signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 8.5.1.3 Mean effective irradiance threshold method. . . . . . . . . . . . . . . . . . . . 309 8.5.2 Indoor method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 8.6 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Annex 8.A. Algorithm to estimate sunshine duration from direct global irradiance measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Annex 8.B. Algorithm to estimate sunshine duration from 1min global irradiance measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 CHAPTER 9. MEASUREMENT OF VISIBILITY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 9.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 9.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 9.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 9.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 9.1.4 Measurement methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 9.2 Visual estimation of meteorological optical range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 9.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 9.2.2 Estimation of meteorological optical range by day. . . . . . . . . . . . . . . . . . . . . . . . 321 9.2.3 Estimation of meteorological optical range at night. . . . . . . . . . . . . . . . . . . . . . . 321 9.2.4 Estimation of meteorological optical range in the absence of distantobjects. . 323 9.2.5 Accuracy of visual observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 9.2.6 Usage of cameras. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 9.3 Instrumental measurement of the meteorological opticalrange. . . . . . . . . . . . . . . . . . . . 324 9.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 9.3.2 Instruments measuring the extinction coefficient. . . . . . . . . . . . . . . . . . . . . . . . . 325 9.3.3 Instruments estimating the scatter coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 9.3.4 Instrument exposure and siting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 9.3.5 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 9.3.5.1 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 9.3.5.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 9.3.6 Accuracy estimates for the measurement of meteorological optical range . . . . 334 9.3.6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 9.3.6.2 Accuracy of transmissometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 9.3.6.3 Accuracy of forward-scatter meters. . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 9.3.6.4 Accuracy of telephotometers and visual extinction meters. . . . . . . . . 335 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 CHAPTER 10. MEASUREMENT OF EVAPORATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1.4 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 10.2 Atmometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 10.2.1 Instrument types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

xiv

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

10.2.2 Measurement taken by atmometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 10.2.3 Sources of error in atmometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 10.3 Evaporation pans and tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 10.3.1 United States Class A pan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 10.3.2 Russian GGI-3000pan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 10.3.3 Russian 20m2 tank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 10.3.4 Measurements taken by evaporation pans and tanks. . . . . . . . . . . . . . . . . . . . . . 341 10.3.5 Exposure of evaporation pans and tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 10.3.6 Sources of error in evaporation pans and tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . 341 10.3.7 Maintenance of evaporation pans and tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 10.4 Evapotranspirometers (lysimeters). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 10.4.1 Measurements taken by lysimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 10.4.2 Exposure of evapotranspirometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 10.4.3 Sources of error in lysimeter measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 10.4.4 Lysimeters maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 10.5 Estimation of evaporation from natural surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 CHAPTER 11. MEASUREMENT OF SOIL MOISTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 11.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 11.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 11.1.2 Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 11.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 11.1.4 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 11.2 Gravimetric direct measurement of soil water content . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352 11.3 Soil water content: indirect methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 11.3.1 Radiological methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 11.3.1.1 Neutron scattering method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 11.3.1.2 Gamma-ray attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 11.3.2 Soil water dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 11.3.2.1 Time-domain reflectometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 11.3.2.2 Frequency-domain measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 11.4 Soil water potential instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 11.4.1 Tensiometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 11.4.2 Resistance blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 11.4.3 Psychrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 11.5 Site selection and sample size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358 11.6 Remote-sensing of soil moisture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 11.6.1 Microwave remote-sensing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 11.6.1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 11.6.1.2 Multi-frequency radiometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 11.6.1.3 Scatterometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361 11.6.1.4 Synthetic aperture radars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 11.6.1.5 Dedicated L-band missions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 11.6.1.6 Soil moisture retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 11.6.2 Thermal infrared remote-sensing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366 CHAPTER 12. MEASUREMENT OF UPPER-AIR PRESSURE, TEMPERATURE AND HUMIDITY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 12.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 12.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 12.1.2 Units used in upper-air measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 12.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 12.1.3.1 Radiosonde data for meteorological operations . . . . . . . . . . . . . . . . . 371

CONTENTS

xv Page

12.2

12.3

12.4

12.5

12.1.3.2 Relationships between satellite and radiosonde upper-air measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 12.1.3.3 Maximum height of radiosonde observations. . . . . . . . . . . . . . . . . . . 375 12.1.4 Accuracy requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 12.1.4.1 Geopotential height: requirements and performance . . . . . . . . . . . . 376 12.1.4.2 Temperature: requirements and performance. . . . . . . . . . . . . . . . . . . 377 12.1.4.3 Relative humidity: requirements and performance. . . . . . . . . . . . . . . 377 12.1.5 Methods of measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 12.1.5.1 Constraints on radiosonde design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 12.1.5.2 Radio frequency used by radiosondes. . . . . . . . . . . . . . . . . . . . . . . . . . 378 12.1.6 Radiosonde errors: general considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 12.1.6.1 Types of error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 12.1.6.2 Potential references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 12.1.6.3 Sources of additional error during radiosonde operations. . . . . . . . . 380 Radiosonde electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 12.2.1 General features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 12.2.2 Power supply for radiosondes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 12.2.3 Methods of data transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 12.2.3.1 Radio transmitter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 Pressure sensors (including height measurements). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 12.3.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 12.3.2 Aneroid capsules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .383 12.3.3 Aneroid capsule (capacitive). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 12.3.4 Silicon sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 12.3.5 Pressure sensor errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 12.3.5.1 Relationship of geopotential height errors to pressure errors . . . . . . 385 12.3.6 Use of geometric height observations instead of pressure sensor observations.387 12.3.6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 12.3.6.2 Method of calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 12.3.7 Sources of error in direct height measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . 389 12.3.7.1 In GPS geometric height measurements. . . . . . . . . . . . . . . . . . . . . . . . 389 12.3.7.2 In radar height measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 Temperature sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 12.4.1 General requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 12.4.2 Thermistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 12.4.3 Thermocapacitors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 12.4.4 Thermocouples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 12.4.5 Scientific sounding instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 12.4.6 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 12.4.7 Temperature errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 12.4.7.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 12.4.7.2 Thermal lag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 12.4.7.3 Radiative heat exchange in the infrared . . . . . . . . . . . . . . . . . . . . . . . . 395 12.4.7.4 Heating by solar radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 12.4.7.5 Deposition of ice or water on the sensor. . . . . . . . . . . . . . . . . . . . . . . . 398 12.4.7.6 Representativeness issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 Relative humidity sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 12.5.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 12.5.2 Thin-film capacitors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 12.5.3 Carbon hygristors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 12.5.4 Goldbeater’s skin sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 12.5.5 Scientific sounding instruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 12.5.6 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 12.5.7 Relative humidity errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 12.5.7.1 General considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 12.5.7.2 Relative humidity at night for temperatures above –20°C. . . . . . . . . 408 12.5.7.3 Relative humidity in the day for temperatures above –20°C. . . . . . . 409 12.5.7.4 Relative humidity at night for temperatures between –20°C and –50°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

xvi

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

12.5.7.5 Relative humidity in the day for temperatures between –20°C and –50°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 12.5.7.6 Relative humidity at night for temperatures between –50°C and –70°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 12.5.7.7 Relative humidity in the day for temperatures between –50°C and –70°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 12.5.7.8 Wetting or icing in cloud. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 12.5.7.9 Representativeness issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 12.6 Ground station equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 12.6.1 General features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 12.6.2 Software for data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 12.7 Radiosonde operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 12.7.1 Control corrections immediately before use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 12.7.2 Deployment methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 12.7.3 Radiosonde launch procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 12.7.4 Radiosonde suspension during flight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 12.7.5 Public safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 12.8 Comparison, calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 12.8.1 Comparisons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 12.8.1.1 Quality evaluation using short-term forecasts . . . . . . . . . . . . . . . . . . . 419 12.8.1.2 Quality evaluation using atmospheric time series. . . . . . . . . . . . . . . . 420 12.8.1.3 Comparison of water vapour measurements with remote-sensing. . 420 12.8.1.4 Radiosonde comparison tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 12.8.2 Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 12.8.3 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 12.9 Computations and reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 12.9.1 Radiosonde computations and reporting procedures. . . . . . . . . . . . . . . . . . . . . . 423 12.9.2 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 12.10 Procurement Issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 12.10.1 Use and update of the results from the WMO Intercomparison of High Quality Radiosonde Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 12.10.2 Some issues to be considered in procurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Annex 12.A. Current breakthrough and optimum accuracy requirements for radiosonde measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 Annex 12.B. Estimates of goal, breakthrough and threshold limits for upper wind, upper-air temperature, relative humidity and geopotential height (derived from the WMO Rolling Review of Requirements for upper-air observations). . . . . . . . . . . . . . . . . . . . . . .428 Annex 12.C. Environmentally friendly radiosondes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 Annex 12.D. Guidelines for organizing radiosonde intercomparisons and for the establishment of test sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 CHAPTER 13. MEASUREMENT OF UPPER WIND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 13.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 13.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 13.1.2 Units of measurement of upper wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 13.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444 13.1.3.1 Uses in meteorological operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444 13.1.3.2 Improvements in reporting procedures . . . . . . . . . . . . . . . . . . . . . . . . 444 13.1.3.3 Accuracy requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 13.1.3.4 Maximum height requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 13.1.4 Methods of measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 13.1.4.1 Tracking using radionavigation signals. . . . . . . . . . . . . . . . . . . . . . . . . 448 13.1.4.2 Tracking using a directional aerial. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449

CONTENTS

xvii Page

13.2 Upper-wind sensors and instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 13.2.1 Optical theodolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 13.2.2 Radiotheodolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 13.2.3 Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 13.2.3.1 Primary radars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 13.2.3.2 Secondary radars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 13.2.4 Navaid tracking systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 13.2.4.1 Availability of navaid signals in the future. . . . . . . . . . . . . . . . . . . . . . . 452 13.2.4.2 Global positioning system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 13.2.4.3 LORAN-C chains. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 13.3 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 13.3.1 General considerations concerning data processing. . . . . . . . . . . . . . . . . . . . . . . 455 13.3.2 Pilot-balloon observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 13.3.3 Observations using a directional aerial. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456 13.3.4 Observations using radionavigation systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456 13.4 Exposure of ground equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 13.5 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 13.5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 13.5.1.1 Target tracking errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 13.5.1.2 Height assignment errors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 13.5.1.3 Target motion relative to the atmosphere. . . . . . . . . . . . . . . . . . . . . . . 459 13.5.2 Errors in pilot-balloon observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 13.5.3 Errors of systems using a directional aerial. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 13.5.4 Errors in the global positioning system windfinding systems . . . . . . . . . . . . . . . 462 13.5.5 Errors in ground-based LORAN-C radionavigation systems. . . . . . . . . . . . . . . . . 465 13.5.6 Representativeness errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 13.6 Comparison, calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 13.6.1 Comparison. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 13.6.1.1 Operational monitoring by comparison with forecast fields . . . . . . . 467 13.6.1.2 Comparison with other windfinding systems . . . . . . . . . . . . . . . . . . . 468 13.6.2 Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468 13.6.3 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 13.7 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 CHAPTER 14. OBSERVATION OF PRESENT AND PAST WEATHER; STATE OF THE GROUND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 14.1 GeneraL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 14.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 14.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 14.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 14.1.4 Observation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 14.2 Observation of present and past weatheR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 14.2.1 Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 14.2.1.1 Objects of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 14.2.1.2 Instruments and measuring devices: precipitation type. . . . . . . . . . . 474 14.2.1.3 Instruments and measuring devices: precipitation intensity and character . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 14.2.1.4 Instruments and measuring devices: multi-sensor approach. . . . . . . 477 14.2.2 Atmospheric obscurity and suspensoids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 14.2.2.1 Objects of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 14.2.2.2 Instruments and measuring devices for obscurity and suspensoid characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 14.2.3 Other weather events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 14.2.3.1 Objects of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 14.2.3.2 Instruments and measuring devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

xviii

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Page

14.2.4 State of the sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.2.4.1 Objects of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.2.4.2 Instruments and measuring devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.3 Observation of state of the ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.3.1 Objects of observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.3.2 Instruments and measuring devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 14.4 Observation of special phenomena. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 14.4.1 Electrical phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 14.4.2 Optical phenomena. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 Annex. Criteria for slight, moderate and heavy precipitation intensity. . . . . . . . . . . . . . . . . . . . 482 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 CHAPTER 15. OBSERVATION AND MEASUREMENT OF CLOUDS. . . . . . . . . . . . . . . . . . . . . . . 487 15.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 15.1.1 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 15.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 15.1.3 Meteorological requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 15.1.4 Observation and measurement methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 15.1.4.1 Cloud amount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 15.1.4.2 Cloud-base height. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 15.1.4.3 Cloud type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 15.2 Estimation and observation of cloud amount, cloud-base height and cloud type by human observer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 15.2.1 Making effective estimations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 15.2.2 Estimation of cloud amount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 15.2.3 Estimation of cloud-base height. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 15.2.4 Observation of cloud type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 15.3 Instrumental measurement of cloud amount. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 15.3.1 Measurement of cloud amount by laser ceilometer. . . . . . . . . . . . . . . . . . . . . . . . 493 15.3.2 Measurement of cloud amount by infrared detector. . . . . . . . . . . . . . . . . . . . . . . 494 15.3.3 Measurement of cloud amount by sky camera. . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 15.4 Instrumental measurement of cloud-base height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 15.4.1 Measurement of cloud-base height by laser ceilometer. . . . . . . . . . . . . . . . . . . . 495 15.4.1.1 Measurement method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 15.4.1.2 Exposure and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496 15.4.1.3 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 15.4.1.4 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 15.4.2 Measurement of cloud-base height by rotating-beam ceilometer . . . . . . . . . . . 498 15.4.2.1 Measurement method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498 15.4.2.2 Exposure and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 15.4.2.3 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 15.4.2.4 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 15.4.3 Measurement of cloud-base height by searchlight . . . . . . . . . . . . . . . . . . . . . . . . 500 15.4.3.1 Measurement method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 15.4.3.2 Exposure and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 15.4.3.3 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 15.4.3.4 Calibration and maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 15.4.4 Balloon measurement of cloud-base height. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 15.4.4.1 Measurement method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 15.4.4.2 Sources of error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 15.5 Instrumental measurement of cloud type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 15.6 Other cloud-related properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 15.6.1 Vertical visibility. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

CONTENTS

xix Page

CHAPTER 16. MEASUREMENT OF ATMOSPHERIC COMPOSITION. . . . . . . . . . . . . . . . . . . . . 506 16.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506 16.1.1 Definitions and descriptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507 16.1.2 Units and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 16.1.3 Measurement principles and techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 16.1.4 Quality assurance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511 16.2 (Stratospheric) ozone measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512 16.2.1 Ozone total column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512 16.2.2 Ozone profile measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 16.2.2.1 Umkehr method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 16.2.2.2 Ozonesonde measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 16.2.2.3 Other measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 16.2.3 Aircraft and satellite observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 16.3 Greenhouse gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 16.3.1 Carbon dioxide (including Δ14C, δ13C and δ18O in CO2, and O2/N2 ratios). . . . . . 515 16.3.2 Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517 16.3.3 Nitrous oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517 16.3.4 Halocarbons and SF6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 16.3.5 Remote-sensing of greenhouse gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 16.4 Reactive gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 16.4.1 Tropospheric (surface) ozone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 16.4.2 Carbon monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520 16.4.3 Volatile organic compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521 16.4.4 Nitrogen oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 16.4.5 Sulphur dioxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 16.4.6 Molecular hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525 16.5 Atmospheric wet deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525 16.5.1 Sample collection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526 16.5.2 Chemical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .527 16.6 Aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530 16.6.1 Aerosol chemical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530 16.6.2 In situ measurements of aerosol radiative properties . . . . . . . . . . . . . . . . . . . . . . 534 16.6.3 Particle number concentration and size distribution. . . . . . . . . . . . . . . . . . . . . . . 536 16.6.4 Cloud condensation nuclei. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 16.6.5 Aerosol optical depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 16.6.6 GAW aerosol lidar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 16.7 Natural radioactivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541 Annex. GAW central facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 References and further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545

FOREWORD

The mission of WMO, as outlined in Article 2 of the WMO Convention, includes facilitating worldwide cooperation in the establishment of networks of stations for the making of meteorological and hydrological observations and related geophysical observations. It also includes promoting the standardization of meteorological and related observations and ensuring the uniform publication of observations and statistics. To support the WMO mission, the World Meteorological Congress adopts updated Technical Regulations which lay down the practices and procedures to be followed by WMO Members. These Technical Regulations are supplemented by Manuals and Guides. Manuals contain standard and recommended practices that Members are required and urged to follow, and Guides describe in detail the practices and procedures that Members are invited to follow. The Guide to Instruments and Methods of Observation (WMO-No. 8) was first published as a provisional ten-chapter guide in 1950. The continuous progress made in standardizing measurement and observational practices and the rapid development of new measurement techniques and technologies have led to the evolution of the Guide into a significantly larger publication and an essential source of information for National Meteorological and Hydrological Services, manufacturers of measuring instruments and related equipment and many other organizations and institutions. The Guide is the authoritative reference document for all matters related to instrumentation and methods of observation in the context of the WMO Integrated Global Observing System. Uniform, traceable and high-quality observational data represent an essential input for most WMO applications, including climate monitoring, nowcasting and severe weather forecasting; these data also facilitate the improvement of the well-being of societies throughout the world. The main purpose of the Guide is to provide guidance on the most effective practices and procedures for, and the capabilities of, instruments and systems that are regularly used to perform meteorological, hydrological and related environmental measurements and observations in order to meet specific requirements for different application areas. The theoretical basis of the techniques and observational methods is outlined in the text and supported by references and further reading for additional background information and details. This newly titled and newly structured 2018 edition of the Guide was approved by the seventeenth session of the Commission for Instruments and Methods of Observation (Amsterdam, the Netherlands, 2018). The new title lacks the word “meteorological”, which was present in the former editions, because the content of the current Guide is not only meteorological in nature, but also pertains to other, related domains. While past editions of the Guide were separated into “parts”, the new edition is split into “volumes” that can be updated and published independently. Since the 2014 edition, almost half of the chapters have been significantly updated, and a new volume on the measurement of cryospheric variables has been introduced. The 2018 edition comprises 40 chapters distributed over the following five thematic volumes: Measurement of Meteorological Variables, Measurement of Cryospheric Variables, Observing Systems, Space-based Observations, and Quality Assurance and Management of Observing Systems. In the process of updating the Guide, WMO benefited from the excellent collaboration that took place between the Commission for Instruments and Methods of Observation and the Global Cryosphere Watch, the Commission for Basic Systems, the Commission for Atmospheric Sciences, the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology, and the WMO Education and Training Programme, which provided significant contributions to the 2018 edition of the Guide.

xxii

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

On behalf of WMO, I would like to take this opportunity to express my sincere gratitude to the Commission for Instruments and Methods of Observation and to all involved experts, whose excellent efforts have enabled the publication of this new edition.

(Petteri Taalas) Secretary-General

CHAPTER 1. GENERAL

1.1

METEOROLOGICAL OBSERVATIONS

1.1.1

General

Meteorological (and related environmental and geophysical) observations are made for a variety of reasons. They are used for the real-time preparation of weather analyses, forecasts and severe weather warnings, for the study of climate, for local weather-dependent operations (for example, local aerodrome flying operations, construction work on land and at sea), for hydrology and agricultural meteorology, and for research in meteorology and climatology. The purpose of the Guide to Meteorological Instruments and Methods of Observation is to support these activities by giving advice on good practices for meteorological measurements and observations. There are many other sources of additional advice, and users should refer to the references at the end of each chapter for a bibliography of theory and practice relating to instruments and methods of observation. The references also contain national practices, national and international standards, and specific literature. They also include reports published by WMO for the Commission for Instruments and Methods of Observation (CIMO) on technical conferences, instrumentation, and international comparisons of instruments. Many other Manuals and Guides issued by WMO refer to particular applications of meteorological observations (see especially those relating to the WMO Integrated Global Observing System (WIGOS) (WMO, 2015, 2017), aeronautical meteorology (WMO, 2014), hydrology (WMO, 2008), agricultural meteorology (WMO, 2010b) and climatology (WMO, 2011a). Quality assurance (QA) and maintenance are of special interest for instrument measurements. Throughout the present Guide many recommendations are made to meet the stated performance requirements. These requirements are described in Annex 1.A. Particularly, VolumeV of the present Guide is dedicated to QA and management of observing systems. It is recognized that quality management and training of instrument specialists is of utmost importance. Therefore, on the recommendation of CIMO,1 regional associations of WMO have set up Regional Instrument Centres (RICs) to maintain standards and provide advice regarding meteorological measurements. These RICs play a key role in the implementation of WMO strategy for traceability assurance, which is set out in Annex 1.B. Their terms of reference are given in Annex1.C. In addition, on the recommendation of the Joint WMO/Intergovernmental Oceanographic Commission Technical Commission for Oceanography and Marine Meteorology (JCOMM)2 (WMO, 2010a) a network of Regional Marine Instrument Centres has been established to provide for similar functions regarding marine meteorology and other related oceanographic measurements. Their terms of reference and locations are given in VolumeIII, Chapter4, Annex4.A of the present Guide.3 Also, to undertake training in meteorology, hydrology and related sciences to meet the needs of the regions, WMO Regional Training Centres4 have been established. The definitions and standards stated in the present Guide (see 1.5.1) will always conform to internationally adopted standards. Basic documents to be referred to are the International Meteorological Vocabulary (WMO, 1992) and the International Vocabulary of Metrology – BasicandGeneral Concepts and Associated Terms (VIM) (Joint Committee for Guides in Metrology (JCGM), 2012).

1 2 3

4

Recommended by the CIMO at its ninth session (1985) through Recommendation19 (CIMO‑IX). Recommended by JCOMM at its third session (2009) through Recommendation1 (JCOMM‑III). Additional information on Regional Marine Instrument Centres can be found at http://​w ww​.jcomm​.info/​index​.php​ ?option​= ​com​_content​&​view​=​article​&​id​=​335:​rmics​&​catid​=​3 4:​capacity​-building. Recent information on Regional Training Centres and their components can be found at https://​w ww​.wmo​.int/​pages/​prog/​dra/​etrp/​r tcs​.php.

2

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Representativeness

1.1.2

The representativeness of an observation is the degree to which it accurately describes the value of the variable needed for a specific purpose. Therefore, it is not a fixed quality of any observation, but results from joint appraisal of instrumentation, measurement interval and exposure against the requirements of some particular application. For instance, synoptic observations should typically be representative of an area up to 100km around the station, but for small-scale or local applications the considered area may have dimensions of 10km or less. In particular, applications have their own preferred timescales and space scales for averaging, station density and resolution of phenomena — small for agricultural meteorology, large for global long-range forecasting. Forecasting scales are closely related to the timescales of the phenomena; thus, shorter-range weather forecasts require more frequent observations from a denser network over a limited area to detect any small-scale phenomena and their quick development. Using various sources (WMO, 2001, 2015; Orlanski, 1975), horizontal meteorological scales may be classified as follows, with a factor two uncertainty: (a) Microscale (less than 100m) for agricultural meteorology, for example, evaporation; (b) Toposcale or local scale (100 m–3km), for example, air pollution, tornadoes; (c) Mesoscale (3–100km), for example, thunderstorms, sea and mountain breezes; (d) Large scale (100–3000km), for example, fronts, various cyclones, cloud clusters; (e) Planetary scale (larger than 3000km), for example, long upper tropospheric waves. Section 1.6 discusses the required and achievable uncertainties of instrument systems. The stated achievable uncertainties can be obtained with good instrument systems that are properly operated, but are not always obtained in practice. Good observing practices require skill, training, equipment and support, which are not always available in sufficient degree. The measurement intervals required vary by application: minutes for aviation, hours for agriculture, and days for climate description. Data storage arrangements are a compromise between available capacity and user needs. Good exposure, which is representative on scales from a few metres to 100km, is difficult to achieve (see 1.3). Errors of unrepresentative exposure may be much larger than those expected from the instrument system in isolation. A station in a hilly or coastal location is likely to be unrepresentative on the large scale or mesoscale. However, good homogeneity of observations in time may enable users to employ data even from unrepresentative stations for climate studies. Annex 1.D discusses site representativeness in further detail and provides guidelines on the classification of surface observing sites on land to indicate their representativeness for the measurement of different variables. This classification has several objectives: (a) To improve the selection of a site and the location of an instrument within the selected site to optimize representativeness by applying some objective criteria; (b) To help in the construction of a network and the selection of its sites: (i)

Not only for meteorological services but also, for example, for road services;

(ii) To avoid inappropriate positioning of instruments; (c) To document the site representativeness with an easy-to-use criterion: (i)

It is clear that a single number is not enough to fully document the environment and representativeness of a site. Additional information is necessary such as a map, pictures or a description of the surroundings;

CHAPTER 1. GENERAL

3

(ii) Despite this numerical value, the site classification is not only a ranking system. Class1 sites are preferred, but sites in other classes are still valuable for many applications; (d) To help users benefit from metadata when using observations data. It is recommended that the metadata be as simple as practical, as well as appropriate for the intended use. 1.1.3

Metadata

The purpose of the present Guide and related WMO publications is to ensure reliability of observations by standardization. However, local resources and circumstances may cause deviations from the agreed standards of instrumentation and exposure. A typical example is that of regions with much snowfall, where the instruments are mounted higher than usual so that they can be useful in winter as well as summer. Users of meteorological observations often need to know the actual exposure, type and condition of the equipment and its operation; and perhaps the circumstances of the observations. This is now particularly significant in the study of climate, in which detailed station histories have to be examined. Metadata (data about data) should be kept concerning all of the station establishment and maintenance matters described in1.3, and concerning changes which occur, including calibration and maintenance history and the changes in terms of exposure and staff (WMO, 2003). Metadata are especially important for elements which are particularly sensitive to exposure, such as precipitation, wind and temperature. One very basic form of metadata is information on the existence, availability and quality of meteorological data and of the metadata about them.

1.2

METEOROLOGICAL OBSERVING SYSTEMS

The requirements for observational data may be met using in situ measurements or remotesensing (including space-borne) systems, according to the ability of the various sensing systems to measure the environmental elements needed. The requirements in terms of global, regional and national scales and according to the application area are described in WMO (2015). WIGOS, designed to meet these requirements, is composed of the surface-based subsystem and the space-based subsystem. The surface-based subsystem comprises a wide variety of types of stations according to the particular application (for example, surface synoptic station, upper-air station, climatological station, and so on). The space-based subsystem comprises a number of spacecraft with on-board sounding missions and the associated ground segment for command, control and data reception. The succeeding paragraphs and chapters in the present Guide deal with the surface-based system and, to a lesser extent, with the space-based subsystem. To derive certain meteorological observations by automated systems, for example, present weather, a socalled “multi-instrument” approach is necessary, where an algorithm is applied to compute the result from the outputs of several sensing instruments.

1.3

GENERAL REQUIREMENTS OF A METEOROLOGICAL STATION

The requirements for elements to be observed according to the type of station and observing network are detailed in WMO (2015). In this section, the observational requirements of a typical climatological station or a surface synoptic network station are considered. The following elements are observed at a station making surface observations (the chapters refer to the present volume): – – – –

Temperature (Chapter 2) Soil temperature (Chapter 2) Atmospheric pressure (Chapter 3) Relative humidity (Chapter 4)

4 – – – – – – – – – – –

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Wind direction and speed (Chapter 5) Precipitation (Chapter 6) Snow cover (Chapter 6) Solar radiation and/or sunshine (Chapters 7, 8) Visibility (Chapter 9) Evaporation (Chapter 10) Present weather (Chapter 14) Past weather (Chapter 14) Cloud amount (Chapter 15) Cloud type (Chapter 15) Cloud-base height (Chapter 15)

Instruments exist that can measure all of these elements, with the exception of cloud type. However, with current technology, instruments for present and past weather, cloud amount and height, and snow cover are not able to make observations of the whole range of phenomena, whereas human observers are able to do so. Some meteorological stations take upper-air measurements (the present volume, Chapters12 and 13), measurements of soil moisture (the present volume, Chapter11), ozone and atmospheric composition (the present volume, Chapter16), and some make use of special instrument systems as described in VolumeIII of the present Guide. Details of observing methods and appropriate instrumentation are contained in the succeeding chapters of the present Guide. 1.3.1

Automatic weather stations

Most of the elements required for synoptic, climatological or aeronautical purposes can be measured by automatic instrumentation (see VolumeIII, Chapter1 of the present Guide). As the capabilities of automatic systems increase, the ratio of purely automatic weather stations (AWSs) to observer-staffed weather stations (with or without automatic instrumentation) increases steadily. The guidance in the following paragraphs regarding siting and exposure, changes of instrumentation, and inspection and maintenance apply equally to AWSs and staffed weather stations. 1.3.2

Observers

Meteorological observers are required for a number of reasons, as follows: (a) To make synoptic and/or climatological observations to the required uncertainty and representativeness with the aid of appropriate instruments; (b) To maintain instruments, metadata documentation and observing sites in good condition; (c) To code and dispatch observations (in the absence of automatic coding and communication systems); (d) To maintain in situ recording devices, including the changing of charts when provided; (e) To make or collate weekly and/or monthly records of climatological data where automatic systems are unavailable or inadequate; (f) To provide supplementary or back-up observations when automatic equipment does not make observations of all required elements, or when it is out of service; (g) To respond to public and professional enquiries.

CHAPTER 1. GENERAL

5

Observers should be trained and/or certified by an authorized Meteorological Service to establish their competence to make observations to the required standards. They should have the ability to interpret instructions for the use of instrumental and manual techniques that apply to their own particular observing systems. Guidance on the instrument training requirements for observers will be given in VolumeV, Chapter5 of the present Guide. 1.3.3

Siting and exposure

1.3.3.1

Site selection

Meteorological observing stations are designed so that representative measurements (or observations) can be taken according to the type of station involved. Thus, a station in the synoptic network should make observations to meet synoptic-scale requirements, whereas an aviation meteorological observing station should make observations that describe the conditions specific to the local (aerodrome) site. Where stations are used for several purposes, for example, aviation, synoptic and climatological purposes, the most stringent requirement will dictate the precise location of an observing site and its associated sensing instruments. A detailed study on siting and exposure is published in WMO (1993). As an example, the following considerations apply to the selection of site and instrument exposure requirements for a typical synoptic or climatological station in a regional or national network: (a) Outdoor instruments should be installed on a level piece of ground, preferably no smaller than 25m x25m where there are many installations, but in cases where there are relatively few installations the area may be considerably smaller. The ground should be covered with short grass or a surface representative of the locality, and surrounded by open fencing or palings to exclude unauthorized persons. Within the enclosure, a bare patch of ground of about 2mx2m is reserved for observations of the state of the ground and of soil temperature at depths of equal to or less than 20cm (see Chapter2 of the present volume) (soil temperatures at depths greater than 20cm can be measured outside this bare patch of ground). An example of the layout of such a station is given in Figure1.1; (b) There should be no steeply sloping ground in the vicinity, and the site should not be in a hollow. If these conditions are not met, the observations may show peculiarities of entirely local significance; (c) The site should be well away from trees, buildings, walls or other obstructions. The distance of any such obstacle (including fencing) from the raingauge should not be less than twice the height of the object above the rim of the gauge, and preferably four times the height; (d) The sunshine recorder, raingauge and anemometer must be exposed according to their requirements, preferably on the same site as the other instruments; (e) It should be noted that the enclosure may not be the best place from which to estimate the wind speed and direction; another observing point, more exposed to the wind, may be desirable; (f) Very open sites which are satisfactory for most instruments are unsuitable for raingauges. For such sites, the rainfall catch is reduced in conditions other than light winds and some degree of shelter is needed; (g) If in the instrument enclosure surroundings, maybe at some distance, objects like trees or buildings obstruct the horizon significantly, alternative viewpoints should be selected for observations of sunshine or radiation; (h) The position used for observing cloud and visibility should be as open as possible and command the widest possible view of the sky and the surrounding country;

6

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(i)

At coastal stations, it is desirable that the station command a view of the open sea. However, the station should not be too near the edge of a cliff because wind eddies created by the cliff will affect the wind and precipitation measurements;

(j)

Night observations of cloud and visibility are best made from a site unaffected by extraneous lighting.

It is obvious that some of the above considerations are somewhat contradictory and require compromise solutions. Detailed information appropriate to specific instruments and measurements is given in the succeeding chapters.

NORTH

3m

Laser Snow

TRH S100

4.85 m

EAST

1.2 m

RG

Mi n. 1.5 m

2.8 m

Cloud

1.2 m Snowboard

m 1.5 n. Mi

WEST

PWx

Power + JB

S50 4m

BP

7.2 m

7m

Site power

5.2 m

Logger + JB

Power + JB

WD

5.25 m

Power + JB

WS

Power + JB

2.4 m

3m

3.25 m

VIS

Min. 1.5 m

RG

Tg 2m

Bare earth S5 2m

Ev

4.4 m

2m

S10

2m

S20

1.75 m

Power + JB

4.4 m

RG

VIS

2.7 m

RAD

2m

SOUTH Notes BP Ev JB PWx RG RAD S5 S10

Barometric pressure Evaporation Junction Box for data Present weather Raingauge Solar radiation (pyranometer) Soil temperature 5 cm Soil temperature 10 cm

S20 S50 SUN Tg TRH VIS WS WD

Soil temperature 20 cm Soil temperature 50 cm Sunshine duration Grass temperature Air temperature and relative humidity in radiation shield Meteorological optical range (visiometer) Wind speed Wind direction

Fence: Ideal = stock proof type. Compromises may be needed to suit security needs

Figure 1.1. Layout example of an observing station in the northern hemisphere showing typical distances between installations and the enclosure. It is important to ensure actual distances are great enough to minimize impact of surrounding obstacles.

CHAPTER 1. GENERAL

1.3.3.2

7

Coordinates of the station

The position of a station referred to in the World Geodetic System 1984 (WGS-84) and its Earth Geodetic Model 1996 (EGM96) must be accurately known and recorded. 5 The coordinates of a station are (as required by WMO, 2017): (a) The latitude in degrees, minutes and integer seconds; (b) The longitude in degrees, minutes and integer seconds; (c) The height of the station above mean sea level (MSL),6 namely, the elevation of the station, in metres (up to twodecimals). These coordinates refer to the plot on which the observations are taken and may not be the same as those of the town, village or airfield after which the station is named. If a higher resolution of the coordinates is desired, the same practice applied to elevation can be followed, as explained below. The elevation of the station is defined as the height above MSL of the ground on which the raingauge stands or, if there is no raingauge, the ground beneath the thermometer screen. If there is neither raingauge nor screen, it is the average level of terrain in the vicinity of the station. If the station reports pressure, the elevation to which the station pressure relates must be separately specified. If a station is located at an aerodrome, other elevations must be specified (see VolumeIII, Chapter2 of the present Guide and WMO, 2014). Definitions of measures of height and MSL are given in WMO (1992). 1.3.3.3

Operating equipment in extreme environments

Continuous observations during and after extreme hydrometeorological events are extremely important, both to support recovery efforts and to prepare for future events. Mitigation strategies for common hazards are described in Annex1.E. 1.3.4

Changes of instrumentation and homogeneity

The characteristics of an observing site will generally change over time, for example, through the growth of trees or erection of buildings on adjacent plots. Sites should be chosen to minimize these effects, if possible. Documentation of the geography of the site and its exposure should be kept and regularly updated as a component of the metadata (see Annex1.F and WMO, 2003). It is especially important to minimize the effects of changes of instrument and/or changes in the siting of specific instruments. Although the static characteristics of new instruments might be well understood, when they are deployed operationally they can introduce apparent changes in site climatology. In order to guard against this eventuality, observations from new instruments should be compared over an extended interval (at least one year; see the Guide to Climatological Practices (WMO, 2011a)) before the old measurement system is taken out of service. The same

5 6

For an explanation of the WGS‑84 and recording issues, see ICAO (2002). MSL is defined in WMO (1992) as the fixed reference level of MSL as described by a well-defined geoid. There are two types of models used in geodesy for defining a position in space. The first is ellipsoid, which is functionally a deformed sphere and is the base model used by many global navigation satellite systems (GNSSs). The second is the geoid, the equipotential surface of the Earth’s gravity field that best fits, in a least squares sense, global MSL. GNSSs provide heights relative to the reference ellipsoid WGS‑84 and must be corrected to the geoid as this difference can be as large as 100 m. The WGS-84 EGM96 includes both the WGS-84 reference ellipsoid and EGM96 geoid. For users that require local height, such as pressure or sea level, an adjustment must be applied from GNSS height to the geoid. In some jurisdictions, the national geodetic authority provides a local correction from the ellipsoid (WGS-84) that is both more precise and of finer resolution than EGM96 geoid.

8

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

applies when there has been a change of site. Where this procedure is impractical at all sites, it is essential to carry out comparisons at selected representative sites to attempt to deduce changes in measurement data which might be a result of changing technology or enforced site changes. 1.3.5

Inspection and maintenance

1.3.5.1

Inspection of stations

All synoptic land stations and principal climatological stations should be inspected no less than once every two years. Agricultural meteorological and special stations should be inspected at intervals sufficiently short to ensure the maintenance of a high standard of observations and the correct functioning of instruments. The principal objective of such inspections is to ascertain that: (a) The siting and exposure of instruments are known, acceptable and adequately documented; (b) Instruments are of the approved type, in good order, and regularly verified against standards, as necessary; (c) There is uniformity in the methods of observation and the procedures for calculating derived quantities from the observations; (d) The observers are competent to carry out their duties; (e) The metadata information is up to date. Further information on the standardization of instruments is given in 1.5. 1.3.5.2

Maintenance

Observing sites and instruments should be maintained regularly so that the quality of observations does not deteriorate significantly between station inspections. Routine (preventive) maintenance schedules include regular “housekeeping” at observing sites (for example, grass cutting and cleaning of exposed instrument surfaces) and manufacturers’ recommended checks on automatic instruments. Routine quality control (QC) checks carried out at the station or at a central point should be designed to detect equipment faults at the earliest possible stage. Depending on the nature of the fault and the type of station, corrective maintenance (instrument replacement or repair) should be conducted according to agreed priorities and timescales. As part of the metadata, it is especially important that a log be kept of instrument faults, exposure changes, and remedial action taken where data are used for climatological purposes. Further information on station inspection and management can be found in WMO (2015).

1.4

GENERAL REQUIREMENTS OF INSTRUMENTS

1.4.1

Desirable characteristics

The most important requirements for meteorological instruments are the following: (a) Uncertainty, according to the stated requirement for the particular variable; (b) Reliability and stability; (c) Convenience of operation, calibration and maintenance;

CHAPTER 1. GENERAL

9

(d) Simplicity of design which is consistent with requirements; (e) Durability; (f) Acceptable cost of instrument, consumables and spare parts; (g) Safe for staff and the environment. With regard to the first two requirements, it is important that an instrument should be able to maintain a known uncertainty over a long period. This is much better than having a high level of initial confidence (meaning low uncertainty) that cannot be retained for long under operating conditions. Initial calibrations of instruments will, in general, reveal deviations from the ideal output, necessitating corrections to observed data during normal operations. It is important that the corrections should be retained with the instruments at the observing site and that clear guidance be given to observers for their use. Simplicity, strength of construction, and convenience of operation and maintenance are important since most meteorological instruments are in continuous use year in, year out, and may be located far away from good repair facilities. Robust construction is especially desirable for instruments that are wholly or partially exposed to the weather. Adherence to such characteristics will often reduce the overall cost of providing good observations, outweighing the initial cost. Appropriate safety procedures must be implemented when using instruments containing dangerous chemicals (see in particular guidance on mercury (the present volume, Chapter3, Annex 3.A) and hazardous chemicals (VolumeIII, Chapter8, 8.5 and 8.6). In the case of radiosondes, environmental pollution should be considered when selecting radiosonde materials; Chapter 12, Annex 12.C of the present volume describes the issues and potential near-future solutions for each radiosonde component. 1.4.2

Impact of the Minamata convention

The Minamata Convention on Mercury of the United Nations Environment Programme (UNEP) came into force globally in August 2017. It bans all production, import and export of observing instruments (thermometers, barometers, and the like) containing mercury (UNEP, 2017). This agreement is a global treaty to eliminate the use of mercury to protect both human health and the environment from its adverse effects. It was agreed at the fifth session of the Intergovernmental Negotiating Committee in Geneva in January 2013. The Convention states that “each party shall not allow, by taking appropriate measures, the manufacture, import or export of mercury-added products listed in Part I of Annex A [of the Convention] after the phase‑out date specified for those products”. More specifically, this list includes the following non-electronic measuring devices, except non-electronic measuring devices installed in large-scale equipment or those used for high-precision measurement, where no suitable mercury-free alternative is available: (a) Barometers; (b) Hygrometers; (c) Manometers; (d) Thermometers; (e) Sphygmomanometers.

10

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

A similar regulation became applicable in Europe on 10 April 2014 (Commission Regulation (EU) No. 847/2012) and a number of manufacturers in Europe are already unable to provide mercurybased instruments. Therefore, mercury-based instruments are no longer recommended and it is strongly encouraged to take appropriate measures to put in place a migration strategy to move away from the use of all instruments containing this element. Due to recent advances in electronic and digital technologies, digital electronic barometers, thermometers and hygrometers are now state of the art. They can provide an economical, accurate and reliable alternative to their dangerous, mercury-based precedents and offer other significant advantages in terms of data storage and real-time data display. 1.4.3

Mechanically recording instruments

In many of the mechanically recording instruments used in meteorology, the motion of the sensing element is magnified by levers that move a pen on a chart on a clock-driven drum. Such recorders should be as free as possible from friction, not only in the bearings, but also between the pen and paper. Some means of adjusting the pressure of the pen on the paper should be provided, but this pressure should be reduced to a minimum consistent with a continuous legible trace. Means should also be provided in clock-driven recorders for making time marks. In the design of recording instruments that will be used in cold climates, particular care must be taken to ensure that their performance is not adversely affected by extreme cold and moisture, and that routine procedures (time marks, and so forth) can be carried out by the observers while wearing gloves. Recording instruments should be compared frequently with instruments of the direct-reading type. An increasing number of instruments make use of electronic recording in magnetic media or in semiconductor microcircuits. Many of the same considerations given for bearings, friction and cold-weather servicing apply to the mechanical components of such instruments.

1.5

MEASUREMENT STANDARDS, TRACEABILITY AND UNITS

1.5.1

Definitions of standards of measurement

The term “standard” and other similar terms denote the various instruments, methods and scales used to establish the uncertainty of measurements. A nomenclature for standards of measurement is given in the International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM), which was prepared conjointly by the Bureau international des poids et mesures (BIPM), the International Electrotechnical Commission (IEC), the International Federation of Clinical Chemistry and Laboratory Medicine, the International Laboratory Accreditation Cooperation, the International Organization for Standardization (ISO), the International Union of Pure and Applied Chemistry, the International Union of Pure and Applied Physics and the International Organization of Legal Metrology, and issued by JCGM. The current version is JCGM200:2012, available at http://​www​.bipm​.org/​en/​publications/​guides/​vim​.html. Some of the definitions are as follows: International System of Units/Système international (SI).  System of units, based on the International System of Quantities, their names and symbols, including a series of prefixes and their names and symbols, together with rules for their use, adopted by the General Conference on Weights and Measures (CGPM). Measurement standard.  Realization of the definition of a given quantity, with stated quantity value and associated measurement uncertainty, used as a reference.

CHAPTER 1. GENERAL

11

Example 1: 1 kg mass measurement standard with an associated standard measurement uncertainty of 3µg Example 2: 100 Ω measurement standard resistor with an associated standard measurement uncertainty of 1µΩ International measurement standard (international standard).  Measurement standard recognized by signatories to an international agreement and intended to serve worldwide. Example: The international prototype of the kilogramme National measurement standard (national standard).  Measurement standard recognized by national authorities to serve in a State or economy as the basis for assigning quantity values to other measurement standards for the kind of quantity concerned. Primary measurement standard (primary standard).  Measurement standard established using a primary reference measurement procedure, or created as an artefact, chosen by convention. Example 1: Primary measurement standard of amount-of-substance concentration prepared by dissolving a known amount of substance of a chemical component to a known volume of solution Example 2: Primary measurement standard for pressure based on separate measurements of force and area Secondary measurement standard (secondary standard).  Measurement standard established through calibration with respect to a primary measurement standard for a quantity of the same kind. Reference measurement standard (reference standard).  Measurement standard designated for the calibration of other measurement standards for quantities of a given kind in a given organization or at a given location. Working measurement standard (working standard).  Measurement standard that is used routinely to calibrate or verify measuring instruments or measuring systems. Notes: 1. A working measurement standard is usually calibrated with respect to a reference measurement standard. 2. In relation to verification, the terms “check standard” or “control standard” are also sometimes used.

Transfer measurement device (transfer device).  Device used as an intermediary to compare measurement standards. Note:

Sometimes, measurement standards are used as transfer devices.

Travelling measurement standard (travelling standard).  Measurement standard, sometimes of special construction, intended for transport between different locations. Collective standard.  A set of similar material measures or measuring instruments fulfilling, by their combined use, the role of a standard. Example: The World Radiometric Reference Notes: 1. A collective standard is usually intended to provide a single value of a quantity. 2. The value provided by a collective standard is an appropriate mean of the values provided by the individual instruments.

12

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Traceability.  A property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. Metrological traceability.  A property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. Metrological traceability chain (traceability chain).  Sequence of measurement standards and calibrations that is used to relate a measurement result to a reference. Calibration.  Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication. Notes: 1. A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainty. 2. Calibration should not be confused with adjustment of a measuring system, often mistakenly called “selfcalibration”, nor with verification of calibration.

Calibration hierarchy.  Sequence of calibrations from a reference to the final measuring system, where the outcome of each calibration depends on the outcome of the previous calibration. 1.5.2

Traceability assurance

Measurements have a useful meaning if the results do not vary significantly with the usage of different instruments, operators or other parameters in the measurement process. This confidence is based on regulations, international agreements and QA in the measurement process. It is universally accepted to assess the quality of measurements by a quantitative statement, which is the measurement uncertainty associated with the measurement result. The confidence in the measurement result and the stated uncertainty relies on the traceability of measurements involving an unbroken and documented chain of comparisons linking measurement result to an internationally agreed measurement standard. Measurements should be traceable to an internationally defined and accepted reference, which is in most cases the SI. Technical and organizational infrastructure has been developed and is maintained by BIPM. Maintenance of national standards and dissemination of traceability at the national level relies on National Metrology Institutes (NMIs) or designated institutes (DIs). The concept of RICs has been established by regional associations to support National Meteorological and Hydrological Services (NMHSs) in the dissemination of traceability to their national meteorological standards and related environmental monitoring instruments. Terms of reference of RICs are presented in Annex 1.C. The responsibility for the implementation of traceability assurance on a national level lies with the NMHS, which should ensure all necessary steps to achieve the objective of the strategy. Lack of traceability assurance strongly reduces confidence in measurements and their usage within the local and global communities. The strategy for traceability assurance is presented in Annex 1.B. Instruments in use face very different environmental conditions than when they are in a controlled laboratory environment. Factors that affect the measured quantity in vivo (influencing quantities, drift in time, and the like) also have to be quantified and documented for each

CHAPTER 1. GENERAL

13

measurement. The estimated influences will add to the uncertainty value. Only then can a measurement result be compared with any other traceable result measured in another place and/ or time. To promote standardization of meteorological and related observations and to ensure the uniform publication of observations and statistics, sets of standard procedures and recommended practices have been developed (Volume V, Chapter 4). 1.5.3

Symbols, units and constants

1.5.3.1

Symbols and units

Instrument measurements produce numerical values. The purpose of these measurements is to obtain physical or meteorological quantities representing the state of the local atmosphere. For meteorological practices, instrument readings represent variables, such as “atmospheric pressure”, “air temperature” or “wind speed”. A variable with symbol a is usually represented in the form a = {a}·[a], where {a} stands for the numerical value and [a] stands for the symbol for the unit. General principles concerning quantities, units and symbols are stated in ISO (2009) and International Union of Pure and Applied Physics (1987). The SI should be used as the system of units for the evaluation of meteorological elements included in reports for international exchange. This system is published and updated by BIPM (2006). Guides for the use of the SI are issued by the United States National Institute of Standards and Technology (NIST, 2008) and ISO (2009). Variables not defined as an international symbol by the International System of Quantities, but commonly used in meteorology can be found in the International Meteorological Tables (WMO, 1966) and relevant chapters in the present Guide. The following units should be used for meteorological observations: (a) Atmospheric pressure, p, in hectopascals (hPa);7 (b) Temperature, t, in degrees Celsius (°C) or T in kelvins (K); Note: The Celsius and kelvin temperature scales should conform to the actual definition of the International Temperature Scale of 1990 (ITS-90; see BIPM, 1990).

(c) Wind speed, in both surface and upper-air observations, in metres per second (ms–1); (d) Wind direction in degrees clockwise from true north or on the scale 0–36, where 36 is the wind from true north and 09 the wind from true east (°); (e) Relative humidity, U, in per cent (%); Note: BIPM recommends: “When any of the terms, %, ppm, etc. are used it is important to state the dimensionless quantity whose value is being specified.” For example, in Chapter 4 this recommendation is followed by using %RH.

(f) Precipitation (total amount) in millimetres (mm) or kilograms per square metre (kgm–2);8 (g) Precipitation intensity, Ri, in millimetres per hour (mmh–1) or kilograms per square metre per second (kgm–2s–1);9 (h) Snow water equivalent in kilograms per square metre (kg m–2); (i) 7

8 9

Evaporation in millimetres (mm);

The unit “pascal” is the principal SI-derived unit for the pressure quantity. The unit and symbol “bar” is a unit outside the SI system; in every document where it is used, this unit (bar) should be defined in relation to the SI. Its continued use is not encouraged. By definition, 1mbar (millibar)=1hPa (hectopascal). Assuming that 1mmequals 1kgm–2 independent of temperature. Recommendation3 (CBS‑XII), Annex1, adopted through Resolution4 (EC‑LIII).

14 (j)

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Visibility in metres (m);

(k) Irradiance in watts per square metre and radiant exposure in joules per square metre (Wm–2, Jm–2); (l)

Duration of sunshine in hours (h);

(m) Cloud height in metres (m); (n) Cloud amount in oktas; (o) Geopotential, used in upper-air observations, in standard geopotential metres (m’). Note: Height, level or altitude are presented with respect to a well-defined reference. Typical references are MSL, station altitude or the 1013.2hPa plane.

The standard geopotential metre is defined as 0.980665 of the dynamic metre; for levels in the troposphere, the geopotential is close in numerical value to the height expressed in metres. 1.5.3.2

Constants

The following constants have been adopted for meteorological use: (a) Absolute temperature of the normal ice point T0 = 273.15 K (t = 0.00 °C); (b) Absolute temperature of the triple point of water T = 273.16 K (t = 0.01 °C), by definition of ITS-90; (c) Standard acceleration of gravity (gn) = 9.80665 m s–2. The values of other constants are given in WMO (1966, 2011b).

1.6

UNCERTAINTY OF MEASUREMENTS

1.6.1

Meteorological measurements

1.6.1.1

General

This section deals with definitions that are relevant to the assessment of accuracy and the measurement of uncertainties in physical measurements, and concludes with statements of required and achievable uncertainties in meteorology. First, it discusses some issues that arise particularly in meteorological measurements. The term measurement is carefully defined in 1.6.1.2, but in most of the present Guide it is used less strictly to mean the process of measurement or its result, which may also be called an “observation”. A sample is a single measurement, typically one of a series of spot or instantaneous readings of a sensing system, from which an average or smoothed value is derived to make an observation. For a more theoretical approach to this discussion, see VolumeV, Chapters2 and 3 of the present Guide. The terms accuracy, error and uncertainty are carefully defined in1.6.1.2, which explains that accuracy is a qualitative term, the numerical expression of which is uncertainty. This is good practice and is the form followed in the present Guide. Formerly, the common and less precise use of accuracy was as in “an accuracy of ±x”, which should read “an uncertainty of x”.

CHAPTER 1. GENERAL

1.6.1.2

15

Definitions of measurements and measurement errors

The following terminology relating to the accuracy of measurements is based on JCGM (2012), which contains many definitions applicable to the practices of meteorological observations. Very useful and detailed practical guidance on the calculation and expression of uncertainty in measurements is given in ISO/IEC(2008)/ JCGM(2008). Measurement.  The process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity. Note:

The operations may be performed automatically.

Measuring instrument.  Device used for making measurements, alone or in conjunction with one or more supplementary devices. Examples: Platinum resistance thermometer (PRT), electronic barometer Note: instrument is sometimes used without the adjective measuring. If the instrument includes a sensor the adjective sensing may be used.

Sensor.  Element of a measuring system that is directly affected by a phenomenon, body, or substance carrying a quantity to be measured. Examples: Sensing coil of a PRT, Bourdon tube of a pressure gauge Note:

Sometimes the term “sensing element” is used for this concept.

Result of a measurement.  A set of quantity values being attributed to a measurand together with any other available relevant information. Notes: 1. When a result is given, it should be made clear whether it refers to the indication, the uncorrected result or the corrected result, and whether several values are averaged. 2. A complete statement of the result of a measurement includes information about the uncertainty of the measurement.

Corrected result.  The result of a measurement after correction for systematic error. Value (of a quantity).  A number and reference (unit) together expressing the magnitude of a quantity. Example: Length of a rod: 5.34 m True value (of a quantity).  The quantity value consistent with the definition of a quantity. Notes: 1. This is a value that would be obtained by a perfect measurement. 2. True values are by nature indeterminate.

Accuracy (of a measurement).  A qualitative term referring to the closeness of agreement between a measured quantity value and a true quantity value of a measurand. The accuracy of a measurement is sometimes understood as the closeness of agreement between measured quantity values that are being attributed to the measurand. It is possible to refer to an instrument or a measurement as having a high accuracy, but the quantitative measure of the accuracy is expressed in terms of uncertainty. Uncertainty.  A non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.

16

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Repeatability.  The closeness of agreement between indications or measured quantity values obtained on the same or similar objects under a set of conditions that includes the same measurement procedure, same operators, same measuring system, same operating conditions and same location, and replicate measurements over a short period of time. Note:

Relevant statistical terms are given in ISO (1994a) and ISO (1994b).

Reproducibility.  The closeness of agreement between indications or measured quantity values obtained on the same or similar objects under a set of conditions that includes different locations, operators and measuring systems, and replicate measurements. Error (of measurement).  Measured quantity value minus a reference quantity value. Instrumental bias.  Average of replicate indications minus a reference quantity value. Random error.  The component of measurement error that in replicate measurements varies in an unpredictable manner. Notes: 1. Random measurement error equals measurement error minus systematic measurement error. 2. A reference quantity value for a random measurement error is the average that would ensue from an infinite number of replicate measurements of the same measurand.

Systematic error.  The component of measurement error that in replicate measurements remains constant or varies in a predictable manner. Notes: 1. Systematic measurement error equals measurement error minus random measurement error. 2. Like true value, systematic error and its causes cannot be completely known.

Correction.  Compensation for an estimated systematic effect. Some definitions are also repeated in Volume V, Chapter 4 of the present Guide for convenience. 1.6.1.3

Characteristics of instruments

Some other properties of instruments which must be understood when considering their uncertainty are taken from JCGM (2012). Sensitivity.  Quotient of the change in an indication of a measuring system and the corresponding change in a value of a quantity being measured. Note:

The sensitivity of a measuring system can depend on the value of the quantity being measured.

Discrimination threshold.  The largest change in a value of a quantity being measured that causes no detectable change in the corresponding indication. Resolution.  The smallest change in a quantity being measured that causes a perceptible change in the corresponding indication. Hysteresis.  The property of a measuring instrument whereby its response to a given stimulus depends on the sequence of preceding stimuli. Stability (of an instrument).  The property of a measuring instrument whereby its metrological properties remain constant in time. Drift.  A continuous or incremental change over time in indication due to changes in metrological properties of a measuring instrument.

CHAPTER 1. GENERAL

17

Step response time.  The duration between the instant when an input quantity value of a measuring instrument or measuring system is subjected to an abrupt change between two specified constant quantity values and the instant when a corresponding indication settles within specified limits around its final steady value. The following other definitions are used frequently in meteorology: Statements of response time.  The time for 90% of the step change is often given. The time for 50% of the step change is sometimes referred to as the half-time. Calculation of response time.  In most simple systems, the response to a step change is: Y = A(1–e–t/τ) (1.1) where Y is the change after elapsed time t; A is the amplitude of the step change applied; t is the elapsed time from the step change; and τ is a characteristic variable of the system having the dimension of time.

The variable τ is referred to as the time constant or the lag coefficient. It is the time taken, after a step change, for the instrument to reach 1/e of the final steady reading.

In other systems, the response is more complicated and will not be considered here (see also VolumeV, Chapter2).

Lag error.  The error that a set of measurements may possess due to the finite response time of the observing instrument. 1.6.2

Sources and estimates of error

The sources of error in the various meteorological measurements are discussed in specific detail in the following chapters of the present Guide, but in general they may be seen as accumulating through the chain of traceability and the measurement conditions. It is convenient to take air temperature as an example to discuss how errors arise, but it is not difficult to adapt the following argument to pressure, wind and other meteorological quantities. For temperature, the sources of error in an individual measurement are as follows: (a) Errors in the international, national and working standards, and in the comparisons made between them. These may be assumed to be negligible for meteorological applications; (b) Errors in the comparisons made between the working, travelling and/or check standards and the field instruments in the laboratory or in liquid baths in the field (if that is how the traceability is established). These are small if the practice is good (say ±0.1K uncertainty at the 95% confidence level, including the errors in (a) above), but may quite easily be larger, depending on the skill of the operator and the quality of the equipment; (c) Non-linearity, drift, repeatability and reproducibility in the field thermometer and its transducer (depending on the type of thermometer element); (d) The effectiveness of the heat transfer between the thermometer element and the air in the thermometer shelter, which should ensure that the element is at thermal equilibrium with the air (related to system time constant or lag coefficient). In a well-designed aspirated shelter this error will be very small, but it may be large otherwise; (e) The effectiveness of the thermometer shelter, which should ensure that the air in the shelter is at the same temperature as the air immediately surrounding it. In a well-designed case this error is small, but the difference between an effective and an ineffective shelter may be 3°C or more in some circumstances;

18

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(f) The exposure, which should ensure that the shelter is at a temperature that is representative of the region to be monitored. Nearby sources and heat sinks (buildings, other unrepresentative surfaces below and around the shelter) and topography (hills, land–water boundaries) may introduce large errors. The station metadata should contain a good and regularly updated description of exposure (see Annex1.F) to inform data users about possible exposure errors. Systematic and random errors both arise at all the above-mentioned stages. The effects of the error sources (d) to (f) can be kept small if operations are performed very carefully and if convenient terrain for siting is available; otherwise these error sources may contribute to a very large overall error. However, they are sometimes overlooked in the discussion of errors, as though the laboratory calibration of the instruments could define the total error completely. Establishing the true value is difficult in meteorology (Linacre, 1992). Well-designed instrument comparisons in the field may establish the characteristics of instruments to give a good estimate of uncertainty arising from stages (a) to (e) above. If station exposure has been documented adequately, the effects of imperfect exposure can be corrected systematically for some parameters (for example, wind; see WMO, 2002) and should be estimated for others. Comparing station data against numerically analysed fields using neighbouring stations is an effective operational QC procedure, if there are sufficient reliable stations in the region. Differences between the individual observations at the station and the values interpolated from the analysed field are due to errors in the field as well as to the performance of the station. However, over a period, the average error at each point in the analysed field may be assumed to be zero if the surrounding stations are adequate for a sound analysis. In that case, the mean and standard deviation of the differences between the station and the analysed field may be calculated, and these may be taken as the errors in the station measurement system (including effects of exposure). The uncertainty in the estimate of the mean value in the long term may, thus, be made quite small (if the circumstances at the station do not change), and this is the basis of climate change studies. 1.6.3

The measurement uncertainties of a single instrument

ISO/IEC (2008)/JCGM (2008) should be used for the expression and calculation of uncertainties. It gives a detailed practical account of definitions and methods of reporting, and a comprehensive description of suitable statistical methods, with many illustrative examples. 1.6.3.1

The statistical distributions of observations

To determine the uncertainty of any individual measurement, a statistical approach is to be considered in the first place. For this purpose, the following definitions are stated (ISO/ IEC(2008)/JCGM(2008); JCGM, 2012): (a) Standard uncertainty; (b) Expanded uncertainty; (c) Variance, standard deviation; (d) Statistical coverage interval. If n comparisons of an operational instrument are made with the measured variable and all other significant variables held constant, if the best estimate of the true value is established by use of a reference standard, and if the measured variable has a Gaussian distribution,10 the results may be displayed as in Figure1.2.

10

However, note that several meteorological variables do not follow a Gaussian distribution. See 1.6.3.2.3.

CHAPTER 1. GENERAL

19

𝜎

T

O

Figure 1.2. The distribution of data in an instrument comparison In this figure, T is the true value, Ō is the mean of the n values O observed with one instrument, and σ is the standard deviation of the observed values with respect to their mean values. In this situation, the following characteristics can be identified: (a) The systematic error, often termed bias, given by the algebraic difference Ō – T. Systematic errors cannot be eliminated but may often be reduced. A correction factor can be applied to compensate for the systematic effect. Typically, appropriate calibrations and adjustments should be performed to eliminate the systematic errors of a measuring instrument. Systematic errors due to environmental or siting effects can only be reduced; (b) The random error, which arises from unpredictable or stochastic temporal and spatial variations. The measure of this random effect can be expressed by the standard deviation σ determined after nmeasurements, where n should be large enough. In principle, σ is a measure for the uncertainty of Ō; (c) The accuracy of measurement, which is the closeness of the agreement between the result of a measurement and a true value of the measurand. The accuracy of a measuring instrument is the ability to give responses close to a true value. Note that “accuracy” is a qualitative concept; (d) The uncertainty of measurement, which represents a parameter associated with the result of a measurement, that characterizes the dispersion of the values that could be reasonably attributed to the measurand. The uncertainties associated with the random and systematic effects that give rise to the error can be evaluated to express the uncertainty of measurement. 1.6.3.2

Estimating the true value

In normal practice, observations are used to make an estimate of the true value. If a systematic error does not exist or has been removed from the data, the true value can be approximated by taking the mean of a very large number of carefully executed independent measurements. When fewer measurements are available, their mean has a distribution of its own and only certain limits within which the true value can be expected to lie can be indicated. In order to do this, it is necessary to choose a statistical probability (level of confidence) for the limits, and the error distribution of the means must be known. A very useful and clear explanation of this notion and related subjects is given by Natrella (1966). Further discussion is given by Eisenhart (1963).

20 1.6.3.2.1

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Estimating the true value – n large

When the number of n observations is large, the distribution of the means of samples is Gaussian, even when the observational errors themselves are not. In this situation, or when the distribution of the means of samples is known to be Gaussian for other reasons, the limits between which the true value of the mean can be expected to lie are obtained from: Upper limit:

LU = X + k ⋅

Lower limit:

LL = X − k ⋅

σ n (1.2)

σ n

(1.3)

where X is the average of the observations Ō corrected for systematic error; σ is the standard deviation of the whole population; and k is a factor, according to the chosen level of confidence, which can be calculated using the normal distribution function. Some values of k are as follows: Level of confidence

90%

95%

99%

k

1.645

1.960

2.575

The level of confidence used in the table above is for the condition that the true value will not be outside the one particular limit (upper or lower) to be computed. When stating the level of confidence that the true value will lie between both limits, both the upper and lower outside zones have to be considered. With this in mind, it can be seen that k takes the value 1.96 for a 95% probability, and that the true value of the mean lies between the limits LU and LL . 1.6.3.2.2

Estimating the true value – n small

When n is small, the means of samples conform to Student’s t distribution provided that the observational errors have a Gaussian or near-Gaussian distribution. In this situation, and for a chosen level of confidence, the upper and lower limits can be obtained from: Upper limit:

LU ≈ X + t ⋅

Lower limit:

LL ≈ X − t ⋅

σ  n

σ  n

(1.4) (1.5)

where t is a factor (Student’s t) which depends upon the chosen level of confidence and the number n of measurements; and σ . is the estimate of the standard deviation of the whole population, made from the measurements obtained, using: n

2

σ  =

2

∑ i =1 ( X i − X ) n −1

=

n ⋅ σ 2 (1.6) n −1 0

where Xi is an individual value Oi corrected for systematic error. Some values of t are as follows: Level of confidence

90%

95%

99%

1

6.314

12.706

63.657

4

2.132

2.776

4.604

8

1.860

2.306

3.355

60

1.671

2.000

2.660

df

CHAPTER 1. GENERAL

21

where df is the degrees of freedom related to the number of measurements by df=n–1. The level of confidence used in this table is for the condition that the true value will not be outside the one particular limit (upper or lower) to be computed. When stating the level of confidence that the true value will lie between the two limits, allowance has to be made for the case in which n is large. With this in mind, it can be seen that t takes the value 2.306 for a 95% probability that the true value lies between the limits LU and LL , when the estimate is made from nine measurements (df=8). The values of t approach the values of k as n becomes large, and it can be seen that the values of k are very nearly equalled by the values of t when df equals 60. For this reason, tables of k (rather than tables of t) are quite often used when the number of measurements of a mean value is greater than 60 or so. 1.6.3.2.3

Estimating the true value – additional remarks

Investigators should consider whether or not the distribution of errors is likely to be Gaussian. The distribution of some variables themselves, such as sunshine, visibility, humidity and ceiling, is not Gaussian and their mathematical treatment must, therefore, be made according to rules valid for each particular distribution (Brooks and Carruthers, 1953). In practice, observations contain both random and systematic errors. In every case, the observed mean value has to be corrected for the systematic error insofar as it is known. When doing this, the estimate of the true value remains inaccurate because of the random errors as indicated by the expressions and because of any unknown component of the systematic error. Limits should be set to the uncertainty of the systematic error and should be added to those for random errors to obtain the overall uncertainty. However, unless the uncertainty of the systematic error can be expressed in probability terms and combined suitably with the random error, the level of confidence is not known. It is desirable, therefore, that the systematic error be fully determined. 1.6.3.3

Expressing the uncertainty

If random and systematic effects are recognized, but reduction or corrections are not possible or not applied, the resulting uncertainty of the measurement should be estimated. This uncertainty is determined after an estimation of the uncertainty arising from random effects and from imperfect correction of the result for systematic effects. It is common practice to express the uncertainty as “expanded uncertainty” in relation to the “statistical coverage interval”. To be consistent with common practice in metrology, the 95% confidence level, or k = 2, should be used for all types of measurements, namely: < expanded uncertainty > = k · σ = 2 · σ (1.7) As a result, the true value, defined in 1.6.1.2, will be expressed as: < true value> = < measured value> ± < expanded uncertainty > = < measured value> ± 2σ 1.6.3.4

Measurements of discrete values

While the state of the atmosphere may be described well by physical variables or quantities, a number of meteorological phenomena are expressed in terms of discrete values. Typical examples of such values are the detection of sunshine, precipitation or lightning and freezing precipitation. All these parameters can only be expressed by “yes” or “no”. For a number of parameters, all of which are members of the group of present weather phenomena, more than two possibilities exist. For instance, discrimination between drizzle, rain, snow, hail and their combinations is required when reporting present weather. For these practices, uncertainty calculations like those stated above are not applicable. Some of these parameters are related to a numerical threshold value (for example, sunshine detection using direct radiation intensity), and the determination of the uncertainty of any derived variable (for example, sunshine duration) can be calculated from the estimated uncertainty of the source variable (for example, direct radiation intensity). However, this method is applicable only for derived parameters, and not for the typical

22

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

present weather phenomena. Although a simple numerical approach cannot be presented, a number of statistical techniques are available to determine the quality of such observations. Such techniques are based on comparisons of two datasets, with one set defined as a reference. Such a comparison results in a contingency matrix, representing the cross-related frequencies of the mutual phenomena. In its most simple form, when a variable is Boolean (“yes” or “no”), such a matrix is a two by two matrix with the number of equal occurrences in the elements of the diagonal axis and the “missing hits” and “false alarms” in the other elements. Such a matrix makes it possible to derive verification scores or indices to be representative for the quality of the observation. This technique is described by Murphy and Katz (1985). An overview is given by Kok (2000). 1.6.4

Accuracy requirements

1.6.4.1

General

The uncertainty with which a meteorological variable should be measured varies with the specific purpose for which the measurement is required. In general, the limits of performance of a measuring device or system will be determined by the variability of the element to be measured on the spatial and temporal scales appropriate to the application. Any measurement can be regarded as made up of two parts: the signal and the noise. The signal constitutes the quantity which is to be determined, and the noise is the part which is irrelevant. The noise may arise in several ways: from observational error, because the observation is not made at the right time and place, or because short-period or small-scale irregularities occur in the observed quantity which are irrelevant to the observations and need to be smoothed out. Assuming that the observational error could be reduced at will, the noise arising from other causes would set a limit to the accuracy. Further refinement in the observing technique would improve the measurement of the noise but would not give much better results for the signal. At the other extreme, an instrument – the error of which is greater than the amplitude of the signal itself – can give little or no information about the signal. Thus, for various purposes, the amplitudes of the noise and the signal serve, respectively, to determine: (a) The limits of performance beyond which improvement is unnecessary; (b) The limits of performance below which the data obtained would be of negligible value. This argument, defining and determining limits (a) and (b) above, was developed extensively for upper-air data by WMO (1970). However, statements of requirements are usually derived not from such reasoning but from perceptions of practically attainable performance, on the one hand, and the needs of the data users, on the other. 1.6.4.2

Required and achievable performance

The performance of a measuring system includes its reliability, capital, recurrent and lifetime cost, and spatial resolution, but the performance under discussion here is confined to uncertainty (including scale resolution) and resolution in time. Various statements of requirements have been made, and both needs and capability change with time. The statements given in Annex1.A are the most authoritative at the time of writing, and may be taken as useful guides for development, but they are not fully definitive. The requirements for the variables most commonly used in synoptic, aviation and marine meteorology, and in climatology are summarized in Annex1.A.11 It gives requirements only for 11

Established by the Commission for Basic Systems (CBS) Expert Team on Requirements for Data from Automatic Weather Stations (2004) and approved by the president of CIMO for inclusion in the present Guide after consultation with the presidents of the other technical commissions.

CHAPTER 1. GENERAL

23

surface measurements that are exchanged internationally. Details on the observational data requirements for Global Data-processing and Forecasting System Centres for global and regional exchange are given in WMO (2010c). The uncertainty requirement for wind measurements is given separately for speed and direction because that is how wind is reported. The ability of individual sensing instruments or observing systems to meet the stated requirements is changing constantly as instrumentation and observing technology advance. The characteristics of typical instruments or systems currently available are given in Annex1.A.12 It should be noted that the achievable operational uncertainty in many cases does not meet the stated requirements. For some of the quantities, these uncertainties are achievable only with the highest quality equipment and procedures. Uncertainty requirements for upper-air measurements are dealt with in the present volume, Chapter 12.

12

Established by the CIMO Expert Team on Surface Technology and Measurement Techniques (2004) and confirmed for inclusion in the present Guide by the president of CIMO.

ANNEX 1.A. OPERATIONAL MEASUREMENT UNCERTAINTY REQUIREMENTS AND INSTRUMENT PERFORMANCE REQUIREMENTS (See explanatory notes at the end of the table; numbers in the top row indicate column numbers.) 1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

1.1 Air temperature

–80 °C to 60 °C

0.1 K

I

0.3 K for ≤ –40 °C 0.1 K for > –40 °C and ≤ 40 °C 0.3 K for > 40 °C

20 s

1 min

0.2 K

1.2 Extremes of air temperature

–80 °C to 60 °C

0.1 K

I

0.5 K for ≤ –40 °C 0.3 K for > –40 °C and ≤ 40 °C 0.5 K for > 40 °C

20 s

1 min

0.2 K

1.3 Sea-surface temperature

–2 °C to 40 °C

0.1 K

I

0.1 K

20 s

1 min

0.2 K

1.4 Soil temperature

–50 °C to 50 °C

0.1 K

I

20 s

1 min

0.2 K

1. Temperature Achievable uncertainty and effective time constant may be affected by the design of the thermometer solar radiation screen. Time constant depends on the airflow over the sensing element

25

CHAPTER 1. GENERAL

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

–80 °C to 35 °C

0.1 K

I

0.1 K

20 s

1 min

0.25 K

Measurement uncertainty depends on the deviation from air temperature

2. Humidity 2.1 Dewpoint temperature

Wet-bulb temperature (psychrometer) 2.2 Relative humidity

0%–100%

1%

I

1%

20 s

1 min

0.2 K

If measured directly and in combination with air temperature (dry bulb). Large errors are possible due to aspiration and cleanliness problems (see also note 11). Threshold of 0°C to be noticed for wet bulb

Solid state and others 40 s

1 min

3%

Time constant and achievable uncertainty of solid-state sensing instruments may show significant temperature and humidity dependence

26

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

500–1080 hPa

0.1 hPa

I

0.1 hPa

2s

1 min

0.15 hPa

Both station pressure and MSL pressure. Measurement uncertainty is seriously affected by dynamic pressure due to wind if no precautions are taken. Inadequate temperature compensation of the transducer may affect the measurement uncertainty significantly. MSL pressure is affected by the uncertainty in altitude of the barometer for measurements on board ships

Not specified

0.1 hPa

I

0.2 hPa

0.2 hPa

Difference between instantaneous values

3. Atmospheric pressure 3.1 Pressure

3.2 Tendency

27

CHAPTER 1. GENERAL

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

4.1 Cloud amount

0/8–8/8

1/8

I

1/8

n/a

2/8

4.2 Height of cloud base

0 m–30 km

10 m

I

10 m for ≤ 100 m 10% for > 100 m

n/a

~10 m

4.3 Height of cloud top

Not available

4. Clouds Period clustering algorithms may be used to estimate low cloud amount automatically Achievable measurement uncertainty can be determined with a hard target. No clear definition exists for instrumentally measured cloud-base height (e.g., based on penetration depth or significant discontinuity in the extinction profile). Significant bias during precipitation

28

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

0–75 m s–1

0.5 m s–1

A

0.5 m s–1 for ≤ 5 m s–1 10% for > 5 m s–1

Distance constant 2–5 m

2 and/or 10 min

0°–360°

A

Damping ratio > 0.3

2 and/or 10 min

0.1–150 m s–1

0.1 m s–1

A

10%

3s

0.5 m s–1 for ≤ 5 m s–1 10% for > 5 m s–1

5. Wind 5.1 Speed

5.2 Direction

5.3 Gusts

Average over 2 and/or 10min Non-linear devices. Care needed in design of averaging process. Distance constant is usually expressed as response length Averages computed over Cartesian components (see Volume V, Chapter3, 3.6 of the present Guide). When using ultrasonic anemometers, no distance constant or time constant is needed. For moving mobile stations, the movement of the station needs to be taken into account, inclusive of its uncertainty Highest 3 s average should be recorded

29

CHAPTER 1. GENERAL

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

6.1 Amount (daily)

0–500 mm

0.1 mm

T

0.1 mm for ≤ 5 mm 2% for > 5 mm

n/a

n/a

The larger of 5% or 0.1 mm

Quantity based on daily amounts. Measurement uncertainty depends on aerodynamic collection efficiency of gauges and evaporation losses in heated gauges

6.2 Depth of snow

0–25 m

1 cm

I

1 cm for ≤ 20 cm 5% for > 20 cm

< 10 s

1 min

1 cm

Average depth over an area representative of the observing site

6.3 Thickness of ice accretion on ships

Not specified

1 cm

I

1 cm for ≤ 10 cm 10% for > 10 cm

0.02–2000 mmh–1

0.1 mmh–1

I

(trace): n/a for 0.02–0.2 mmh–1 0.1 mmh–1 for 0.2–2 mmh–1 5% for > 2 mmh–1

< 30 s

1 min

Under constant flow conditions in laboratory, 5% above 2 mmh-1, 2% above 10mmh-1 In field, 5mmh-1 and 5% above 100mmh-1

Uncertainty values for liquid precipitation only. Uncertainty is seriously affected by wind. Instruments may show significant non-linear behaviour. For < 0.2mmh–1: detection only (yes/no) instrument time constant is significantly affected during solid precipitation using catchment type of gauges

0–24 h

60 s

T

n/a

60 s

6. Precipitation

6.4 Precipitation intensity

6.5 Precipitation duration (daily)

Threshold value of 0.02mmh-1

30

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

0–24 h

60 s

T

0.1 h

20 s

n/a

The larger of 0.1 h or 2%

7.2 Net radiation, radiant exposure (daily)

Not specified

1 J m–2

T

0.4 MJ m–2 for ≤ 8 MJ m–2 5% for > 8 MJ m–2

20 s

n/a

15%

7.3 Global downward/ upward solar radiation

Not specified

1 J m–2

T

2%

20 s

n/a

5% (daily) 8% (hourly)

7.4 Downward/ upward long-wave radiation at Earth surface

Not specified

1 J m–2

T

5%

20 s

n/a

10%

7. Radiation 7.1 Sunshine duration (daily)

Radiant exposure expressed as daily sums (amount) of (net) radiation. Best achievable operational uncertainty is obtained by combining the measurements of two pyranometers and two pyrgeometers Daily total exposure

31

CHAPTER 1. GENERAL

1

2

3

4

5

6

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

8.1 Meteorological optical range (MOR)

10 m–100 km

1m

I

50 m for ≤ 600 m 10% for > 600 m– ≤ 1500 m 20% for > 1 500 m

< 30 s

8.2 Runway visual range

10–2 000 m

1m

A

10 m for ≤ 400 m 25 m for > 400 m– ≤ 800m 10% for > 800 m

0–40 000 cd m–2

1 cd m–2

I

7

8

9

Achievable measurement uncertainty

Remarks

1 and 10 min

The larger of 20 m or 20%

Achievable measurement uncertainty may depend on the cause of obscuration. Quantity to be averaged: extinction coefficient (see VolumeV, Chapter3, 3.6 of the present Guide). Preference for averaging logarithmic values

< 30 s

1 and 10min

The larger of 20m or 20%

In accordance with WMO-No.49, VolumeII, AttachmentA (2004ed.) and the International Civil Aviation Organization (ICAO) Doc9328-AN/908 (seconded., 2000). New versions of these documents may exist, specifying other values.

30 s

1 min

10%

Instrument Output time averaging time constant

8. Visibility

8.3 Background luminance

Related to 8.2 Runway visual range

32

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1

2

3

4

5

6

7

8

9

Variable

Range

Reported resolution

Mode of measurement/ observation

Required measurement uncertainty

Instrument time constant

Output averaging time

Achievable measurement uncertainty

Remarks

9.1 Significant wave height

0–50 m

0.1 m

A

0.5 m for ≤ 5 m 10% for > 5 m

0.5 s

20 min

0.5 m for ≤ 5 m 10% for > 5 m

Average over 20min for instrumental measurements

9.2 Wave period

0–100 s

1s

A

0.5 s

0.5 s

20 min

0.5 s

Average over 20min for instrumental measurements

9.3 Wave direction

0–360°

A

10°

0.5 s

20 min

20°

Average over 20min for instrumental measurements

0–100 mm

0.1 mm

T

0.1 mmfor ≤ 5 mm 2% for > 5 mm

n/a

9. Waves

10. Evaporation 10.1 Amount of pan evaporation

Notes: 1. Column 1 gives the basic variable. 2. Column 2 gives the common range for most variables; limits depend on local climatological conditions. 3. Column 3 gives the most stringent resolution as determined by the Manual on Codes (WMO-No.306). 4. In column 4: I = Instantaneous: to exclude the natural small-scale variability and the noise, an average value over a period of 1min is considered as a minimum and most suitable; averages over periods of up to 10min are acceptable. A = Averaging: average values over a fixed period, as specified by the coding requirements. T = Totals: totals over a fixed period, as specified by coding requirements. 5. Column 5 gives the recommended measurement uncertainty requirements for general operational use, that is, of level II data according to FM12, 13, 14, 15 and its BUFR equivalents. They have been adopted by all eight technical commissions and are applicable for synoptic, aeronautical, agricultural and marine meteorology, hydrology, climatology, and the like. These requirements are applicable for both manned weather stations and AWSs as defined in the Manual on the WMO Integrated Global Observing System (WMO-No.1160). Individual applications may have less stringent requirements. The stated value of required measurement uncertainty represents the uncertainty of the reported value with respect to the true value and indicates the interval in which the true value lies with a stated probability. The recommended probability level is 95% (k = 2), which corresponds to the 2σ level for a normal (Gaussian) distribution of the variable. The assumption that all known corrections are taken into account implies that the errors in reported values will have a mean value (or bias) close to zero. Any residual bias should be small compared with the stated measurement uncertainty requirement. The true value is the value which, under operational conditions, perfectly characterizes the variable to be measured/observed over the representative time interval, area and/or volume required, taking into account siting and exposure.

CHAPTER 1. GENERAL

Notes (cont.) 6. Columns 2 to 5 refer to the requirements established by the CBS Expert Team on Requirements for Data from AWSs in 2004. 7. Columns 6 to 8 refer to the typical operational performance established by the CIMO Expert Team on Surface Technology and Measurement Techniques in 2004. 8. Achievable measurement uncertainty (column 8) is based on instrument performance under nominal and recommended exposure that can be achieved in operational practice. It should be regarded as a practical aid to users in defining achievable and affordable requirements. 9. n/a = not applicable. 10. The term “uncertainty” has preference over “accuracy” (that is, uncertainty is in accordance with ISO/IEC/JCGM standards on the uncertainty of measurements (ISO/IEC(2008); JCGM(2008)). 11. Dewpoint temperature, relative humidity and air temperature are linked, and thus their uncertainties are linked. When averaging, preference is given to absolute humidity as the principal variable.

33

ANNEX 1.B. STRATEGY FOR TRACEABILITY ASSURANCE

1.

INTRODUCTION

Traceability of measurement and calibration results plays a key role for many application areas, ranging obviously from the assessment of climate variability and changes, but also to aspects that may have strong economic and legal impacts in the context of issuance of warnings for severe weather to protect lives and livelihoods. Ensuring metrological traceability enables full confidence in the validity of measurement results, which leads to confidence in the implications of the measurement data: in the forecasts and warnings derived from the measurements; in climate analyses and trends derived from the measurements. And this in turn leads to improvements in disaster risk reduction (DRR), climate change mitigation, advice for policy developers, human health and safety, and property protection. The lack of traceability of measurement results was recognized as a major concern by CIMO because the full potential of WIGOS would be brought into question without regular traceability. Therefore, CIMO stressed the need to sensitize NMHSs to the necessity of regular instrument calibrations, in addition to preventive maintenance and periodical instrument checks, as an essential tool to ensure the required traceability and quality of measurement results. Numerous developing-country Members have no calibration laboratory at all to ensure the traceability of their instruments. Some Members are also facing challenges with the calibration of their network instruments and are replacing a comprehensive calibration strategy with a policy of carrying out field verification checks to identify instruments that do not conform to the required uncertainties and to perform complete laboratory calibrations only on these instruments. Field verification checks should cover the full measurement range, similarly to on-site regular calibrations, and they should be distinguished from the field inspections (see VolumeV, Chapter4, 4.3.4 of the present Guide), which are usually performed at one point (ambient conditions) and considered as “one-point” calibrations. The strategy presented in this annex seeks to build upon best available practices to strengthen calibration services and improve traceability assurance across WMO Members. It focuses on providing widely acceptable guidelines to increase confidence in measurement results.

2.

OBJECTIVE OF THE STRATEGY

The main objective of the calibration strategy for traceability assurance is to ensure the proper traceability of measurement and calibration results to the SI, through an unbroken chain of calibrations, each contributing to the measurement uncertainty. This strategy applies to meteorological measurements for which a traceability chain to the SI is well established (for example, measurements of temperature, atmospheric pressure, humidity, wind speed, precipitation and solar radiation). The strategy aims to provide guidance on how to effectively and efficiently achieve this objective.

3.

RESPONSIBILITY FOR IMPLEMENTING THE STRATEGY

The responsibility for traceability assurance lies with WMO Members, who should ensure all the required calibrations as well as other necessary steps to achieve the objective of the strategy.

CHAPTER 1. GENERAL

35

It is up to each NMHS to choose the most suitable approach for its traceability assurance, but ensuring the metrological traceability of all measurement results is strongly recommended.

4.

WAYS OF TRACEABILITY ASSURANCE

Simplifying the ISO/JCGM definition, metrological traceability could be described as a direct link between a result of a measurement made in the field and a result obtained by the calibration process in a calibration laboratory. It ensures that different measurement methods and instruments used in different countries at different times produce reliable, repeatable, reproducible, compatible and comparable measurement results. When a measurement result is metrologically traceable, it can be confidently linked to the internationally accepted measurement references. At the top of the metrological traceability chain there is an internationally defined and accepted reference, in most cases the SI, whose technical and organizational infrastructure has been developed and maintained by BIPM (www​.bipm​.org). The framework through which NMIs demonstrate the international equivalence of their measurement standards and the calibration and measurement certificates they issue is called the Comité international des poids et mesures (CIPM) Mutual Recognition Arrangement (CIPM MRA). The outcomes of the MRA are the internationally recognized (peer-reviewed and approved) Calibration and Measurement Capabilities (CMCs) of the participating institutes. Approved CMCs and supporting technical data are publicly available from the CIPM MRA Key Comparison Database (http://​kcdb​.bipm​.org/​). NMIs are responsible for maintenance of national standards and dissemination of traceability on the national level, either by themselves or by DIs. DIs are experienced institutes operating at the top of the national metrology system, but are not part of formal NMI structure. They are designated to be responsible for certain national standards and associated services that are not covered by the regular activities of NMIs. Further dissemination of traceability relies on accredited calibration laboratories whose implemented quality management system is accredited by a national accreditation body. National accreditation bodies are usually signatories of the International Laboratory Accreditation Cooperation Mutual Recognition Arrangement, which ensures the acceptance of and confidence in calibration certificates across national borders. Whenever possible, all the measurements within any particular country have to be traceable to the SI. Taking into account all the aforementioned, as well as WMO Members’ capabilities and needs, the following scenarios of traceability assurance (or lack of) can be identified (numbers indicate subsequent sections treating the subject): 4.1 Fully assured traceability – target, high confidence level in measurements; 4.2 Assured traceability (without accreditation) – good confidence level but some risks; improvement recommended; 4.3 Partially assured traceability – poor confidence and high risk; improvement required; 4.4 Lack of traceability – level of confidence cannot be assessed; urgent need for improvement.

36

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

4.1

Fully assured traceability – target, high confidence level in measurements

This traceability assurance (Figure 1.B.1) ensures fully traceable meteorological measurement results provided by particular NMHSs, to international standards. The whole traceability chain is covered by accreditation according to ISO/IEC 17025 and /or by CIPM MRA. The NMHS field instruments have to be calibrated in the accredited calibration laboratory regularly, ensuring the highest achievable measurement uncertainties. In the case that the calibration laboratory is also accredited for on-site calibrations that cover the whole range of meteorological parameters, those calibrations can be performed, but particular care concerning the required and achievable uncertainties must be taken into account. If on-site calibrations are not covered by accreditation they must not be used for regular traceability assurance, but as field verification checks only. Field checks are not part of traceability assurance. They can only be used as an additional QC aiming to identify instruments performing outside of required uncertainties. The following preconditions must be met to achieve this status: –

NMHS has a calibration laboratory;

Laboratory personnel are well trained and competent to properly operate laboratory standards and equipment;

Calibration standards and equipment meet the target uncertainties required for calibrations of meteorological instruments;

BIPM/SI units

CIPM MRA CIP

BIPM/SI units NMI/DI MI/D

NMI/DI

C 17025 ISO/IEC

ISO/IEC 17025

Cal lab

RIC

ISO/IEC 17025

NMHS/Cal lab

Measuring instrument

Measuring instrument

Measuring instrument

Figure 1.B.1. Fully assured traceability – target, high confidence level in measurements

CHAPTER 1. GENERAL

37

Calibration standards and equipment are regularly calibrated and maintained;

Quality management system, including all the calibration procedures, working instructions and forms, is well documented and applied in laboratory work;

Calibration laboratory is accredited according to ISO/IEC 17025;

Calibration laboratory participates in interlaboratory comparisons.

A determined engagement of the NMHS management board to support continuous strengthening of the calibration laboratory should be stated. This should be followed by a clear policy on the needs for regular calibrations of meteorological instruments for which standards exist, under the responsibility of the NMHS, including the defined calibration intervals, as well as policy on implementation of calibration results. Traceability of the laboratory standards and equipment has to be assured, by the means of calibrations at an NMI, DI, an accredited WMO RIC, or other accredited calibration laboratory, aiming at meeting the requirements of the Member in terms of target uncertainty. The NMHS calibration laboratory should also, jointly with other relevant departments, develop procedures aimed at avoiding gaps in field measurements due to calibration activities. This should be achieved by a small reserve of calibrated instruments that can be used as a replacement set for the instruments in the network. Those recovered should be calibrated in the laboratory, forming, as a consequence, a new replacement set, and so on, to cover the whole network. Additional QC could be assured by performing non-accredited on-site calibrations or field verification checks, but only to identify instruments performing outside uncertainty specifications. The instruments identified must be calibrated according to the accredited calibration methods. A set of travelling standards and/or portable calibration devices used for non-accredited on-site calibrations or field checks must be regularly calibrated in the accredited calibration laboratory, and checked before and after field use. 4.2

Assured traceability (without accreditation) – good confidence level but some risks; improvement recommended

This type of traceability assurance (Figure 1.B.2) is still appropriate and acceptable, but does not ensure fully traceable meteorological measurement results. It is applicable to NMHSs with calibration facilities, but without accreditation according to ISO/IEC 17025. Although these calibration laboratories are not accredited, their calibration standards have to be calibrated by accredited calibration laboratories, accredited RICs, or by laboratories that are signatories of CIPM MRA. The least appropriate way, but still acceptable, could be a calibration done by non-accredited RIC, but that RIC must demonstrate fully assured traceability of its calibration standards. The NMHS field instruments have to be calibrated either in the calibration laboratory (if it exists), or on site by portable calibration devices that are themselves calibrated at accredited laboratories and that cover the whole range of meteorological parameters. All calibrations have to be performed regularly ensuring the highest achievable measurement uncertainty. Field verification checks can be used as an additional QC, aiming to identify instruments performing outside required uncertainties, but not for the traceability assurance. The following preconditions must be met to achieve this status: –

NMHS has a calibration laboratory, or at least portable calibration devices covering the whole range of measured meteorological parameters;

38

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Laboratory personnel are well trained and competent to properly operate calibration standards and equipment;

Calibration standards and equipment meet the target uncertainties required for calibrations of meteorological instruments;

Calibration standards and equipment are regularly calibrated and maintained.

In addition, the following are highly recommended: –

Quality management system, including all the calibration procedures, working instructions and forms, should be documented and applied in laboratory work;

Although not accredited, calibration facilities should follow the requirements of ISO/IEC 17025;

Participation in the interlaboratory comparisons, which will be of great benefit.

Traceability of the laboratory standards and equipment has to be assured by the means of calibrations at an NMI, DI, RIC, or other accredited calibration laboratory. Non-accredited RICs must demonstrate traceability of their standards to the SI through an accredited laboratory, NMI or DI. A determined engagement of the NMHS management board to support continuous strengthening of the calibration facilities is desired. It should be followed by a defined policy on

CIPM MRA CIP

BIPM/SI units NMI/DI MI/D

NMI/DI

ISO/IEC 17025

Cal lab

RIC

ISO/IEC 17025

NMHS/Cal lab (portable calibration device)

Measuring instrument

Measuring instrument

Measuring instrument

Figure 1.B.2. Assured traceability (without accreditation) – good confidence level but some risks; improvement recommended

39

CHAPTER 1. GENERAL

the needs for regular calibrations of all meteorological instruments under the responsibility of the NMHS, including the calibration intervals, as well as policy on implementation of calibration results. The procedures aiming to avoid gaps in field measurements due to calibration activities should be developed. A possible solution is that the NMHS has at its disposal a small reserve of calibrated instruments that can be used as a replacement set for the instruments in the network. Those recovered should be calibrated regularly forming, as a consequence, a new replacement set, and so on, to cover the whole network. Additional QC could be assured by performing field verification checks, but only to identify instruments out of uncertainty specifications. A set of travelling standards or portable calibration devices used for field checks has to be regularly calibrated in the calibration laboratory, and checked before and after field use. 4.3

Partially assured traceability – poor confidence and high risk; improvement required

This way of traceability assurance (Figure 1.B.3) is the least appropriate and should be followed only when the two aforementioned scenarios are not applicable. It is applicable to NMHSs without a calibration laboratory and portable calibration devices, but with a field inspection kit. The field inspection kit must be regularly calibrated by accredited calibration laboratories, accredited RICs, calibration laboratories that are signatories of CIPM MRA, or in the worst case by

CIPM MRA CIP

BIPM/SI units NMI/DI MI/D

NMI/DI

ISO/IEC 17025

Cal lab

RIC/Cal lab

ISO/IEC 17025

NMHS/Field inspection kit

Measuring instrument

Measuring instrument

Measuring instrument

Figure 1.B.3. Partially assured traceability – poor confidence and high risk; improvement required

40

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

non-accredited RICs or calibration laboratories. The latter should only be used in the absence of all the aforementioned options and only when those laboratories can demonstrate fully assured traceability of their calibration standards. A field inspection is not equivalent to a regular laboratory calibration or a field verification check, but could be an acceptable means of ensuring the quality of network observations. The field inspection can be considered as a “one-point calibration”. To enable at least partially assured traceability, Members are encouraged to achieve the following: –

A field inspection kit should be acquired with the required metrological characteristics regarding field instruments and with a calibration certificate issued by an accredited calibration laboratory;

A cost-effective field inspection kit should include travelling instruments for the measurement of, as a minimum, pressure, temperature, humidity and rainfall;

The field inspection kit should be regularly calibrated by an accredited calibration laboratory, accredited RIC, or by an NMI or DI. In the case that accredited calibration services are not available, the chosen calibration laboratory must demonstrate fully assured traceability;

The field inspection kit should be checked before and after field use and cross-checked whenever more than one kit exists;

Personnel designated to operate the field inspection kit should be well trained and competent to perform field inspections;

Technical procedures for operating the field inspection kit should be documented;

Field inspections should be performed on a regular time base;

The results of field inspections must be documented.

4.4

Lack of traceability – not appropriate

Lack of metrological traceability leads to a lack of reliability of meteorological measurements, and consequently highly reduces confidence in the implications of measurement data such as weather forecasts, warnings and climate analyses. Ultimately, this brings into question the usefulness of meteorological measurements for the global community. So the consequences of untraceable measurement results are severe. Therefore, measurement traceability is essential and WMO Members are urged to assure traceability of all the measurements under their responsibility.

ANNEX 1.C. REGIONAL INSTRUMENT CENTRES

Note: Information on RIC capabilities and activities is available athttps://​w ww​.wmo​.int/​pages/​prog/​w ww/​IMOP/​ instrument​-reg​- centres​.html.

The Commission for Instruments and Methods of Observation recommended,1 at its seventeenth session held in 2018, the following terms of reference for all RICs. Regional Instrument Centres shall have the following capabilities to carry out their corresponding functions: Capabilities: (a) An RIC shall have the necessary facilities and laboratory equipment to perform the functions necessary for the calibration of meteorological and related environmental instruments; (b) An RIC shall maintain a set of meteorological standard instruments and establish the traceability of its own measurement standards and measuring instruments to the SI; (c) An RIC shall have competent managerial and technical staff to fulfil its functions; (d) An RIC shall have technical procedures for calibration of meteorological and related environmental instruments using calibration equipment employed by the RIC; (e) An RIC shall have and maintain a quality management system, preferably according to the ISO/IEC 17025 standard; (f) An RIC shall participate in, and/or organize inter-laboratory comparisons of standard calibration instruments and methods; (g) An RIC shall, as appropriate, utilize the available resources and capabilities to the Members’ best interest; (h) An RIC shall, as far as possible, apply international standards applicable for calibration laboratories, such as ISO/IEC 17025 standard;2 (i) An RIC shall ensure it is assessed by a recognized authority or by a WMO evaluation team, at least every four years, to verify its capabilities and performance. Corresponding functions: (a) An RIC shall assist Members of the Region, and possibly of other Regions, in calibrating their national meteorological standards and related environmental monitoring instruments; (b) An RIC shall participate in, and/or organize, inter-laboratory comparisons, and support instrument intercomparisons following relevant WMO recommendations; (c) According to relevant recommendations on the WMO Quality Management Framework, an RIC shall make a positive contribution to Members regarding the quality of measurements; (d) An RIC shall advise Members on enquiries regarding instrument performance, maintenance and the availability of relevant guidance materials;

1 2

Recommendation 2 (CIMO-17). RICs with accreditation for at least one parameter are called “ISO/IEC 17025-accredited RICs”. Those without accreditation are strongly encouraged to achieve it as soon as possible.

42

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(e) An RIC shall actively participate, or assist, in the organization of workshops on calibration and maintenance of meteorological and related environmental instruments; (f) An RIC shall contribute to the standardization of meteorological and related environmental measurements; (g) An RIC shall conduct or support the regular assessment of Members’ needs for RIC services; (h) An RIC shall regularly inform Members and report,3 on an annual basis, to the WMO Secretariat on the services offered to Members and activities carried out.

3

A word file RIC-Reporting Form (.docx), available at WMO/IMOP website, is recommended.

ANNEX 1.D. SITING CLASSIFICATIONS FOR SURFACE OBSERVING STATIONS ON LAND (This annex presents the text of a common ISO/WMO standard. It is also published, with identical content, as ISO 19289:2014(E)) Note: In this annex the word “sensor” is not used in the way it is defined in 1.6.1.2 of this chapter. According to this definition it should be replaced with the word “instrument”. As this Annex is just referencing the text of the ISO standard this has not been changed.

INTRODUCTION The environmental conditions of a site1 may influence measurement results. These conditions must be carefully analysed, in addition to assessing characteristics of the instrument itself, so as to avoid distorting the measurement results and affecting their representativeness, particularly when a site is supposed to be representative of a large area (that is, 100 to 1000km2).

1.

SCOPE

This annex 2 indicates exposure rules for various sensors. But what should be done when these conditions are not fulfilled? There are sites that do not respect the recommended exposure rules. Consequently, a classification has been established to help determine the given site’s representativeness on a small scale (impact of the surrounding environment). Hence, a class1 site can be considered as a reference site. A class5 site is a site where nearby obstacles create an inappropriate environment for a meteorological measurement that is intended to be representative of a wide area (at least tens of km2). The smaller the siting class, the higher the representativeness of the measurement for a wide area. In a perfect world, all sites would be in class1, but the real world is not perfect and some compromises are necessary. A site with a poor class number (large number) can still be valuable for a specific application needing a measurement in this particular site, including its local obstacles. The classification process helps the actors and managers of a network to better take into consideration the exposure rules, and thus it often improves the siting. At least, the siting environment is known and documented in the metadata. It is obviously possible and recommended to fully document the site, but the risk is that a fully documented site may increase the complexity of the metadata, which would often restrict their operational use. That is why this siting classification is defined to condense the information and facilitate the operational use of this metadata information. A site as a whole has no single classification number. Each parameter being measured at a site has its own class, and is sometimes different from the others. If a global classification of a site is required, the maximum value of the parameters’ classes can be used.

1 2

A “site” is defined as the place where the instrument is installed. Whereas this is referred to as an annex in the present Guide, it is referred to as a standard in the ISO document.

44

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The rating of each site should be reviewed periodically as environmental circumstances can change over a period of time. A systematic yearly visual check is recommended: if some aspects of the environment have changed, a new classification process is necessary. A complete update of the site classes should be done at least every five years. In the following text, the classification is (occasionally) completed with an estimated uncertainty due to siting, which has to be added in the uncertainty budget of the measurement. This estimation is coming from bibliographic studies and/or some comparative tests. The primary objective of this classification is to document the presence of obstacles close to the measurement site. Therefore, natural relief of the landscape may not be taken into account, if far away (that is, > 1km). A method to judge if the relief is representative of the surrounding area is the following: does a move of the station by 500m change the class obtained? If the answer is no, the relief is a natural characteristic of the area and is not taken into account. Complex terrain or urban areas generally lead to high class numbers. In such cases, an additional flag “S” can be added to class numbers 4 or 5 to indicate specific environment or application (that is,4S).

2.

AIR TEMPERATURE AND HUMIDITY

2.1

General

Sensors situated inside a screen should be mounted at a height determined by the meteorological service (within 1.25 to 2m as indicated in the present volume, Chapter 2, 2.1.4.2.1). The height should never be less than 1.25m. The respect of the higher limit is less stringent, as the temperature gradient versus height is decreasing with height. For example, the difference in temperature for sensors located between 1.5 and 2m is less than 0.2°C. The main discrepancies are caused by unnatural surfaces and shading: (a) Obstacles around the screen influence the irradiative balance of the screen. A screen close to a vertical obstacle may be shaded from the solar radiation or “protected” against the night radiative cooling of the air, by receiving the warmer infrared (IR) radiation from this obstacle or influenced by reflected radiation; (b) Neighbouring artificial surfaces may heat the air and should be avoided. The extent of their influence depends on the wind conditions, as wind affects the extent of air exchange. Unnatural or artificial surfaces to take into account are heat sources, reflective surfaces (for example buildings, concrete surfaces, car parks) and water or moisture sources (for example, ponds, lakes, irrigated areas). Shading by nearby obstacles should be avoided. Shading due to natural relief is not taken into account for the classification (see above). The indicated vegetation growth height represents the height of the vegetation maintained in a “routine” manner. A distinction is made between structural vegetation height (per type of vegetation present on the site) and height resulting from poor maintenance. Classification of the given site is therefore made on the assumption of regular maintenance (unless such maintenance is not practicable).

45

CHAPTER 1. GENERAL

Class 1

2.2

(a) Flat, horizontal land, surrounded by an open space, slope less than ⅓ (19°); (b) Ground covered with natural and low vegetation (< 10 cm) representative of the region; (c) Measurement point situated: (i)

At more than 100 m from heat sources or reflective surfaces (buildings, concrete surfaces, car parks, and the like);

(ii) At more than 100 m from an expanse of water (unless significant of the region); (iii) Away from all projected shade when the sun is higher than 5°. A source of heat (or expanse of water) is considered to have an impact if it occupies more than 10% of the surface within a circular radius of 100m surrounding the screen, makes up 5% of an annulus of 10–30m, or covers 1% of a 10m radius area.

Heat sources (building, car parks, concrete surface)

S = surface of heat sources

≤ 19º

S ≤ 5%

Low vegetation < 10 cm 100 m

10 m S ≤ 1%

≥ 100 m

Lake...

30 m

S ≤ 10%

5º ≥ 100 m

Figure 1.D.1. Criteria for air temperature and humidity for class 1 sites

Class 2

2.3

(a) Flat, horizontal land, surrounded by an open space, slope inclination less than ⅓ (19°); (b) Ground covered with natural and low vegetation (< 10 cm) representative of the region; (c) Measurement point situated: (i)

At more than 30 m from artificial heat sources or reflective surfaces (buildings, concrete surfaces, car parks, and the like);

(ii) At more than 30 m from an expanse of water (unless significant of the region); (iii) Away from all projected shade when the sun is higher than 7°.

46

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

A source of heat (or expanse of water) is considered to have an impact if it occupies more than 10% of the surface within a radius of 30m surrounding the screen, makes up 5% of an annulus of 5–10m, or covers 1% of a 5m radius area.

S = surface of heat sources

Heat sources (building, car parks, concrete surface)

S ≤ 5%

≤ 19º Vegetation < 10 cm

30 m

5m S ≤ 1%

10 m

≥ 30 m

Lake...

S ≤ 10% 7º ≥ 30 m

Figure1.D.2. Criteria for air temperature and humidity for class 2 sites Class 3 (additional estimated uncertainty added by siting up to 1 °C)

2.4

(a) Ground covered with natural and low vegetation (< 25 cm) representative of the region; (b) Measurement point situated: (i)

At more than 10 m from artificial heat sources and reflective surfaces (buildings, concrete surfaces, car parks, and the like);

(ii) At more than 10 m from an expanse of water (unless significant of the region); (iii) Away from all projected shade when the sun is higher than 7°. A source of heat (or expanse of water) is considered to have an impact if it occupies more than 10% of the surface within a radius of 10m surrounding the screen or makes up 5% of a 5m radius area.

S = surface of heat sources

Heat sources (building, car parks, concrete surface)

Vegetation < 25 cm S ≤ 5%

≥ 10 m

Lake...

≥ 10 m

5m S ≤ 10% 7º 10 m

Figure1.D.3.Criteria for air temperature and humidity for class 3 sites

47

CHAPTER 1. GENERAL

Class 4 (additional estimated uncertainty added by siting up to 2 °C)

2.5

(a) Close, artificial heat sources and reflective surfaces (buildings, concrete surfaces, car parks, and the like) or expanse of water (unless significant of the region), occupying: (i)

Less than 50% of the surface within a 10 m radius around the screen;

(ii) Less than 30% of the surface within a 3 m radius around the screen; (b) Away from all projected shade when the sun is higher than 20°.

S = surface of heat sources Heat sources (building, car parks, concrete surface) 20º

S ≤ 30% 3m

< 10 m

S ≤ 50% 10 m

Figure1.D.4.Criteria for air temperature and humidity for class 4 sites

2.6

Class 5 (additional estimated uncertainty added by siting up to 5 °C)

Site not meeting the requirements of class 4.

3.

PRECIPITATION

3.1

General

Wind is the greatest source of disturbance in precipitation measurements, due to the effect of the instrument on the airflow. Unless raingauges are artificially protected against wind, for instance by a wind shield, the best sites are often found in clearings within forests or orchards, among trees, in scrub or shrub forests, or where other objects act as an effective windbreak for winds from all directions. Ideal conditions for the installation are those where equipment is set up in an area surrounded uniformly by obstacles of uniform height. An obstacle is an object with an effective angular width of 10° or more. The choice of such a site is not compatible with constraints in respect of the height of other measuring equipment. Such conditions are practically unrealistic. If obstacles are not uniform, they are prone to generate turbulence, which distorts measurements; this effect is more pronounced for solid precipitation. This is the reason why more realistic rules of elevation impose a certain distance from any obstacles. The orientation of such obstacles with respect to prevailing wind direction is deliberately not taken into account. Indeed, heavy precipitation is

48

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

often associated with convective factors, whereby the wind direction is not necessarily that of the prevailing wind. Obstacles are considered of uniform height if the ratio between the highest and lowest height is less than 2. Reference for the heights of obstacles is the catchment’s height of the raingauge. 3.2

Class 1

(a) Flat, horizontal land, surrounded by an open area, slope less than ⅓ (19°). The raingauge shall be surrounded by low obstacles of uniform height, that is subtending elevation angles between 14° and 26° (obstacles at a distance between 2 and 4times their height); (b) Flat, horizontal land, surrounded by an open area, slope less than ⅓ (19°). For a raingauge artificially protected against wind, the instrument does not necessarily need to be protected by obstacles of uniform height. In this case, any other obstacles must be situated at a distance of at least 4times their height.

26.5º

14º

d≥2h

d≤4h

19º

or:

h

Site

Obstacle

d≥4h ≤ 19º

≥ 10º

(site < 14º)

Figure1.D.5.Criteria for precipitation for class 1 sites

3.3

Class 2 (additional estimated uncertainty added by siting up to 5%)

(a) Flat, horizontal land, surrounded by an open area, slope less than ⅓ (19°); (b) Possible obstacles must be situated at a distance at least twice the height of the obstacle (with respect to the catchment’s height of the raingauge).

h

Site

Obstacle

d≥2h ≤ 19º (site ≤ 26.5º)

Figure1.D.6.Criteria for precipitation for class 2 sites

≥ 10º

49

CHAPTER 1. GENERAL

3.4

Class 3 (additional estimated uncertainty added by siting up to 15%)

(a) Land is surrounded by an open area, slope less than ½ (≤ 30°); (b) Possible obstacles must be situated at a distance greater than the height of the obstacle.

h

Site d≥h ≤ 30º (site ≤ 45º)

Figure1.D.7.Criteria for precipitation for class 3 sites

3.5

Class 4 (additional estimated uncertainty added by siting up to 25%)

(a) Steeply sloping land (> 30°); (b) Possible obstacles must be situated at a distance greater than one half (½) the height of the obstacle.

h Site d 30º (site > 45º)

Figure1.D.8.Criteria for precipitation for class 4 sites

3.6

Class 5 (additional estimated uncertainty added by siting up to 100%)

Obstacles situated closer than one half (½) their height (tree, roof, wall, and the like).

Figure1.D.9.Criteria for precipitation for class 5 sites

50

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

4.

SURFACE WIND

4.1

General

Conventional elevation rules stipulate that sensors should be placed 10 m above ground surface level and on open ground. Open ground here represents a surface where obstacles are situated at a minimum distance equal to at least 10times their height. 4.2

Roughness

Wind measurements are disturbed not only by surrounding obstacles; terrain roughness also plays a role. WMO defines wind blowing at a geometrical height of 10m and with a roughness length of 0.03m as the surface wind for land stations. This is regarded as a reference wind for which exact conditions are known (10m height and roughness length of 0.03m). Therefore, roughness around the measuring site has to be documented. Roughness should be used to convert the measuring wind to the reference wind, but this procedure can be applied only when the obstacles are not too close. Roughness-related matters and correction procedure are described in the present volume, Chapter5. The roughness classification, reproduced from the annex of Chapter5, is recalled here: Terrain classification from Davenport (1960) adapted by Wieringa (1980) in terms of aerodynamic roughness lengthz0 Class index

Short terrain description

z0 (m)

1

Open sea, fetch at least 5 km

0.0002

2

Mud flats, snow; no vegetation, no obstacles

3

Open flat terrain; grass, few isolated obstacles

0.03

4

Low crops; occasional large obstacles, x/H > 20

0.10

5

High crops; scattered obstacles, 15 < x/H < 20

0.25

6

Parkland, bushes; numerous obstacles, x/H ≈ 10

0.5

7

Regular large obstacle coverage (suburb, forest)

1.0

8

City centre with high- and low-rise buildings

≥2

0.005

Note: Here x is a typical upwind obstacle distance and H is the height of the corresponding major obstacles. For more detailed and updated terrain class descriptions see Davenport et al. (2000).

4.3

Environmental classification

The presence of obstacles, including vegetation, (almost invariably) means a reduction in average wind readings, but less significantly affects wind gusts. The following classification assumes measurement at 10m, which is the standard elevation for meteorological measurement. When measurements are carried out at lower height (such as measurements carried out at 2m, as is sometimes the case for agroclimatological purposes), a class4 or 5 (see below) is to be used, with flag S (Specific situation).

51

CHAPTER 1. GENERAL

Where numerous obstacles higher than 2 m are present, it is recommended that sensors be placed 10m above the average height of the obstacles. This method allows the influence of the adjacent obstacles to be minimized. This method represents a permanent solution for partly eliminating the influence of certain obstacles. It inconveniently imposes the necessity for higher masts that are not standard and consequently are more expensive. It must be considered for certain sites and where used, the height of obstacles to be taken into account is that above the level situated 10m below the sensors (for example, for an anemometer installed at a 13m height, the reference “ground” level of the obstacles is at a 3m height; an obstacle of 7m is considered to have an effective height of 4m). In the following, an object is considered to be an obstacle if its effective angular width is over 10°. Tall, thin obstacles, that is with an effective angular width less than 10° and a height greater than 8m, also need to be taken into account when considering class1 to 3, as mentioned below. Under some circumstances, a cluster of tall, thin obstacles will have a similar effect to a single wider obstacle and will need to be considered as such. Changes of altitude (positive or negative) in the landscape which are not representative of the landscape are considered as obstacles. 4.4

Class 1

(a) The mast should be located at a distance equal to at least 30 times the height of surrounding obstacles; (b) Sensors should be situated at a minimum distance of 15 times the width of thin obstacles (mast, thin tree) higher than 8m. Single obstacles lower than 4 m can be ignored. Roughness class index is less than or equal to 4 (roughness length ≤ 0.1 m).

h

Site d ≥ 30 h (site ≤ 1.9º) Large obstacle

Thin obstacle > 8 m Width

≥ 10º ≥ 15 Width Obstacles lower than 4 m ignored

Figure1.D.10.Criteria for surface wind for class 1 sites 4.5

Class 2 (additional estimated uncertainty added by siting up to 30%, possibility to apply correction)

(a) The mast should be located at a distance of at least 10 times the height of the surrounding obstacles; (b) Sensors should be situated at a minimum distance of 15 times the width of thin obstacles (mast, thin tree) over 8m high.

52

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Single obstacles lower than 4 m can be ignored. Roughness class index is less than or equal to 5 (roughness length ≤ 0.25 m).

Thin obstacle > 8 m Width Site

h

d ≥ 10 h (site ≤ 5.7º)

≥ 15 Width

Obstacles lower than 4 m ignored

≥ 10º

Figure1.D.11.Criteria for surface wind for class 2 sites

Note: When the mast is located at a distance of at least 20 times the height of the surrounding obstacles, a correction (see the present volume, Chapter5) can be applied. For nearer obstacles, a correction may be applied in some situations.

4.6

Class 3 (additional estimated uncertainty added by siting up to 50%, correction cannot be applied)

(a) The mast should be located at a distance of at least 5 times the height of surrounding obstacles; (b) Sensors should be situated at a minimum distance of 10 times the width of thin obstacles (mast, thin tree) higher than 8m. Single obstacles lower than 5 m can be ignored.

Width h

Site d ≥ 5 h (site ≤ 11.3º)

>8m

Obstacles lower than 5 m ignored

≥ 10 Width

Figure1.D.12.Criteria for surface wind for class 3 sites

4.7

Class 4 (additional estimated uncertainty added by siting greater than50%)

(a) The mast should be located at a distance of at least 2.5 times the height of surrounding obstacles; (b) No obstacle with an angular width larger than 60° and a height greater than 10m, within a 40m distance.

53

CHAPTER 1. GENERAL

Single obstacles lower than 6 m can be ignored, only for measurements at 10 m or above.

No > 10 m

h

Site

> 60º

d ≥ 2.5 h (site ≤ 21.8º)

40 m Obstacles lower than 6 m ignored

Figure1.D.13.Criteria for surface wind for class 4 sites

4.8

Class 5 (additional estimated uncertainty cannot be defined)

Site not meeting the requirements of class 4.

5.

GLOBAL AND DIFFUSE RADIATION

5.1

General

Close obstacles have to be avoided. Shading due to the natural relief is not taken into account for the classification. Non-reflecting obstacles below the visible horizon can be neglected. An obstacle is considered as reflecting if its albedo is greater than 0.5. The reference position for elevation angles is the sensitive element of the instrument. 5.2

Class 1

(a) No shade projected onto the sensor when the sun is at an angular height of over 5°. For regions with latitude ≥ 60°, this limit is decreased to 3°; (b) No non-shading reflecting obstacles with an angular height above 5° and a total angular width above 10°.

JGODDPD LDODODOD

JGODDPD LDODODOD JFOSOKSOS SOSOS

KDDFJSISOSOSK SLSL

KSKSKSKSKSKSKS

JFOSOKSOS SOSOS

SOSOSOSOSOSOSOSS

KDDFJSISOSOSK SLSL

S SLSLSLSLSSLSLSLSLSLS K

KSKSKSKSKSKSKS

SKSKSK

SOSOSOSOSOSOSOSS

S SLSLSLSLSSLSLSLSLSLS K

SKSKSK

5º No shade

No non-shading obstacles with total angular width > 10º

Figure1.D.14.Criteria for global and diffuse radiation for class 1 sites

5.3

Class 2

(a) No shade projected onto the sensor when the sun is at an angular height of over 7°. For regions with latitude ≥ 60°, this limit is decreased to 5°;

54

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(b) No non-shading reflecting obstacles with an angular height above 7° and a total angular width above 20°.

JGODDPD LDODODOD

JGODDPD LDODODOD JFOSOKSOS SOSOS

KDDFJSISOSOSK SLSL

JFOSOKSOS SOSOS

SOSOSOSOSOSOSOSS

KSKSKSKSKSKSKS

KDDFJSISOSOSK SLSL

S SLSLSLSLSSLSLSLSLSLS K

SOSOSOSOSOSOSOSS

KSKSKSKSKSKSKS

SKSKSK

S SLSLSLSLSSLSLSLSLSLS K

SKSKSK

7º No non-shading obstacles with total angular width > 20º

Figure1.D.15.Criteria for global and diffuse radiation for class 2 sites

5.4

Class 3

(a) No shade projected onto the sensor when the sun is at an angular height of over 10°. For regions with latitude ≥ 60°, this limit is decreased to 7°; (b) No non-shading reflecting obstacles with an angular height above 15° and a total angular width above 45°.

JGODDPD LDODODOD

JGODDPD LDODODOD

JFOSOKSOS SOSOS

SOSOSOSOSOSOSOSS

JFOSOKSOS SOSOS

SOSOSOSOSOSOSOSS

KDDFJSISOSOSK SLSL

S SLSLSLSLSSLSLSLSLSLS

KDDFJSISOSOSK SLSL

S SLSLSLSLSSLSLSLSLSLS

KSKSKSKSKSKSKS

K

KSKSKSKSKSKSKS

K

SKSKSK

SKSKSK

15º

10º No non-shading obstacles with total angular width > 45º

Figure1.D.16.Criteria for global and diffuse radiation for class 3 sites

5.5

Class 4

No shade projected during more than 30% of the daytime, for any day of the year.

JGODDPD LDODODOD JFOSOKSOS SOSOS

KDDFJSISOSOSK SLSL

KSKSKSKSKSKSKS

SOSOSOSOSOSOSOSS

S SLSLSLSLSSLSLSLSLSLS K

SKSKSK

≤ 30% of daytime

No shade projected for more than 30% of daytime

Figure1.D.17.Criteria for global and diffuse radiation for class 4 sites

5.6

Class 5

Shade projected during more than 30% of the daytime, for at least one day of the year.

55

CHAPTER 1. GENERAL

6.

DIRECT RADIATION AND SUNSHINE DURATION

6.1

General

Close obstacles have to be avoided. Shading due to the natural relief is not taken into account for the classification. Obstacles below the visible horizon can be neglected. The reference position for angles is the sensitive element of the instrument. 6.2

Class 1

No shade projected onto the sensor when the sun is at an angular height of over 3°.

Figure1.D.18.Criteria for direct radiation and sunshine duration for class 1 sites

6.3

Class 2

No shade projected onto the sensor when the sun is at an angular height of over 5°.

Figure1.D.19.Criteria for direct radiation and sunshine duration for class 2 sites

6.4

Class 3

No shade projected onto the sensor when the sun is at an angular height of over 7°.

Figure1.D.20.Criteria for direct radiation and sunshine duration for class 3 sites

56 6.5

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Class 4

No shade projected during more than 30% of the daytime, for any day of the year.

≤ 30% of daytime

No shade for more than 30% of daytime

Figure1.D.21.Criteria for direct radiation and sunshine duration for class 4 sites

6.6

Class 5

Shade projected during more than 30% of the daytime, for at least one day of the year.

ANNEX 1.E. OPERATING EQUIPMENT IN EXTREME ENVIRONMENTS

Extreme weather events and harsh climatic environments have direct impacts on observing networks and may lead to interruption of core NMHS functions. The damage to real-time observing and monitoring systems during a weather event can severely limit the effectiveness of forecasting and warning services. The loss of delayed-mode observations affects the capacity to plan for extreme events and understand their climatology. The WMO DRR country-level survey (2006)1 identified droughts, flash and river floods, extreme winds, severe storms, tropical cyclones, storm surges, forest and wildfires, heatwaves, landslides and aviation hazards as the top ten hazards of concern to all Members. Maintenance of highquality observational records (historical and real time) is critical for DRR applications. These observations are critical for: (a) Risk identification; (b) Risk reduction through the provision of early warnings to support emergency preparedness and response as well as climate services for medium- and long-term sectoral planning; (c) Risk transfer through insurance and other financial tools. Thus, interruptions in monitoring caused by damage to instruments and observing networks as a result of natural hazards hamper NMHS capacities to deliver effective services, not only during and following a disaster, but also in the long term if these systems are not rebuilt. In this regard, CIMO stressed, at its sixteenth session, that it is critical to ensure that instrumentation and observing networks are designed according to standards that will withstand the impact of extreme weather events. There are a number of factors that influence the robustness of equipment, both infrastructure and sensors in the field. The most straightforward and efficient way of ensuring the availability of a system is to design robustness into the system from the beginning. Factors to be considered are: –

Data availability – one of the first factors to consider. Are there other similar sources of information nearby? Is this the only information available to the forecasters and therefore critical in extreme events? If so, more effort will be needed in the design and planning of the station to ensure availability of data. What type of outages can you tolerate? Does it matter that the data are not available on a regular basis for five minutes? Does it matter if they are not available for a day? All these questions inform the way the system is designed for robustness and how the system is supported.

Threats – what are the extreme weather events that will impact the weather station at a particular location? In an ideal world, all parameters would be monitored to the highest standard. However, funding realities generally mean that this is not possible. Identify the critical parameters and concentrate on ensuring their availability.

Environmental impacts – every location presents its own challenges. Review topography to ensure any ground work will not be subject to water erosion. Include in your consideration soil type, local pollution sources, proximity to the sea and salt corrosion, risk of vandalism, and the like. These threats impact both the design and the maintenance requirements.

Once the need for the observation is appreciated, and the strengths and weaknesses of the location have been assessed, then a range of mitigation strategies can be considered to maximize the availability of observations and minimize operational cost. These approaches fall into one of several categories listed in Table 1.E.1.

1

http://​w ww​.wmo​.int/​pages/​prog/​drr/​natRegCap​_en​.html.

58

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Table 1.E.1. General approaches for mitigating the impact of extreme environment on observation instrumentation and infrastructure Approach Site redundancy

Method Increase the density of measurement locations and equipment in critical areas

Strength

Weakness

Increased density of measurements reduces the impact of the loss of information from a single site

Increase in capital costs and maintenance efforts.

Potential to use lower-cost solutions

Risk of overall lowerquality data and reliability

Allows network QC, potentially reducing maintenance costs and predicting system failures Instrument redundancy

Use of environmentally appropriate infrastructure materials

Design

Duplicate sensitive or vulnerable instruments at a particular site

Increased availability of data

Increase in capital costs

Choose materials that are designed to survive in extreme environments (e.g., marine and high-grade steel, UV-resistant plastics, highoil-containing timbers)

Depending on usage, these materials will last longer and be stronger

Use appropriately rated enclosures and glands

Reduces the risk of damage to equipment caused by water ingress or dust

Short-term costs can be slightly higher

Use of structural engineers for design of infrastructure such as masts

Ensures the infrastructure will withstand extreme weather conditions

Short-term costs can be slightly higher

Greater flexibility to manage outages and maintenance Tend to be more expensive both as raw materials and in construction

Reduces maintenance burden

Lengthens the life of infrastructure by minimizing the stress caused by environmental impacts Reduces over engineering and associated costs

Specific examples of event types and the threat they pose to infrastructure and instruments in the immediate and longer term are given in Table 1.E.2. Methods of mitigation of these threats are also provided. These mitigations are in line with the four approaches of Table 1.E.1. While extensive, the mitigations are not exhaustive; they are a compilation of general knowledge and experience of a variety of NMHSs. In applying any of these methods, the user will need to consider the impact on measurements in their situation. While mitigation may work for a particular problem, it may also cause issues for other parameters. The user needs to consider the specific environment before employing any of these solutions.

CHAPTER 1. GENERAL

59

Table 1.E.2. Extreme weather hazards, examples of their associated infrastructure and sensor vulnerabilities, and mitigating actions Event type Cause

Hail

Considerations

Characteristic of weather systems such as thunderstorms What size and intensity of hail would the system need to cope with? * Generally hail less than 2.5 cm diameter is not considered to be significant hail, while > 4.5 cm hail will create a significant dent in a car, and > 7 cm will smash a windscreen * Less than 5% of hail is greater than 2.5 cm diameter

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Impact

* Damage to: radomes - dints and holes; observer shelters - breakage of louvers; electronics enclosures - dints and holes; masts - dints, nicks or snapping

* Use high strength materials (including steel, carbon fibre) for the outer skin materials of enclosures, and the like, and that the structures are strong and well supported

* Use component-designed radomes, shelters, enclosures, and the like, that allow for panel changes

* Deterioration of coated surfaces

* Use high strength materials that do not require painting or other coating methods

* Damage to solar panels

* Install removable high-strength, stiff and structurally supported covers

* Deterioration of painted surfaces

* Use high-strength and corrosion resistant materials that do not require painting or other coating methods

Vulnerability or impact to sensors

Mitigation

Impact

* Mechanical anemometers, damage to cups in particular. Small and light weight plastic cups are particularly vulnerable

* Use heavy-duty instruments constructed from strong materials. Depending on the use, specialized materials such as carbon fibre may be considered

* Ultrasonic anemometers, damage to arms and detectors causing misalignment

* Use heavy-duty instruments mounts and arms constructed from strong materials. Depending on the use, specialized materials such as carbon fibre may be considered

* Radiation instruments, damage to their domes

* Use alternate technologies such as pitot tube anemometers that rely on aerodynamic design and have minimally exposed components

60

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event Type

Flood

Cause

Result of significant weather systems, including thunderstorms, cyclones, and the like. Flooding may occur well down stream of the weather event

Considerations

Is the system expected to resume function post immersion? What maximum rainfall amount and rate would be expected? Is the site vulnerable to upstream flooding?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Water ingress

* Ground-mounted equipment undermined or washed away

* Use mounting systems that stabilize the surrounding soil by spreading the load. There are commercial solutions that use a submerged "tripod" arrangement that minimizes soil disturbance while spreading the load

* Design and align foundations parallel to any expected surface flow to minimize hydrostatic pressure

* Equipment damaged by exposure/immersion in water

* Use materials such as marine grade stainless steel, galvanized iron or steel, appropriate plastics; avoid the use of aluminium

* Ensure all connectors are wrapped in water proofing tape to prevent corrosion

* Use preventative coatings and impregnating materials, e.g., fish oil, paint

* In marine environments, use sacrificial anodes

* Corrosion of metal components, particularly connectors, welds, joints

* Perform regular data monitoring and regular inspections and maintenance of infrastructure and equipment in vulnerable environments to manage maintenance regime

* Coat welds, joints and nuts with grease, e.g., silicon, even butter

Power surge

* Loss of data due to power or communications failure

* Include redundant communications via an alternate supplier

Debris

* Damage from large debris in stream flow impacting towers and screens

* Reinforce lower sections of towers to expected flood height

* Damage to protective coatings

* Ensure all powder or other coating is pit and chip free

Corrosion

Contamination

61

CHAPTER 1. GENERAL

Event Type

Flood

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Water ingress

* Any non-submersible sensor

* Mount the data acquisition system enclosure as high as practical to avoid being submerged (water stage station for example)

* Design and align foundations parallel to any expected surface flow to minimize hydrostatic pressure

* Use appropriately "IP" (ingress protection or international protection) rated seals and enclosures for equipment, typically IP67 and above for waves and splash

* Equipment in close proximity to high water flow (direct contact or erosion) becomes submerged

* Use appropriately IP-rated seals and enclosures for equipment, typically IP67 and above for waves and splash

* Equipment damaged by exposure/immersion in water, particularly connectors, welds, joints

* Use appropriately IP-rated seals and enclosures for equipment, typically IP67 and above for waves and splash

* Avoid metals that do not passivate or are susceptible to corrosion for example low grade steel

* Inspect equipment regularly to ensure all paint and coated surfaces are pit and chip free

* Protect connectors and clamps using grease/oil impregnated tape or similar

* Carefully select metal types at joints or use of isolating separators and lubricants (high viscosity grease) to ensure that electrolysis is minimized

Contamination

* Foreign chemical or dirt build-up on sensing elements, such as relative humidity sensing elements

* Perform regular inspections and data monitoring to manage maintenance regime

Debris

* Damage from large debris in stream flow impacting towers and screens

* In flood prone areas, elevate instruments and enclosures above flood level

Corrosion

62

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Land/mudslide

Cause

Result of rainfall in combination with unstable ground conditions

Considerations

* What is the slope of the land? * Is the area subject to a long period of moderate rainfall?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Water ingress

* Ground-mounted equipment undermined or washed away

* Use mounting systems that stabilize the surrounding soil by spreading the load. There are commercial solutions that use a submerged "tripod" arrangement that minimizes soil disturbance while spreading the load

* Use appropriately IP-rated seals and enclosures for equipment, typically IP67 and above for waves and splash

* Mount data acquisition system enclosure as high as practical when the sensor can be submerged (water stage station for example)

* Ground-mounted equipment undermined or washed away

* Use mounting systems that stabilize the surrounding soil by spreading the load. There are commercial solutions that use a submerged "tripod" arrangement that minimizes soil disturbance while spreading the load

* Design and align foundations parallel to any expected surface follow to minimize hydrostatic pressure

Mud

* Nearly all, total destruction

* Site equipment on local mounds, or sculpture land to redirect mud and water around equipment

Debris

* Nearly all, total destruction

* Reinforce lower sections of towers to expected land/mud slide height

See also "Flood"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Water ingress

* Failure of any non-submersible sensor

* Mount data-acquisition system enclosure as high as practical when the sensor can be submerged (water stage station for example)

* Use appropriately IP-rated sensor enclosures and seals, typically IP67 and above for waves and splash

Water current

* Instruments break away or are submerged in mud

* Mount instruments at height greater than expected 20- to 50-year event

Mud

* Nearly all, total destruction

* Perform regular inspections and data monitoring to manage maintenance regime

Water current

63

CHAPTER 1. GENERAL

Event type

High winds

Cause

Extreme weather systems such as cyclone, thunderstorm, and the like, with winds over 100 km h (approx. 27.8 m s-1)

Considerations

* What maximum average wind and maximum instantaneous wind would a system need to withstand?

-1

* Is there much material that could become flying debris during an event? Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Wind

* Damage to: radomes - dints and holes; observer shelters - breakage of louvers; electronics enclosures - dints and holes; masts - dints, nicks or snapping

* Use high strength materials (including steel, carbon fibre) for the outer skin materials of enclosures, and the like, and ensure the structures are strong and well supported

* Use component-designed radomes, shelters, enclosures, and the like, that allow for panel changes

* Major structural damage due to debris

* Use guy wires on tower/tripod mast to minimize damage from vibration, attached to suitable anchors, e.g., concrete or physical anchors

* Structural damage due to drag and wind pressure

* Ensure all compartments/doors close securely; consider inclusion of door-open warning alarms.

* Where practical, design infrastructure to reduce wind load using curved and low profile surfaces

* Consider the aerodynamics of the design to minimize drag and to stabilize the construction

* Undermining of infrastructure supports through erosion and wind stress

* Perform regular inspections, particularly after major events, to ensure the structural integrity of foundations and mounts

* Creation of micro-fractures, degradation of welded joints and loosening of clamps, and the like, due to wind vibration

* Perform regular inspections, particularly after major events, to ensure the structural integrity of foundations and mounts

* Provide additional support for major infrastructure such as guy wires for masts to limit flexing during high winds

Debris

* Towers severely damaged

* Use towers/tripods with appropriate wind load rating

* Attach masts to suitable anchors, e.g., concrete or physical anchors

64

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

High winds

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Wind

* Damage to instruments due to wind force and small debris

* Use heavy-duty instruments

* Use wind instruments with few moving parts, such as "pitot tube" instruments which use pressure difference, and ultrasonic wind instruments, to eliminate vulnerabilities associated with moving parts; however, these may still be damaged by flying debris

* Inspect and ensure instruments are appropriately and securely mounted prior to the event, and that raingauges and screens are appropriately bolted down

* Tie down or remove any loose objects or material that could act as flying debris during a storm. Inspect surroundings for trees or bushes with branches that are likely to break or fall during a high wind event; arrange for their removal

* Use high strength rope or wire to support anemometer arm

* Ensure cabling is well secured and supported

* Consider the aerodynamics of the design to minimize drag and to stabilize the construction

Debris

* Damage to instruments due to flying debris

* Clean up the area around the equipment and remove any material that could become a projectile

65

CHAPTER 1. GENERAL

Event type

Thunderstorms

Cause

Strong winds, lightning and rainfall from larger storms

Considerations

* Are the systems expected to operate after a lightning strike?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Lightning

* Electrical surge

* Use electrical surge protection on the power circuit and individual surge protection on each monitored channel (e.g., temperature, wind)

* Use appropriate earthing of infrastructure through a collector (e.g., Franklin rod or spline ball), to a conductor for dissipation to ground. Note: all connections must maintain high conductivity and bends should be no greater than 45 degrees

Water

* Corrosion

* Use suitable materials such as stainless or galvanized steel and appropriate plastics

* Use preventative coatings and impregnating materials, e.g., fish oil, paint

* In marine environments use sacrificial anodes

* Perform regular inspection and maintenance of infrastructure and equipment in vulnerable environments

See also "Flood"

See also "High wind"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Lightning

* Instruments with exposure and no tolerance for direct or indirect lightning strikes

* Use grounding rod/plate, finial, and the like, on weather station tower/tripod

* Use surge-suppression devices between instruments and data-acquisition system to protect the data-acquisition system

* Induced noise

* Avoid long unshielded cables

Water

* Corrosion of connectors, and the like

* Protect connectors and clamps using grease/oil impregnated tape or similar

* Carefully select metal types at joints or use isolating separators and lubricants (high-viscosity grease) to ensure that electrolysis is minimized

* Foreign chemical build-up on sensing elements, such as relative humidity sensing elements

* Perform regular inspection and data monitoring to manage maintenance regime

See also "Flood"

See also "High wind"

66

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Tropical cyclone

Cause

Characteristic of a weather system

Considerations

* Does rotating winds present any additional risk?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Wind

* Rotating winds

* For infrastructure that may rotate in high winds, design mounts and cables so that it does not drive or turn cables beyond limits

* Tie down or remove any loose objects or material that could act as flying debris during a storm. Inspect surroundings for trees or bushes with branches that are likely to break or fall during a high wind event; arrange for their removal

See also "High wind"

Debris

See also "High wind"

Event type

Tornado

Cause

A weather sub-system characterized by high winds and blowing debris

Considerations

* Do rotating winds present any additional risk?

Dominant hazard

Impact to infrastructure examples

Mitigation

Wind

* Rotating winds

* For infrastructure that may rotate in high winds, design mounts and cables so that it does not drive or turn cables beyond limits

See also "High wind"

Debris

See also "High wind"

Event type

Storm surge

Cause

Results of cyclones and severe weather

Considerations

Constraints

Current

See also "Tsunami"

Water

See also "Flood"

Debris

See also "Flood"

67

CHAPTER 1. GENERAL

Event type

Tsunami

Cause

Independent of meteorological factors, resulting from geological movement, underwater land slip or meteor

Considerations

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Current

* Erosion or loss of footings

* Ensure tower and screen footings are reinforced to deal with the force of water travelling between 2 and 20 m s-1. Note the run-up for a tsunami is significantly greater than the height of the wave

* Secure masts and large infrastructure to nearby structures with additional ropes

* Tie down or remove any loose objects or material that could act as flying debris during a storm. Inspect surroundings for trees or bushes with branches that are likely to break or fall during a high wind event; arrange for their removal

Water

See also "Flood"

Debris

See also "Flood"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Current

* Nearly all, total destruction

* In tsunami-prone areas, mount instruments above likely tsunami run-up height

See also "Flood"

See also "Flood"

68

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Snow/blizzard/icing

Cause

Extreme cold weather systems, and associated with prolonged cold and windy weather

Considerations

* Are systems expect to operate after freeze/thaw situations? * Are instruments or infrastructure specified to cope with sustained depressed temperatures?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Cold and ice accretion

* Deterioration of shelters, masts, and the like, due to the weight of ice/snow

* Investigate the use of ice phobic coatings and materials

* Weakening of screens and enclosures caused by the expansion of freezing water in joints, cracks and crevices

* Ensure screens and enclosures are well maintained; use materials that are tolerant to expansion stress and less prone to rot such as non-brittle plastics

* Towers/masts

* Use towers/masts that are slightly flexible and/or that will vibrate slightly to loosen snow and ice

* Snow/Ice cover on solar panels resulting in eventual loss of power

* Tilt solar panels as close to vertical as possible to prevent snow/ice accretion

Wind

* Failure of infrastructure (e.g., mast) in high wind due to ice accretion

* Choose materials that maintain elasticity below expected minimum temperature

* De-ice on a regular schedule

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Cold and ice accretion

* Ice build-up on instruments, e.g., mechanical anemometers, ultrasonic sensors, rain sensors and gauges

* Use heated instruments (e.g., anemometers) and heat cycling instruments (e.g., humidity) if practical. Ensure the heater does not interfere with other instruments

* Use a continuous flow of air (ideally dry air) to prevent water or snow to settle, or ice to form

* Apply heat tape directly to surfaces (electrical resistance elements embedded in a flexible sheet or nichrome wire); most effective on sensors without moving parts

* Use instruments that have ice phobic surfaces or coatings

* Spray a low freezing-point fluid (such as glycol or ethanol) on sensors during icing events; not suitable for humidity sensors

* Mount wind sensor on slightly flexible mast (e.g., "wind surfer" mast)

* In heavy icing conditions none of these methods are effective

69

CHAPTER 1. GENERAL

Event type

Snow/blizzard/icing

* Snow/ice cover on pyranometers/radiation sensors

* Use a continuous flow of air (ideally dry air) to prevent water or snow to settle, or ice to form

* Snow/ice accretion on infrastructure impacting the measurement environment, e.g., snow/ice cover on temperature sensors and screens causes incorrect data (due to a much higher time constant), and causes turbulence around anemometers

* Prevent icing or de-ice on a regular basis using methods above such as ice-phobic materials, low freezing point fluids

* Minimize the surface area of the infrastructure

70

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Avalanche

Cause

Result of snowfall build-up in combination with certain ground and atmospheric conditions

Considerations

* What is the slope of the terrain near the site?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Mass and Debris

* Destruction of infrastructure in path of avalanche

* Site station higher on mountain sides

* Snow/ice cover on solar panels resulting in eventual loss of power

* Construct tower with multiple solar cells at various height

* Include back up batteries and alarms for loss of voltage and current supply

See also "Land/mudslide"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Mass and debris

* Nearly all

* Snow/ice cover on optical sensors

* Construct tower with multiple sensor suites at various heights

* Snow/ice cover on pyranometers/radiation sensors

* For light coverage consider automated cleaning

* Snow/ice cover on temperature sensors and screens causes incorrect data (due to a much higher time constant)

* Prevent icing or de-ice on a regular basis using methods above such as ice-phobic materials, low freezing point fluids

See also "Snow/blizzard/icing"

See also "Land/mudslide"

71

CHAPTER 1. GENERAL

Event type

Dust storm

Cause

Result of high winds in combination with certain ground conditions

Considerations

How long do we expect systems to operate unattended or maintained? What IP rating would we expect?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Oblation

* Equipment that can be damaged by sandblasting or burying

* Avoid the use of coated materials; choose polished metal

* Failure or deterioration of protective coatings that may lead to pitting or overall corrosion

* Inspect painted, plastic or powder coasted surfaces for chips, crazing or cracking

Dirt

* Build-up of dust/sand in enclosures

* Use enclosures with an IP6X or higher

* Design mounts and frames to minimize the build-up of sand and dirt

* Clogging of aspirated screen

* Perform regular inspections and clearing

* Loss of power or communications

* Include back up batteries and alarms for loss of voltage and current supply

* Include redundant communications via an alternate supplier

Dominant hazard

Vulnerability or impact to sensors

Mitigation

Oblation

* Aspirated equipment drawing dust

* Stop aspiration when wind or particle count is above a set point

Dirt

* Clogging of non-aspirated equipment

* Increase inspection frequency of equipment to remove dust build up

* Use well-sealed (high IP-rated) enclosures for data acquisition system, e.g., IP68

* Increase replacement frequency of filter in dusty environments

* Design sensors to minimize the surface area and presence of crevices and pockets where dirt can build up

* Optical and solar radiation equipment

* For light coverage, consider daily automated cleaning

72

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Fire

Cause

Result of hot weather, lightning or vandalism

Considerations

* How hot is a typical fire likely to burn?

* How long is a fire likely to keep burning? Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Heat and combustion

* Deformation of metal and plastic components

* Avoid plastics with low melting temperature and lightweight metals

* Failure of electronics in extreme heat

* Use enclosures that provide some insulation such as a double skin

* Ensure electronics are correctly rated for use in the climate they are being deployed in, e.g., 20 °C–30 °C above the climatic maximum temperature

* Damage reducing IP rating

* Inspect and replace seals

* Destruction of any combustible materials

* Construct with non-combustible materials such as metal and concrete

* Avoid cracks and crevices in the design of housings, and the like, where embers and sparks can lodge. Openings should be screened or sealed where practical

* Failure of structural integrity of masts and other infrastructure following the event

* Perform regular inspections for stress fractures, fatigue and grain growth in metal components

* Failure or deterioration of protective coatings that may lead to pitting or overall corrosion

* Inspect painted, plastic or powder coated surfaces for chips, crazing or cracking

Debris

* Damage from falling debris

* Site equipment in fire prone areas with appropriate clearance from potential falling structures and trees

Dust

See also "Dust storm"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Heat and combustion

* Deformation of casings/enclosures and failure of electronics

* Avoid plastics with low melting temperature and lightweight metals

* Use enclosures that provide some insulation such as a double skin, but avoid combustible insulation materials

* Damage reducing IP rating

* Inspect and replace seals

* Destruction of any combustible materials

* Construct with non-combustible materials such as metal and concrete

73

CHAPTER 1. GENERAL

Event type

Fire

* Sensors damaged by heat effects (sensing elements or housings)

* Ensure sensors are correctly rated for use in the climate they are being deployed in; e.g., 20 °C–30 °C above the climatic maximum for electronics and 5 °C–10 °C above for the measurement range

* Failure of structural integrity of masts and other infrastructure following the event

* Perform regular inspections for stress fractures, fatigue and grain growth in metal components

* Failure or deterioration of protective coatings that may lead to pitting or overall corrosion

* Inspect painted, plastic or powder coated surfaces for chips, crazing or cracking

Debris

* Damage from falling debris

* Site equipment in fire prone areas with appropriate clearance from potential falling structures and trees

Dust

See also "Dust storm"

Event type

Drought

Cause

Result of prolonged periods of low or no rain

Considerations

Do footings need to accommodate dynamic soils?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Dust

* Degradation of equipment foundations in clay soils (cracking, erosion)

* Use mounting systems that stabilize the surrounding soil such as a physical anchor which causes minimal soil disturbance while spreading the load

Erosion

See also "Dust storm"

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Dust

* Failure of electronics

* Check for dry joints in electronics

Erosion

* Clogged filters

* Perform more frequent filter changes in dusty conditions

74

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Heatwave/solar radiation

Cause

Prolonged periods of elevated temperatures and/or intense sunlight

Considerations

Can instruments or infrastructure cope with sustained elevated temperatures?

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Heat

* Few, unless exterior surfaces have low temperature tolerance

* Avoid plastics with low melting temperature and lightweight metals

* Failure of electronics due to overheating

* Use canvas or similar to shade electronics and reduce thermal stress on systems

* Where practical, bury the electronics box. Note: Ensure that no water ingress can occur

* Use passive cooling such as with a vent and chimney design. Note: Ensure the risk of water ingress is not increased, by placing the vent above expected water levels; use filters / screens to prevent dust and animals from gaining access

* Use active cooling such as with fans (note cautions above regarding water, dust and animal ingress)

* Use active coolers such as Peltier coolers or air-conditioning

* Ageing of welds and joints

* Perform more frequent inspections for metal fatigue and deterioration

Irradiation

* Structural deterioration due to UV exposure

* Use UV resistant materials such as metals, hardwood or UV stabilized plastics

* Discolouration and ageing of plastic components

* Perform more frequent inspections to detect distortion and deterioration of enclosures and screens in particular

* Use canvas or similar to shade electronics and reduce thermal stress on systems

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Heat

* Sensors damaged by heat effects (sensing elements or housings)

* Ensure instruments are correctly rated for use in the climate they are being deployed in; e.g., 20 °C–30 °C above the climatic maximum temperature for electronics and 5 °C–10 °C above for the measurement range

* Failure of instruments due to overheating

* Where measurements will not be compromised, use Peltier coolers or airflow (passive and active)

Irradiation

* Structural deterioration due to UV exposure

* Use UV resistant materials such as metals, hardwood or UV stabilized plastics

75

CHAPTER 1. GENERAL

Event type Cause

Earthquake/volcano

Independent of meteorological factors

Considerations Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Eruption

* Volcano: burying by fallout; destruction from direct contact with flow

* Maximize use of fire resistant materials

Land movement

* Earthquake: most infrastructure

* Use mounting systems that stabilize the surrounding soil such as a physical anchor which causes minimal soil disturbance while spreading the load

* Ash cover on solar panels: eventual loss of power

* Include back up batteries and alarms for loss of voltage and current supply

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Eruption

* Volcano: dust contamination

* See “Dust” above

Land movement

* Earthquake: weighing gauges, loosely mounted instruments, e.g., tipping-bucket raingauge

* Ash cover on optical sensors

* For light coverage consider automated cleaning

* Ash cover on pyranometers/radiation sensors

* For light coverage consider automated cleaning

76

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Event type

Security

Cause

Vandalism

Considerations

Dominant hazard

Vulnerability or impact to infrastructure

Mitigation

Vandalism

* Theft or wanton damage

* Use fencing

* Use non-removable fittings for high value items such as solar panels

* In remote areas, encourage engagement from the local community regarding the value of the service provided by the equipment

Wildlife

* Chewing of cables

* Use strong conduit or armoured cables

* Crushing of infrastructure by animals rubbing against the equipment

* Use appropriate livestock fencing

Dominant hazard

Vulnerability or impact to instruments

Mitigation

Vandalism

* Theft or wanton damage

* Use fencing

* In remote areas, encourage engagement from the local community regarding the value of the service provided by the equipment

Wildlife

* Bird attacks on ultrasonic sensors

* Use bird spikes on the edges of roosting points

* Contamination and corrosion by bird droppings

* Use bird spikes on the edges of roosting points

* Crushing or misalignment of sensors by animals rubbing against the equipment

* Use appropriate livestock fencing

ANNEX 1.F. STATION EXPOSURE DESCRIPTION

The accuracy with which an observation describes the state of a selected part of the atmosphere is not the same as the uncertainty of the instrument, because the value of the observation also depends on the instrument’s exposure to the atmosphere. This is not a technical matter, so its description is the responsibility of the station observer or attendant. In practice, an ideal site with perfect exposure is seldom available and, unless the actual exposure is adequately documented, the reliability of observations cannot be determined (WMO, 2002). Station metadata should contain the following aspects of instrument exposure: (a) Height of the instruments above the surface (or below it, for soil temperature); (b) Type of sheltering and degree of ventilation for temperature and humidity; (c) Degree of interference from other instruments or objects (masts, ventilators); (d) Microscale and toposcale surroundings of the instrument, in particular: (i)

The state of the enclosure’s surface, influencing temperature and humidity; nearby major obstacles (buildings, fences, trees) and their size;

(ii) The degree of horizon obstruction for sunshine and radiation observations; (iii) Surrounding terrain roughness and major vegetation, influencing the wind; (iv) All toposcale terrain features such as small slopes, pavements, water surfaces; (v) Major mesoscale terrain features, such as coasts, mountains or urbanization. Most of these matters will be semi-permanent, but any significant changes (growth of vegetation, new buildings) should be recorded in the station logbook, and dated. For documenting the toposcale exposure, a map with a scale not larger than 1:25 000 showing contours of ≈ 1m elevation differences is desirable. On this map the locations of buildings and trees (with height), surface cover and installed instruments should be marked. At map edges, major distant terrain features (for example, built-up areas, woods, open water, hills) should be indicated. Photographs are useful if they are not merely close-ups of the instrument or shelter, but are taken at sufficient distance to show the instrument and its terrain background. Such photographs should be taken from all cardinal directions. The necessary minimum metadata for instrument exposure can be provided by filling in the template given on the next page for every station in a network (see the figure below). An example of how to do this is shown in WMO (2003). The classes used here for describing terrain roughness are given in the present volume, Chapter5. A more extensive description of metadata matters is given in WMO (2017).

78

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Station

Update Latitude

Elevation 0

Longitude

200 m Enclosure

N

Building Road x xx Trees, bushes (12) Height (m) of obstacle +3

Elevation contour

Radiation horizon 1: 6 1: 10 1: 20 N Temperature and humidity:

8° 4° 0°

E

Surface cover under screen

W

S Sensor height Artificial ventilation?

N yes/no

Soil under screen Precipitation: Wind:

Gauge rim height Anemometer height

(if “no” above: building height Terrain roughness class: to N

Free-standing? , width

,to E

, to S,

yes/no

, length to W

Remarks:

General template for station exposure metadata

REFERENCES AND FURTHER READING

Brooks, C.E.P. and N. Carruthers, 1953: Handbook of Statistical Methods in Meteorology. MO 538, Meteorological Office, London. Bureau International des Poids et Mesures, 2006: The International System of Units (SI). BIPM, Sèvres/Paris. Bureau International des Poids et Mesures/Comité Consultatif de Thermométrie, 1990: The International Temperature Scale of 1990 (ITS-90) (H. Preston Thomas). Metrologia, 27:3–10. Davenport, A.G., 1960: Rationale for determining design wind velocities. Journal of the Structural Division, American Society of Civil Engineers, 86(5):39–68. Davenport, A.G., C.S.B. Grimmond, T.R. Oke and J. Wieringa, 2000: Estimating the roughness of cities and sheltered country. Preprints of the Twelfth American Meteorological Society Conference on Applied Climatology (Asheville, NC, United States), pp. 96–99. Eisenhart, C., 1963: Realistic evaluation of the precision and accuracy of instrument calibration systems. National Bureau of Standards–C, Engineering and Instrumentation, Journal of Research, 67C(2). International Civil Aviation Organization, 2002: World Geodetic System – 1984 (WGS-84) Manual. ICAO Doc9674–AN/946. Quebec, ICAO. International Organization for Standardization, 1994a: Accuracy (Trueness and Precision) of Measurement Methods and Results – Part1: General Principles and Definitions, ISO5725-1:1994/Cor.1:1998. Geneva. ———, 1994b: Accuracy (Trueness and Precision) of Measurement Methods and Results – Part2: Basic Method for the Determination of Repeatability and Reproducibility of a Standard Measurement Method, ISO5725-2:1994/Cor.1:2002. Geneva. ———, 2009: Quantities and Units – Part 1: General, ISO 80000-1:2009. Geneva. International Organization for Standardization/International Electrotechnical Commission, 2008: Uncertainty of Measurement – Part3: Guide to the Expression of Uncertainty in Measurement, ISO/ IEC Guide98-3:2008, Incl. Suppl.1:2008/Cor 1:2009, Suppl.1:2008, Suppl.2:2011. Geneva. (equivalent to: Joint Committee for Guides in Metrology, 2008: Evaluation of Measurement Data – Guide to the Expression of Uncertainty in Measurement, JCGM100:2008, Corrected in 2010). International Union of Pure and Applied Physics, 1987: Symbols, Units, Nomenclature and Fundamental Constants in Physics. SUNAMCO DocumentIUPAP-25 (E.R. Cohen and P.Giacomo), reprinted from Physica 146A, pp.1–68. Joint Committee for Guides in Metrology, 2012: International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM). JCGM200:2012. Kok, C.J., 2000: On the Behaviour of a Few Popular Verification Scores in Yes/No Forecasting. Scientific Report. WR-2000-04. KNMI, De Bilt. Linacre, E., 1992: Climate Data and Resources – A Reference and Guide. Routledge, London. Murphy, A.H. and R.W. Katz (eds.), 1985: Probability, Statistics and Decision Making in the Atmospheric Sciences. Westview Press, Boulder. National Institute of Standards and Technology, 2008: Guide for the Use of the International System of Units (SI) (A. Thompson and B.N. Taylor). NIST Special Publication No.811, Gaithersburg, United States of America. Natrella, M.G., 1966: Experimental Statistics. National Bureau of Standards Handbook91, Washington DC. Orlanski, I., 1975: A rational subdivision of scales for atmospheric processes. Bulletin of the American Meteorological Society, 56:527–530. United Nations Environment Programme, 2017: Minamata Convention on Mercury, http://​www​ .mercuryconvention​.org/​Portals/​11/​documents/​Booklets/​COP1​%20version/​Minamata​ -Convention​-booklet​- eng​-full​.pdf. Wieringa, J., 1980: Representativeness of wind observations at airports. Bulletin of the American Meteorological Society, 61:962–971. World Meteorological Organization, 1966: International Meteorological Tables (S. Letestu, ed.) (1973amendment). (WMO-No.188, TP.94). Geneva. ———, 1970: Performance Requirements of Aerological Instruments: an Assessment Based on Atmospheric Variability (C.L. Hawson). Technical Note No.112 (WMO-No.267, TP.151). Geneva. ———, 1992: International Meteorological Vocabulary (WMO-No.182). Geneva. ———, 1993: Siting and Exposure of Meteorological Instruments (J. Ehinger). Instruments and Observing Methods Report No.55 (WMO/TD-No.589). Geneva. ———, 2001: Lecture Notes for Training Agricultural Meteorological Personnel (J. Wieringa and J. Lomas) (WMONo.551). Geneva.

80

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

———, 2002: Station exposure metadata needed for judging and improving the quality of observations of wind, temperature and other parameters (J. Wieringa and E. Rudel). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2002) Instruments and Observing Methods Report No.75 (WMO/TDNo.1123). Geneva. ———, 2003: Guidelines on Climate Metadata and Homogenization (P. Llansó, ed.). World Climate Data and Monitoring Programme (WCDMP) Series Report No.53 (WMO/TD-No.1186). Geneva. ———, 2008: Guide to Hydrological Practices (WMO-No.168), VolumeI. Geneva. ———, 2010a: Abridged Final Report with Resolutions and Recommendations of the Third Session of the Joint WMO/ IOC Technical Commission for Oceanography and Marine Meteorology (WMO-No.1049). Geneva. ———, 2010b: Guide to Agricultural Meteorological Practices (WMO-No.134). Geneva. ———, 2010c: Manual on the Global Data-processing and Forecasting System (WMO-No.485), VolumeI, AppendixII-2. Geneva. ———, 2011a: Guide to Climatological Practices (WMO-No.100). Geneva. ———, 2011b: Technical Regulations (WMO-No.49), VolumeI, AppendixA. Geneva. ———, 2014: Guide to Meteorological Observing and Information Distribution Systems for Aviation Weather Services (WMO-No.731). Geneva. ———, 2015 (updated in 2017): Manual on the WMO Integrated Global Observing System (WMO-No.1160). Geneva. ———, 2017 (updated in 2018): Guide to the WMO Integrated Global Observing System (WMO-No.1165). Geneva.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.1

GENERAL

2.1.1

Definition

Thermodynamic temperature, T, is a physical quantity characterizing the average energy of random molecular motion within a substance. Direct measurement of T using so-called primary thermometers is experimentally difficult and is only intermittently carried out even at national measurement institutes. Instead, the BIPM Consultative Committee for Thermometry (CCT) recommends the use of the ITS-90 to produce practical approximations to thermodynamic temperature (BIPM 1989, 1990).1 ITS-90 summarises our knowledge of primary thermometry in 1990 and recommends the value of freezing points, melting points, or triple points of pure substances that can be used to calibrate standard PRTs (SPRTs). In the temperature range of meteorological interest (-80°C to 60°C), ITS-90 specifies the way in which the electrical resistance of SPRTs varies in between these fixed-point temperatures. The approximations to thermodynamic temperature produced by ITS-90 have been shown to be in error by less than ±0.01°C over the entire range of meteorological interest (Underwood etal., 2017). For meteorological purposes, temperatures are measured for a number of media. The most common variable measured is air temperature (at various heights). Other variables are ground surface temperature, subsurface soil temperature, minimum air temperature above grass surface and fresh- and seawater temperature. WMO (1992) defines air temperature as “the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation”. Although this definition cannot be used as the definition of the thermodynamic quantity itself, it is suitable for most applications. 2.1.2

Units and scales

Thermodynamic temperature T is measured in units of kelvin (K). One K is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Thus, the triple point of water occurs at 0.01°C and a partial water vapour pressure of 611.657 Pa exactly by definition; the temperature (t), in degrees Celsius defined by equation2.1, is used for most meteorological purposes: t/°C = T/K – 273.15 (2.1) Often the equilibrium between melting ice and air-saturated water (the “ice point”) is used for calibration. At standard atmospheric pressure (101.325 kPa), the ice point occurs at 273.150K (0.000°C) and varies by -9.91x10 -5KkPa-1. The variation thus amounts to less than ±0.001°C for atmospheric pressure changes from 111kPa to 92kPa (Harvey etal., 2013). A temperature difference of 1°C is equal to a temperature difference of 1 K. Note that the symbolK is used without the degree symbol. In the thermodynamic scale of temperature, measurements are expressed as differences from absolute zero (0K), the temperature at which the molecules of any substance possess no thermal energy. ITS-90 provides a practical approximation to thermodynamic temperature (see annex), which is based on assigned values for the temperatures of a number of reproducible equilibrium states (see annex table) and on specified standard instruments calibrated at those temperatures (Nicholas and White, 1993; Quinn, 1990). Most thermometers for meteorological applications are calibrated by comparison against either a thermometer calibrated according to ITS-90, or a secondary standard that has in turn been calibrated according to ITS-90 (BIPM/CCT, 1990; Nicholas and White, 1993; Bentley, 1998). 1

The authoritative body for this scale is BIPM; see http://​w ww​.bipm​.org. CCT is the executive body responsible for establishing and realizing the ITS.

82

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

2.1.3

Meteorological requirements

2.1.3.1

General

Meteorological requirements for temperature measurements primarily relate to the following: (a) The air near the Earth’s surface; (b) The surface of the ground; (c) The soil at various depths; (d) The surface levels of the sea and lakes (see VolumeIII, Chapter4 of the present Guide); (e) The upper air (see Chapter12 of the present volume). These measurements are required, either jointly or independently and locally or globally, for input to numerical weather prediction (NWP) models, for synoptical analyses, for hydrological and agricultural purposes, and as indicators of climatic variability. Local temperature also has direct physiological significance for the day-to-day activities of the world’s population. Measurements of temperature may be required as continuous records or may be sampled at different time intervals. This chapter deals with requirements relating to(a), (b) and (c). 2.1.3.2

Measurement uncertainty

The range, reported resolution and required uncertainty for temperature measurements are detailed in Chapter1 of the present volume. Meteorological thermometers should be calibrated against a laboratory standard and may be used with corrections being applied to their readings as necessary. It is necessary to limit the size of the corrections to keep residual errors within bounds. Also, the operational range of the thermometer will be chosen to reflect the local climatic range. All thermometers should be issued with a certificate confirming compliance with the appropriate uncertainty or performance specification, or a calibration certificate that gives the corrections that must be applied to meet the required uncertainty. The initial, as well as regular testing and calibration, should be performed by a laboratory accredited according to ISO/IEC 17025. 2.1.3.3

Response times

For routine meteorological observations there is no advantage in using thermometers with a very short time constant or lag coefficient, since the temperature of the air continually fluctuates up to one or two degrees within a few seconds. Thus, obtaining a representative reading with such a thermometer requires taking the mean of a number of readings, whereas a thermometer with a longer time constant tend to smooth out the rapid fluctuations. Too long a time constant, however, may result in errors when long-period changes of temperature occur. It is recommended that the time constant, defined as the time required by the thermometer to register 63.2% of a step change in air temperature, should be approximately 20s. Nevertheless, the time constant will become shorter at high airflow over the sensor. 2.1.3.4

Recording the circumstances in which measurements are taken

Temperature is one of the meteorological quantities whose measurements are particularly sensitive to exposure. For climate studies in particular, temperature measurements are affected by the state of the surroundings, by vegetation, sources of such as buildings and other objects, by ground cover, by the condition of, and changes in, the design of the radiation shield or screen, and by other changes in equipment (WMO, 2011). It is important that records are kept, not only

CHAPTER 2. MEASUREMENT OF TEMPERATURE

83

of the temperature data, but also of the circumstances in which the measurements are taken. Such information is known as metadata (data about data; see the present volume, Chapter1, Annex1.F). 2.1.4

Methods of measurement and observation

Radiation from the sun, clouds, ground and other surrounding objects passes through the air without appreciably changing its temperature, but a thermometer exposed freely in the open can absorb considerable radiation. As a consequence, its temperature may differ from the true air temperature. The difference depends on the balance between the absorption and emission of radiation and the thermal contact with the air. The effect of radiation can be minimized by using shiny thermometers – which reflect rather than absorb radiation – and which have a small diameter, so that they are effectively cooled by the air (Çengal and Ghajar, 2014; Incropera and de Witt, 2011; Erell etal., 2005; Harrison, 2015). For very fine wires used in an open-wire resistance thermometer, the difference from true air temperature may be very small or even negligible. It has been found (Harrison and Pedder, 2001; Harrison and Rogers, 2006; Harrison, 2010) that a thermometer made of 500mm length of 0.025mm diameter platinum wire held over a frame and exposed directly to the sun showed a warming due to irradiance of less than 0.07°C/100W m-2 for wind speeds greater than 1m s-1. Such a thermometer would typically show less than 1°C of error in full sunlight. Similar effects have been shown for very thin thermocouples (Bugbee etal., 1995). However, with the more usual operational thermometers, the temperature difference may reach 25K under extremely unfavourable conditions. Therefore, to ensure that the thermometer is as close to true air temperature as possible, it is necessary to protect it from radiation by a screen or shield that usually also serves to support the thermometer (see2.5). This screen also shelters the thermometer from precipitation while allowing the free circulation of air around it, and prevents accidental damage. If there is precipitation on the sensor, then evaporation will cool the sensor to an extent which depends on the local airflow. This cooling is similar to the behaviour of the wet-bulb thermometer in a psychrometer (see the present volume, Chapter4). Maintaining free circulation may, however, be difficult to achieve under conditions of rime ice accretion. Practices for reducing observational errors under such conditions will vary and may involve the use of special designs of screens or temperature-measuring instruments, including artificial ventilation. Nevertheless, in the case of artificial ventilation, care should be taken when moisture may be drawn onto the thermometer. In precipitation, drizzle and fog, moisture deposition in combination with evaporation may give rise to anomalous readings. An overview of concepts of temperature measurement applicable for operational practices is given by Sparks (1970). Actual best practice in thermometer exposure is exemplified by “triply redundant” aspirated sensors (Diamond etal., 2013). 2.1.4.1

General measurement principles

Temperature measurements of an object or substance can be categorized as either contact or non-contact. In contact thermometry a thermometer is placed in physical contact with an object, and ideally (in thermodynamic equilibrium) it attains the same temperature as the object, and so the temperature of the object can be inferred from the temperature of the thermometer itself. Any physical property of a substance that is a function of temperature can be used as the basis of a thermometer. The properties most widely used in meteorological thermometers are the change in electrical resistance of metals with temperature and thermal expansion of liquids and solids. Electrical thermometers are the recommended instruments for temperature measurement. They are already in widespread use in meteorology for measuring temperatures and provide the potential for automatic and continuous measurements. The most frequently used measurement

84

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

principle is the temperature dependence of the electrical resistance of a metal. Thermocouples are seldom used in meteorological observation systems. They are based on the principle of the “Seebeck effect” generating a temperature-dependent voltage. The principle of the thermal expansion of metal is used in mechanical thermographs with bimetallic or Bourdon-tube sensors. These instruments are used when accuracy is not as critical, but trends are to be observed. They are considered to be obsolete and should be replaced by alternatives if possible. The large difference between the thermal expansion of liquids and glass is exploited in liquidin-glass thermometers. Mercury or alcohol have been used for centuries for temperature measurement in such devices. Mercury-in-glass thermometers, used in a range of -30°C to 50°C, have been widespread but are no longer recommended. Taking into account the Minamata Convention on Mercury (see2.1.4.5), NMHSs are encouraged to take appropriate measures to replace mercury-in-glass thermometers with modern alternatives as soon as possible. In non-contact thermometry, the thermal radiation emitted from the surface of an object is used to estimate its temperature. This radiation is typically most intense in the IR or microwave region of the electromagnetic spectrum. Additionally the temperature of air may be measured without physical contact over a region of space by characterizing the transmission of sound, ultrasound or electromagnetic waves through the air (WMO, 2002a). Non-contact thermometers are not commonly used for meteorological measurements but can have advantages in some specialized applications. There is considerable research aimed at developing non-contact techniques for air-temperature measurement. Ultrasonic anemometers yield a parameter called “acoustic temperature” that can follow the fluctuations in air temperature at up to 100readings per second. These rapid measurements are useful for estimating heat flux (Schotanus etal., 1983) but the overall accuracy is poor (Richiardone etal., 2002). Other acoustic and optical techniques have been developed (for example, Underwood etal., 2017) but are not yet suitable for operational metrology. Thermometers that indicate the prevailing temperature are often known as ordinary thermometers, while those which indicate extreme temperature over a period of time are called maximum or minimum thermometers. If the temperature measurement is taken with electrical thermometers, the maximum and minimum temperature can be determined from the measured data if a continuous recording and sufficient measuring frequency is provided. As the only liquid for liquid-in-glass maximum thermometers is mercury, electrical alternatives should be used. There are various standard texts on instrument design and laboratory practice for the measurement of temperature (for example, Harrison, 2015; Jones, 1992). Considering the concepts of thermometry, care should be taken that, for meteorological applications, only specific technologies are applicable because of constraints determined by the typical climate or environment. 2.1.4.2

General exposure requirements

2.1.4.2.1

Measuring air temperatures

In order to achieve representative results when comparing thermometer readings at different places and at different times, a standardized exposure of the screen and, hence, of the thermometer itself is also indispensable. For general meteorological work, the observed air temperature should be representative of the free air conditions surrounding the station over as large an area as possible, at a height of between 1.25 and 2m above ground level. For reasons of comparability the measurement should be taken over natural ground, preferably over grass. The height above ground level is specified because large vertical temperature gradients may exist in the lowest layers of the atmosphere that can influence the temperature measurement. The most appropriate site for the measurements is, therefore, over level ground, freely exposed to sunshine and wind and not shielded by, or close to, trees, buildings and other obstructions.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

85

Sites on steep slopes or in hollows are subject to exceptional conditions and should be avoided. In towns and cities, local peculiarities are expected to be more marked than in rural districts. Temperature observations on the top of buildings are of doubtful significance and use because of the variable vertical temperature gradient and the effect of the building itself on the temperature distribution. The siting classification for surface observing stations on land (see the present volume, Chapter1, Annex1.D) provides additional guidance on the selection of a site and the location of a thermometer within a site to optimize representativeness. 2.1.4.2.2

Measuring soil temperatures

The standard depths for soil temperature measurements are 5, 10, 20, 50 and 100cm below the surface; additional depths may be included (for example, 2cm). The site for such measurements should be a level plot of bare ground (about 2mx2m) and typical of the surrounding soil for which information is required. When the ground is covered with snow, it is desirable to measure the temperature of the snow cover as well. Where snow is rare, the snow may be removed before taking the readings and then replaced. When describing a site for soil temperature measurements, the soil type, soil cover and the degree and direction of the ground’s slope should be recorded. Whenever possible, the physical soil constants, such as bulk density, thermal conductivity and the moisture content at field capacity, should be indicated. The level of the water table (if within 5m of the surface) and the soil structure should also be included. This is important to estimate the soil heat flow in NWP. At agricultural meteorological stations, the continuous recording of soil temperatures and air temperatures at different levels in the layer adjacent to the soil (from ground level up to about 10m above the upper limit of prevailing vegetation) is desirable. 2.1.4.2.3

Measuring minimum temperatures (grass or bare soil)

The grass minimum temperature is the lowest temperature reached overnight by a thermometer freely exposed to the sky just above short grass. Grass minimum temperatures should be measured at 5 cm above grass or a surface representative of the locality. If bare soil minimum temperatures are observed, these measurements should be made at 5cm above the natural bare-ground level. When the ground is covered with snow, the thermometer should be supported immediately above the surface of the snow, as near to it as possible without actually touching. At a station where snow is persistent and of varying depth, it is possible to use a support that allows the thermometers to be raised or lowered to maintain the correct height above the snow surface. 2.1.4.3

Sources of error – general comments

Errors in the measurement of temperature may be caused by the following: (a) Direct and indirect radiation from different sources, for example, the sun, clouds, soil and surrounding objects and lakes; (b) Uncertainty of the sensing element, the instrument and for electrical measurements made by other technical devices in the data chain; (c) Insufficient ventilation of the screen (wind speed under 1m s-1) especially in conditions of high solar radiation; (d) Psychrometric cooling due to wet surfaces on the screen and/or the sensor;

86

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(e) Contamination of the sensor, for example, by dirt, sea spray; (f) Incorrect operation, for example, failure to achieve stable equilibrium or reading errors from the observer. The time constant of the sensor, the time averaging of the output and the data requirement should be consistent. The different types of temperature sensors vary in their susceptibility to, and the significance of, each of the above; further discussion will be found in the appropriate sections of this chapter. Due to its high relevance for temperature measurements, radiation errors are discussed in more detail in the following paragraphs. Radiation errors are caused by direct heating of a thermometer by electromagnetic radiation (EMR) that passes freely through the air. The heating effects arise from both direct irradiation – due to visible light leaking into a thermometer enclosure – and thermal irradiation due to differences in temperature between the thermometer and its surroundings. The absorption of radiation in these two bands determines the magnitude of the radiative “load” on a sensor. The load is determined by the intensity of the irradiation in each band, and the emissivity of the sensor surface – which generally varies with the wavelength of the irradiation. However, the heating load is always minimized by having a low-emissivity, polished (that is, shiny) surface. The measurement of air temperature with contact sensors is particularly sensitive to radiative loading because of the weak thermal contact between the sensors and the air, especially when the air is slow moving. The heat flow between the thermometer and the air is characterized by a heat transfer coefficienth (Çengal and Ghajar, 2014; Incropera and de Witt, 2011) which depends on the speed of the air flowing past the thermometer and the diameter of the thermometer. For a wide range of thermometers with a cylindrical or spherical form, the heat transport improves as the square root of the air speed past the thermometer and inversely as the square root of the diameter of the thermometer (Ney etal., 1960; Erell etal., 2005; Harrison, 2015). Thus for any air speed, the error caused by a radiant heat load will be reduced by a factor of two if the diameter of the sensor is reduced by a factor of four. The strength of radiative coupling between a thermometer and its environment is stronger than is often considered. For cylindrical sensors with a stainless steel case, a screen that is 3°C warmer than the air, in a wind speed of 0.1m s-1 will result in ~0.5°C error for a 6 mm-diameter sensor, but only ~0.2°C error for a 1mm-diameter sensor. 2.1.4.4

Maintenance – general comments

The following maintenance procedures should be considered: (a) Sensors and housings should be kept clean to reduce radiation errors; (b) If artificially ventilated screens are used, the fan status should be checked regularly, either manually or, preferably, automatically; (c) Regular calibration is required for all temperature sensors and, if applicable, for the electrical interfaces. Field checks should be performed between calibration intervals; (d) If analog–digital converters (ADCs) are used they should be checked regularly with ohmic resistance to determine whether they still fulfil requirements. Detailed maintenance requirements specific to each type of thermometer described in this chapter are included in the appropriate section.

87

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.1.4.5

Implications of the Minamata Convention for temperature measurement

The UNEP Minamata Convention on Mercury came into force globally in August 2017 and bans all production, import and export of mercury-in-glass thermometers (see the present volume, Chapter1, 1.4.1). Therefore, mercury-in-glass thermometers are no longer recommended and it is strongly encouraged to take appropriate measures to replace them with modern alternatives as soon as possible. Electrical resistance thermometers provide an economical, accurate and reliable alternative to their dangerous, mercury-based precedents and offer significant advantages in terms of data storage and real-time data display.

2.2

ELECTRICAL THERMOMETERS

2.2.1

General description

Electrical instruments are in widespread use in meteorology for measuring temperatures. Their main virtue lies in their ability to provide an output signal suitable for use in remote indication, recording, storage, or transmission of temperature data. The most frequently used sensors are PRTs, but semiconductor thermometers (thermistors) and thermocouples are also used. 2.2.1.1

Metal resistance thermometers

Across the entire meteorological temperature range from –80°C to 60°C the electrical resistance of most pure metals is an almost linear function of temperature. Although many pure metals could be used for thermometry, platinum metal is most widely used for electrical resistance thermometers because of its exceptional resistance to corrosion. Ultra-pure, strain-free platinum is used for so-called SPRTs that are used for interpolating between fixed points in realizations of ITS-90 in standard laboratories. However, these thermometers are too delicate for use in the field. The most common format of PRT is called a Pt100 because the sensors are engineered to have a resistance R0 close to 100Ω at 0°C. These sensors use slightly less pure platinum and are much more robust that SPRTs. Typically, the sensors consist of platinum wires wound around a ceramic core and held inside a ceramic, glass, or stainless steel outer casing (Figure2.1(a)). Alternatively, thin films of platinum can be deposited in a labyrinthine pattern onto a ceramic substrate and then typically packaged in stainless steel (Figure2.1(b)). From – 80°C to 60°C, the electrical resistance of a PRT can be represented by the Callender-van Dusen equation:

(

)

R = R0 1 + At + Bt 2 + C ( t − 100 ) t 3 (2.2)

where t is the temperature in°C. Pt100 sensors are commonly specified by the tolerance within which they conform to standards such as IEC 60751 (DIN EN 60751) or ASTM E1137. For a thermometer that conforms closely to the

(a)

(b) Thin Platinum Wire Coil

Connecting Wires

Glass or Alumina Protection

Labyrinthine Platinum Track

Alumina Substrate

Alumina Tube Contact Pads

Figure 2.1. (a) Wire-wound PRT; (b) thin-film PRT

88

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(a) (a)

(b)

(b) 0.400

Pt 100 Sensitivity (Ω °C-1)

Pt 100 Resistance (Ω)

120 110 100 90 80 70 60 -80 -70 -60 -50 -40 -30 -20 -10 0 10

0.398 0.396 0.394 0.392 0.390 0.388 0.386 0.384 -80 -70 -60 -50 -40 -30 -20 -10 0

20 30 40 50

10 20 30 40 50

Temperature (°C)

Temperature (°C)

Figure 2.2. (a) The resistance versus temperature for an IEC 60751-compliant Pt 100 sensor; (b) the sensitivity versus temperature for an IEC 60751-compliant Pt 100 sensor IEC 60751 specification, the Callendar–van Dusen coefficients are R0=100Ω, A = 3.908 x 10 -3°C-1 and B = -5.80 x 10 -7°C-2, while the C coefficient takes different values above and below 0°C. Below 0°C, its value is C = 4.27 x 10 -12°C-4, while C is exactly zero above 0°C. The resistance and sensitivity of an IEC standard Pt100 sensor are shown in Figure 2.2. The sensitivity of Pt100 thermometers describes the change in resistance due to temperature change and is commonly specified by an α (alpha) value defined by:

α =

R100 − R0 (2.3) 100°C * R0

The sensitivity (Figure 2.2(b)) is almost independent of temperature, being 0.3952Ω°C -1 at -40°C, 0.3909Ω°C ‑1 at 0°C and 0.3863Ω°C -1 at 40°C, a variation of just 2.3% across a range of 80K. For IEC60751-compliant thermometers, α has a value close to 3850 x 10 -6°C -1. The tolerance classes of IEC60751 or ASTME1137 are shown in Table2.1 and graphed in Figure2.3. Sensors are also available with smaller tolerance typically specified as a fraction of one of the standards shown in Table2.1. Table 2.1. The tolerance classes of IEC 60751 or ASTM E1137. IEC 60751 (2008)

ASTM E1137

Tolerance class

Definition

Tolerance grade

Definition

F0.3 (Old Class B)

±(0.3 + 0.005 |t|)

B

±(0.25 + 0.0042 |t|)

F0.15 (Old Class A)

±(0.15 + 0.002 |t|)

A

±(0.13 + 0.0017|t|)

Note: In the IEC specification the F indicates a thin-film sensor and is replaced with a W for a wirewound sensor. |t| indicates the absolute value of the temperature in degrees Celsius.

For example, an IEC60751 W0.3 (old Class B) sensor describes a wirewound sensor which conforms to the IEC60751 curve within ±(0.3 + 0.005t)°C, so at 20°C a class B sensor would be guaranteed to fall within 0.400°C of the IEC curve. The Callendar–van Dusen equation (equation2.2) has no simple inverse equation expressing temperature as a function of resistance, t(R). There are two solutions to this difficulty. First, for temperatures greater than -40°C, the C term in equation2.2 corresponds to less than 0.01°C and may reasonably be neglected in many applications. In this case, the Callendar–van Dusen equation may be approximated as:

(

)

R = R0 1 + At + Bt 2 (2.4)

89

CHAPTER 2. MEASUREMENT OF TEMPERATURE

0.80

Positive limit of IEC60751 Class B (W0.3) Tolerance

0.60 0.40

Tolerance (°C)

0.20 0.00

Positive and Negative limit of IEC60751 Class A (W0.15) Tolerance

-0.20 -0.40

Positive limit of IEC60751 Class B (W0.3) Tolerance

-0.60 -0.80 -80

-70

-60

-50

-40

-30

-20

-10

10

20

30

40

50

Temperature (°C)

Figure 2.3. Tolerance bands for an IEC 60751 compliant Pt100 sensor. Moreover, its inverse can be calculated using the standard quadratic formulae:

t =−

A A R  4B  + 1− 1 −  (2.5) 2 R0  2B 2B A 

The error from using this formula is still less than 0.1°C at -80°C. Alternatively, equation2.4 may be used to generate an initial estimate of the temperature, which is then iteratively refined by repeated use of the forward equation forR. Before calibration for deployment in a meteorological setting, Pt100 sensors are usually “aged” (by manufacturers) by temperature cycling the sensor between, typically, the ice point and 20°C. The aim of this procedure is to discover any manufacturing faults before deployment and to relieve any strain in the wires that will eventually be released in the field. 2.2.1.2

Thermistors

Another type of resistance element in common use is the thermistor. Although thermistors are available with positive temperature coefficients of resistance, the most common and useful form has large negative coefficients of resistance. The composition of thermistors is a proprietary secret, but they typically consist of sintered metallic oxides in the form of small discs, rods or spheres and are often glass-coated to prevent chemical reactions with the air, particularly moisture. The temperature dependence of the resistance, R, of a thermistor can be qualitatively described by:

 R = R0 exp  − β  

 1 1  −   (2.6)  T0 T  

where R0 is the resistance of the thermistor at absolute temperature T0 (in kelvin), and T is the temperature of the thermistor in kelvin. Thermistors are typically specified for the value R0 at a temperature of 25°C, that is, T0= 25 + 273.15 = 298.15 K, and β is specified in kelvin. Typical values are R0≈1kΩ and β≈4000K (for example, see Figure2.4). There are three key differences in the behaviour of thermistors when compared with Pt100 sensors.

90

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The first is the high resistance of the thermistors, often sufficiently high that the resistance of the connecting wires may be neglected. This is almost never true for Pt100 sensors for which a fourwire measurement technique is always necessary. The second is the high sensitivity compared with Pt100 sensors. Although this is an advantage at any specific temperature, the fact that the sensitivity varies with temperature is problematic and leads to a non-linear behaviour, and the very high dynamic range of the sensors also presents signal processing problems. Finally, the sensors can be very small, and so they can have a small time constants and high heattransfer coefficients (Erell etal., 2005). However, very small thermistors have the disadvantage that, for a given power dissipation, the self-heating effect is greater than for larger thermometers. Thus, care must be taken to keep the power dissipation small. It should be noted that although equation2.6 describes the general behaviour of thermistors and is useful for interpolation across small temperature intervals, it is not accurate enough to be used for meteorological applications. Several expressions of the general form: B C D  R = R0 exp  A + + +  (2.7) T T2 T3  

are commonly used to describe the behaviour of thermistors more accurately than equation2.6. One special case of equation2.7 where the coefficient C is set to zero is known as the Steinhart– Hart equation. The coefficients R0, A, B, C (if used) and D must be determined for each sensor by calibration. An equivalent inverse expression for the temperature is: 2 3   R   R   R   T =  A '+ B 'ln   + C 'ln   + D 'ln     R0   R0   R0   

−1

(2.8)

where the parameters A’, B’, C’ and D’ are completely different from the parameters from the parameters A, B, C and D in equation2.7. In both equations2.7 and 2.8, it must be remembered that the temperature T must be expressed in kelvin.

100

Resistance (kΩ)

10

1

0.1 -60

-50

-40

-30

-20

-10

10

20

30

40

50

60

Temperature (°C)

Figure 2.4. The resistance versus temperature for a thermistor R 0 = 1 kΩ and β≈3 700 K. (Note the vertical axis is logarithmic.)

91

CHAPTER 2. MEASUREMENT OF TEMPERATURE

In general the inverse formula is not as accurate as the forward formula, and if a computer is used to calculate the temperature, it is often advantageous to iterate the forward formula (equation2.7) to find the correct result. Many iteration schemes have been devised, and optimized for accuracy and speed, but a typical approach is outlined below. In this procedure, the correct temperature is first guessed as being in the midpoint of the calibration range, and the resistance corresponding to this guess is calculated using equation2.7. If this resistance is greater than the measured resistance, then a new temperature guess is made in the upper half of the calibration range. If this resistance is less than the measured resistance, then a new temperature guess is made in the lower half of the calibration range. A new resistance is calculated from the second temperature guess and depending on the value of this resistance compared to the measured resistance, a third temperature guess is made. Iteration criteria must be met to stop the iteration process. In this way, the temperature may be inferred from a measured resistance using only the forward formula2.7. 2.2.1.3

Thermocouples

The Seebeck effect describes the phenomenon whereby a temperature gradient in a metal gives rise to an accompanying electric field. The magnitude of the accompanying electric field is always proportional to the temperature gradient, but depends on a material-dependent and temperature-dependent Seebeck coefficient (Nicholas and White, 1993; Bentley, 1998). Thus, a wire of pure material in a temperature gradient spontaneously acquires a voltage across its ends, the magnitude of which is equal to the integrated Seebeck voltage along the wire. A thermocouple (Figure2.5) is made from two wires of different materials with differing Seebeck coefficients joined at one end – the so-called “hot junction”. Typically the open ends of the junctions are connected to the terminals of a high-resolution voltmeter. The thermo-voltage of a thermocouple is generated continuously along the entire length of the wires. We can consider each wire in the thermocouple to experience a sequence of small temperature changes, ΔTi (Figure2.5(a)). Each small temperature difference ΔTi gives rise to a voltage ΔVi that is proportional to ΔTi and a material and temperature dependent Seebeck coefficient,S (Figure2.5(b)). The thermo-voltage measured across the open ends of the thermocouples is proportional to the temperature difference between the open ends of the wire and the thermocouple junction, even though no voltage is generated at the junction. Notice that terms “hot” and “cold” junctions are entirely conventional as thermocouples can measure temperatures even when the “hot junction” is colder than the “cold junction”. It is important to stress that the thermo-voltage is generated along the entire length of the thermocouple wires and that no voltage is generated at the junction itself – where the

(a)

“Hot” Junction

“Cold” Junction

(b)

“Hot” Junction

“Cold” Junction Thermovoltage

Wire 1 Wire 2

Wire 1 Wire 2

Figure 2.5. In a temperature gradient (a) voltages are generated (b) along the entire length of both wires.

92

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

temperature is measured. The thermocouple junction merely joins the two wires in the thermocouple together and may be made by welding or soldering together the two wires. All that is required is to create a suitably robust electrical connection between the wires. Thermocouple wires can be made very narrow and the junctions can be made very small, with three potential benefits. First, thermocouples can be made with response times in air much less than one second. Second, the small size also improves heat exchange with the air resulting in lower errors when the thermocouple is irradiated (Bugbee etal., 1996). Finally, the effect of thermal conduction along the thermocouple wires is reduced. Thermocouples are characterized as being made from either “base metals” (typically alloys of either copper or nickel) or “noble metals” (typically alloys of platinum, rhodium or gold). In meteorological applications there is no advantage to the use of noble metal thermocouples. Thermocouples are typically purchased as a manufactured item with the two wires preselected from standard alloy combinations which are specified by a “letter-type”. The most commonly used types in meteorological applications according to IEC60584-1:2013 are: –

Type K: made from two nickel alloys, chromel and alumel;

Type J: made from iron and constantan (copper–nickel alloy);

Type T: made from copper and constantan.

In the meteorological temperature range there is little advantage to using one type over another. Type K is the most common specification, and at 20°C it produces a signal of approximately 40µV°C-1. Type J has a slightly higher sensitivity (approximately 50µV°C-1) but the pure iron leg is potentially subject to corrosion if exposed for long periods. Type T has a similar sensitivity to Type K but the copper wire has a high thermal conductivity, which can lead to errors in some circumstances. Tables of thermo-voltage versus temperature for standard thermocouple types are available from manufacturers and standards bodies. 2.2.2

Measurement procedures

2.2.2.1

Electrical resistance thermometers

Electrical resistance thermometers may be connected to a variety of electrical measurement circuits. Historically, many variations of resistance bridge circuits were used in either balanced or unbalanced form. In such circuits, a single voltage or current measurement enables the comparison of the unknown resistance of the thermometer with known temperatureindependent standard resistors. The excellent resolution and linearity of modern ADCs, and the measurement component within voltmeters and multimeters, enables alternative approaches. The unknown resistance of the thermometer is estimated from two measurements; a measurement of the current flowing through the temperature sensor; and a measurement of the voltage across the temperature sensor. This allows the measurement of a resistance of approximately 100Ω with an uncertainty of just a few mΩ. The resistance of the wires connecting the sensor to the ADC must also be considered. Typically such wires have an electrical resistance of a few tenths of an ohm per metre. For a Pt100 sensor, an additional resistance of 0.39 Ω is equivalent to an error of 1°C. And so for Pt100 sensors a four-wire measurement configuration must be used in which an additional pair of wires sense the voltage (Figure2.6).

93

CHAPTER 2. MEASUREMENT OF TEMPERATURE

Pt 100 Sensor Current ~1 mA

Ohm Meter 108.01 Ω

Voltage Sensing

Figure 2.6. A four-wire arrangement for reading a Pt100 sensor For a thermistor, the error caused by the connecting wires varies strongly with temperature, but tends to be much less significant than for Pt100 sensors. For the sensor with R0=1kΩ and β=3700K illustrated in Figure2.4, at -20°C the sensor resistance is 9.08kΩ and the sensitivity is -542 Ω°C -1, a change of 6.0%°C -1, and at 20°C the sensor resistance is 1.24kΩ and the sensitivity is -55Ω°C -1, a change of 4.4%°C -1. Thus, at -20°C an additional resistance of 0.39Ω is equivalent to an error of 0.0007°C, and at 20°C an error of 0.007°C, which in many cases can be considered negligible. To maintain the advantages of thermistors, such as rapid response and high heat transfer coefficient, but avoid the disadvantages of the high dynamic range and varying sensitivity, thermistors are often used in “linearizing” circuits. A large number of such circuits exist, but the simplest consists of a parallel constant resistor (White, 2015, 2017). 2.2.2.2

Thermocouples

Historically thermocouples were used in a wide variety of configurations that required the open ends of the thermocouple – the so-called “cold-junction” – to be immersed in melting ice (Figure2.7(a)). The temperature of the hot junction was then deduced from standard tables for the pair of metals used in the thermocouple. In practice it is inconvenient to maintain an ice point and this is now only rarely done. Instead, the measurements are referenced to the temperature of the terminals of the digital voltmeter. This technique – known as cold-junction compensation – requires a measurement of the temperature of the voltmeter terminals using a thermistor or PRT (Figure2.7(b)). The additional thermocouple voltage that would have been expected if an ice point had been used is then calculated and added to the measured voltage. The sum is then used to determine the temperature using interpolation of standard tables. Where cold-junction compensation is used, special care must be taken close to voltmeter junctions where small temperature differences between the terminals can generate spurious voltages.

(a)

“Hot” Junction Voltmeter 0.803 mV

“Cold” Junction

(b) “Cold”Junction Compensating Thermometer

“Hot” Junction

0.803 mV

Ice-water mixture

Figure 2.7. Arrangements for reading a thermocouple: (a) one of several traditional arrangements in which the cold junction is immersed in melting ice; (b) a modern alternative in which a thermistor or Pt100 thermometer measures the temperature of the “cold junction”.

94

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(a)

(b)

T1 Voltmeter

T1 Voltmeter

0.803 mV

0.803 mV

T2

T2

Figure 2.8. Use of thermocouple in differential mode: (a) difference between the two junctions; (b) thermo pile Typically a purchased thermocouple is not long enough to connect to a voltmeter in an environmentally isolated casing, and a so-called “extension cable” is required for this purpose. As noted above, the thermo-voltage is generated along the entire length of the thermocouple, including any extension cable. Since the temperature gradients in the cable are often largest close to the enclosure containing the data acquisition equipment, the thermo-voltages generated in the extension cable are as significant as those generated close to the hot junction of the thermocouple. For this reason thermocouples are generally not chosen for routine meteorological use. In meteorology, thermocouples are used for thermometry for two special applications. The first application is when a thermometer with a low mass and very small time constant is required for special research tasks (see for example, Bugbee etal., 1996). The second application is for the measurement of small temperature differences; for this the thermocouple is wired as two thermocouples in opposition (Figure2.8(a)). In this configuration the measured voltage is sensitive only to the difference in temperature between the two junctions. A modification of the differential thermocouple is the “thermo pile” (Figure2.8(b)), which consists of a large number of differential thermocouples wired in series. The output voltage from a thermopile of N junctions is just Ntimes the thermocouple output from a single thermocouple, and for 10junctions this can approach 400µV°C-1. This allows high resolution detection of very small temperature differences such as those that occur in a pyranometer. 2.2.3

Exposure and siting

The general requirements relating to the exposure and siting of thermometers are described in 2.1.4.2. Additional requirements include the following: (a) The measurement of extreme values: Separate maximum and minimum thermometers may no longer be required if the electrical thermometer is connected to a continuously operating data recording system; (b) The measurement of surface temperatures (bare soil or grass minimum thermometer): The radiative properties of electrical thermometers will be different from liquid-in-glass thermometers. Electrical thermometers exposed as grass minimum (or other surface) thermometers will, therefore, record different values compared to similarly exposed conventional thermometers. These differences may be minimized by placing the electrical thermometer within a glass sheath with the same diameter as the superseded thermometers; (c) The measurement of soil temperatures: Electrical thermometers are deployed in brass plugs, inserted at the required depth into an undisturbed vertical soil face, the latter having been exposed by trenching. Electrical connections are brought out through plastic tubes via the trench, which is then refilled in such a way to restore, as far as possible, the original strata and drainage characteristics.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.2.4

Sources of error

2.2.4.1

Electrical resistance thermometers

95

The main sources of error in a temperature measurement taken with electrical resistance thermometers are the following: (a) Self-heating of the thermometer element; (b) Inadequate compensation for lead resistance; (c) Inadequate compensation for non-linearities in the sensor or processing instrument; (d) Sudden changes in switch contact resistances. Self-heating occurs because the passage of a current through the resistance element produces heat and, thus, the temperature of the thermometer element becomes higher than that of the surrounding medium. For a 1mA (10mA) current in a Pt100 sensor, the heating is approximately 0.1mW (10mW). For a sensor with a diameter of 6mm, 30mm long, in a wind speed of 1m s-1, the heat transfer coefficient will be approximately 40Wm-2 K-1 and the resultant sensor heating will be between 0.004K and 0.4K. The resistance of the connecting leads will introduce an error in the temperature reading. This will become more significant for long leads, for example, when the resistance thermometer is located at some distance from the measuring instrument; the reading errors will also vary as the temperature of the cables changes. To reduce errors, it is highly recommended to use four-wire measurements of Pt100 thermometers (see Figure2.6). Neither the electrical resistance thermometer nor the thermistor is linear over an extended temperature range. While for electrical resistance thermometers the output may be considered to be approximately linear for a limited range, appropriate provision must be made to compensate for such non-linearities with thermistors (White, 2016). Sudden changes in switch contact resistance can occur as switches age. They may be variable and can go undetected unless regular system calibration checks are performed (see2.2.5). 2.2.4.2

Thermocouples

The main uncertainty arising in the use of thermocouples arises from the distributed nature of the thermo-voltage generation. As shown in Figure2.5, the measured voltage is generated along the entire length of the thermocouple along with its extension wires. This requires extreme uniformity in the alloy composition of thin wires. Additionally, the thermocouple must then be calibrated with the temperature gradients that the thermocouple will experience operationally. Additionally, the secondary measurement of temperature used for the cold-junction compensation introduces an unknown error based on the environment of the ADC within the voltmeter or data acquisition system. For meteorological deployments, proper calibration is not practical and the effectiveness of the cold-junction compensation cannot be assessed without additional knowledge. For these reasons, thermocouples are not recommended for standard meteorological deployment.

96

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

2.2.5

Comparison and calibration

2.2.5.1

Electrical resistance thermometers

Laboratory calibrations of thermometers should be carried out regularly by calibration laboratories with ISO/IEC17025 accreditation. Thermometers should be compared against standard thermometers usually in a stirred liquid bath, climatic chamber or a dry-block calibrator in the temperature range of interest. More details can be found in2.6. Since the measurement instrument is an integral part of the electrical thermometer, its calibration should be checked by substituting the resistance thermometer by an accurate, calibrated resistance reference and by applying resistances equivalent to fixed temperature increments (for example, 10K) over the operational temperature range. The error at any point should not exceed 0.1K. This work would normally be performed by a servicing technician. 2.2.5.2

Thermocouples

The calibration and checking of thermocouples require the hot and cold junctions to be maintained at accurately known temperatures and the gradient between these temperatures to be varied to assess non-uniformity of the Seebeck coefficient. The techniques and instrumentation necessary to undertake this work are very specialized and will not be described here (Bentley, 1998; Nicholas and White, 1993; ASTM, 1993). 2.2.6

Corrections

When initially issued, electrical thermometers should be provided with either: (a) A dated certificate confirming compliance with the appropriate standard; (b) A dated calibration certificate giving the actual resistance or temperature (using the IEC standard Callendar–van Dusen parameters) at fixed points in the temperature range. These resistances/temperatures should be used when checking the uncertainty of the measuring instrument or system interface before and during operation. The magnitude of the resistance difference from the nominal value should not, in general, be greater than an equivalent temperature error of 0.1 or 0.2K. After each calibration, a Pt100 sensor will have a table of values of resistance, Ri, at a set temperatures Ti. These values may be used to generate a table of corrections (either ΔRi or ΔTi) to be used with the thermometer. Based on this calibration data, conformance to the specified standard (for example, IEC60751), class or tolerance band can be checked. However, to properly assess conformance within a tolerance band, users should additionally consider the effect of measurement uncertainty, uT, associated with the calibration. There are three cases to consider: –

In the first case, the correction ±uT falls entirely within the conformance band. In this case, the thermometer can be judged as being compliant with the specification. If this is the case, the thermometer can be returned to use and its temperature inferred from the standard IEC60751 curve specified by the standard Callendar–van Dusen coefficients. For the example in Figure2.9, this would be the case for an uncertainty of uT=0.05°C.

In the second case, the correction ±uT falls entirely outside the conformance band. The thermometer can be judged as being non-compliant with the specification. In this case, the thermometer cannot be returned to use and the sensor would typically be discarded.

In the third case, the correction ±uT overlaps the conformance band so that there is a significant possibility that the sensor is non-compliant. In this case, the action taken is a matter of judgement depending on the degree of overlap. If the likelihood of nonconformance is judged to be sufficiently small, the thermometer might be returned to use

97

CHAPTER 2. MEASUREMENT OF TEMPERATURE

and its temperature inferred from the standard IEC60751 curve specified by the standard Callendar–van Dusen coefficients. For the example in Figure2.9, this would be the case for an uncertainty of uT=0.15°C. Note that conformance or non-conformance may depend on the coverage factor ascribed to the uncertainty. In the simplest scheme that applies corrections, the corrections at a particular temperature are presumed to apply to all temperatures that are closer to that calibration point than any other calibration point. The temperature is thus inferred from the resistance using a standard curve, and then the correction is added. This scheme (Figure2.9(a)) has the disadvantage of generating a discontinuity in the temperature versus resistance curve midway between the calibration points. An improvement on the simple correction scheme is a linear interpolation (Figure2.9(b)). The correction that applies to a particular measurement is calculated by linearly interpolating between the corrections from the calibration points above and below the particular temperature chosen. This has the advantage of generating a continuous temperature versus resistance curve. A more sophisticated treatment of the data would be to use the data to generate custom Callendar–van Dusen parameters associated with that particular thermometer. This procedure is described in (Nicholas and White, 1993). 2.2.7

Maintenance

Regular field checks should identify any changes in system calibration. These may occur as a result of long-term changes in the electrical characteristics of the thermometer, degradation of the electrical cables or their connections, changes in the contact resistance of switches or changes in the electrical characteristics of the measuring equipment. Identification of the exact source and correction of such errors requires specialized equipment and training and should be undertaken only by a maintenance technician.

2.3

LIQUID-IN-GLASS THERMOMETERS

Mercury-in-glass thermometers have been in widespread use, but as a result of the Minamata Convention on Mercury (see2.1.4.5) are no longer recommended. NMHSs are encouraged to take appropriate measures to replace mercury-in-glass thermometers with modern alternatives

(b)

(a)

0.30

0.30

0.20

Correction (°C)

Correction (°C)

0.20

Positive limit of IEC60751 Class A (W0.15) Tolerance

0.10 0.00 -0.10 -0.20 -0.30

Negative limit of IEC60751 Class A (W0.15) Tolerance

-80 -70 -60 -50 -40 -30 -20 -10

10 20

Temperature (°C)

30 40 50

Positive limit of IEC60751 Class A (W0.15) Tolerance

0.10 0.00 -0.10 -0.20

Negative limit of IEC60751 Class A (W0.15) Tolerance

-0.30 -80 -70 -60 -50 -40 -30 -20 -10

10 20

Temperature (°C)

Figure 2.9. Schemes for applying corrections derived from the calibration data: (a) stepwise; (b) linear

30 40 50

98

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(for example, electrical resistance thermometers). Considering the historical development of thermometry and the residual use of mercury-in-glass thermometers, the following text includes also mercury-in-glass thermometers. 2.3.1

General description

For routine observations of air temperature, including maximum, minimum and wet-bulb temperatures, liquid-in-glass thermometers are still commonly used. Such thermometers make use of the differential expansion of a pure liquid with respect to its glass container to indicate the temperature. The stem is a tube which has a fine bore attached to the main bulb; the volume of liquid in the thermometer is such that the bulb is filled completely but the stem is only partially filled at all temperatures to be measured. The change in volume of the liquid with respect to its container are indicated by change in the liquid column; by calibration with respect to a standard thermometer, a scale of temperature can be marked on the stem, or on a separate scale tightly attached to the stem. The liquid used depends on the required temperature range; mercury has been used for temperatures above its freezing point (–38.9°C), while ethyl alcohol or other pure organic liquids are used for lower temperatures. The glass should be one of the normal or borosilicate glasses approved for use in thermometers. The glass bulb is made as thin as practical, while maintaining reasonable strength, to facilitate the conduction of heat to and from the bulb and its contents. A narrower bore provides greater movement of liquid in the stem for a given temperature change, but reduces the useful temperature range of the thermometer for a given stem length. The thermometer should be suitably annealed before it is graduated in order to minimize the slow changes that occur in the glass with ageing. There are four main types of construction for meteorological thermometers, as follows: (a) The sheathed type with the scale engraved on the thermometer stem; (b) The sheathed type with the scale engraved on an opal glass strip attached to the thermometer tube inside the sheath; (c) The unsheathed type with the graduation marks on the stem and mounted on a metal, porcelain or wooden back carrying the scale numbers; (d) The unsheathed type with the scale engraved on the stem. The stems of some thermometers are lens-fronted to provide a magnified image of the liquid thread. Types (a) and (b) have the advantage over types (c) and (d) that their scale markings are protected from wear. For types(c) and (d), the markings may have to be reblackened from time to time; on the other hand, such thermometers are easier to make than types(a) and (b). Types(a) and (d) have the advantage of being less susceptible to parallax errors (see2.3.4). An overview of thermometers, designed for use in meteorological practices is given by Her Majesty’s Stationary Office/Meteorological Office (1980). Whichever type is adopted, the sheath or mounting should not be unduly bulky as this would keep the heat capacity high. At the same time, the sheath or mounting should be sufficiently robust to withstand the normal risks associated with handling and transit. For mercury-in-glass thermometers, especially maximum thermometers, it is important that the vacuum above the mercury column be nearly perfect. All thermometers should be graduated for total immersion, with the exception of thermometers for measuring soil temperature. The special requirements of thermometers for various purposes are dealt with hereafter under the appropriate headings.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.3.1.1

99

Ordinary (station) thermometers

Historically a very accurate mercury-in-glass-type thermometer has been used. Its scale markings have an increment of 0.2K or 0.5K, and the scale is longer than that of the other meteorological thermometers. The ordinary thermometer is mounted in a thermometer screen to avoid radiation errors. A support keeps it in a vertical position with the bulb at the lower end. The form of the bulb is that of a cylinder or a sphere. A pair of ordinary thermometers can be used as a psychrometer if one of them is fitted with a wet-bulb2 sleeve (see the present volume, Chapter4, 4.3). 2.3.1.2

Maximum thermometers

The recommended type for maximum thermometers has been a mercury-in-glass thermometer with a constriction in the bore between the bulb and the beginning of the scale. This constriction prevents the mercury column from receding with falling temperatures. However, observers can reset by holding it firmly, bulb-end downwards, and swinging their arm until the mercury column is reunited. A maximum thermometer should be mounted at an angle of about 2° from the horizontal position, with the bulb at the lower end to ensure that the mercury column rests against the constriction without gravity forcing it to pass. It is desirable to have a widening of the bore at the top of the stem to enable parts of the column which have become separated to be easily united. As the only liquid suitable for liquid-in-glass maximum thermometers is mercury, electrical alternatives should be used to measure maximum temperature (see2.2). 2.3.1.3

Minimum thermometers

As regards minimum thermometers, the most common instrument is a spirit thermometer with a dark glass index, about 2cm long, immersed in the spirit. Since some air is left in the tube of a spirit thermometer, a safety chamber should be provided at the upper end which should be large enough to allow the instrument to withstand a temperature of 50°C or greater without being damaged. Minimum thermometers should be supported in a similar manner to maximum thermometers, in a near-horizontal position. Various liquids can be used in minimum thermometers, such as ethyl alcohol, pentane and toluol. It is important that the liquid should be as pure as possible since the presence of certain impurities increases the tendency of the liquid to polymerize with exposure to light and after the passage of time; such polymerization causes a change in calibration. In the case of ethyl alcohol, for example, the alcohol should be completely free of acetone. Minimum thermometers are also exposed to obtain grass minimum temperature (see2.1.4.2.3). 2.3.1.4

Soil thermometers

For measuring soil temperatures at depths of 20cm or less, mercury-in-glass thermometers, with their stems bent at right angles, or any other suitable angle, below the lowest graduation, have been in common use. The thermometer bulb is sunk into the ground to the required depth, and the scale is read with the thermometer in situ. These thermometers are graduated for immersion up to the measuring depth. Since the remainder of the thermometer is kept at air temperature, a safety chamber should be provided at the end of the stem for the expansion of the mercury. For measuring temperature at depths of over 20cm, mercury-in-glass thermometers have been used mounted on wooden, glass or plastic tubes, with their bulbs embedded in wax or metallic paint. The thermometer–tube assemblies are then suspended or slipped in thin-walled metal or

2

Wet-bulb temperatures are explained in the present volume, Chapter 4.

100

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

plastic tubes sunk into the ground to the required depth. In cold climates, the tops of the outer tubes should extend above the ground to a height greater than the expected depth of snow cover. The technique of using vertical steel tubes is unsuitable for measuring the diurnal variation of soil temperature, particularly in dry soil, and calculations of soil thermal properties based on such measurements could be significantly in error because they will conduct heat from the surface layer. The large time constant due to the increased heat capacity enables the thermometers to be removed from the outer tubes and read before their temperature has had time to change appreciably from the soil temperature. When the ground is covered by snow, and in order that the observer may approach the line of thermometers without disturbing the snow cover, it is recommended that a lightweight bridge be constructed parallel to the line of thermometers. The bridge should be designed so that the deck can be removed between readings without affecting the snow cover. 2.3.2

Measurement procedures

2.3.2.1

Reading ordinary thermometers

Thermometers should be read as rapidly as possible in order to avoid changes of temperature caused by the observer’s presence. Since the liquid meniscus, or index, and the thermometer scale are not on the same plane, care must be taken to avoid parallax errors. These will occur unless the observer ensures that the straight line from his/her eye to the meniscus, or index, is at a right angle to the thermometer stem. Since thermometer scales are not normally subdivided to less than one fifth of a degree, readings to the nearest tenth of a degree, which are essential in psychrometry, must be made by estimation. Corrections for scale errors, if any, should be applied to the readings. Maximum and minimum thermometers should be read and set at least twice daily. Their readings should be compared frequently with those of an ordinary thermometer in order to ensure that no serious errors develop. 2.3.2.2

Measuring grass minimum temperatures

The temperature is measured with a minimum thermometer such as that described in 2.3.1.3. The thermometer should be mounted on suitable supports so that it is inclined at an angle of about 2° from the horizontal position, with the bulb lower than the stem, 50mm above the ground. Normally, the thermometer is exposed at the last observation hour before sunset, and the reading is taken the next morning. The instrument is kept within a screen or indoors during the day. However, at stations where an observer is not available near sunset, it may be necessary to leave the thermometer exposed throughout the day. In strong sunshine, exposing the thermometer in this way can cause the spirit to distil and collect in the top of the bore. This effect can be minimized by fitting a cotton sock on a black metal shield over the safety chamber end of the thermometer; this shield absorbs more radiation and consequently reaches a higher temperature than the rest of the thermometer. Thus, any vapour will condense lower down the bore at the top of the spirit column.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.3.3

101

Thermometer siting and exposure

Both ordinary thermometers and maximum and minimum thermometers are always exposed in a thermometer screen as described in2.2.3. Extreme thermometers are mounted on suitable supports so that they are inclined at an angle of about 2° from the horizontal position, with the bulb being lower than the stem. The siting and exposure of grass minimum thermometers is as prescribed in2.1.4.2.3 and2.3.2.2. 2.3.4

Sources of error in liquid-in-glass thermometers

The main sources of error common to all liquid-in-glass thermometers are the following: (a) Elastic errors; (b) Errors caused by the emergent stem; (c) Parallax and gross reading errors; (d) Changes in the volume of the bulb produced by exterior or interior pressure; (e) Capillarity; (f) Errors in scale division and calibration; (g) Inequalities in the expansion of the liquid and glass over the range considered. The last three errors can be minimized by the manufacturer and included in the corrections to be applied to the observed values. Some consideration needs to be given to the first three errors. Error (d) does not usually arise when the thermometers are used for meteorological purposes. 2.3.4.1

Elastic errors

There are two kinds of elastic errors, namely reversible and irreversible errors. The first is of importance only when a thermometer is exposed to a large temperature range in a short period of time. Thus, if a thermometer is checked at the steam point and shortly afterwards at the ice point, it will read slightly too low at first and then the indicated temperature will rise slowly to the correct value. This error depends on the quality of the glass employed in the thermometer, and may be as much as 1K (with glass of the highest quality it should be only 0.03K) and would be proportionately less for smaller ranges of temperature. The effect is of no importance in meteorological measurements, apart from the possibility of error in the original calibration. The irreversible changes may be more significant. The thermometer bulb tends to contract slowly over a period of years and, thus, causes the zero to rise. The greatest change will take place in the first year, after which the rate of change will gradually decrease. This alteration can be reduced by subjecting the bulb to heat treatment and by using the most suitable glass. Even with glass of the highest quality, the change may be about 0.01K per year at first. For accurate work, and especially with inspector or check thermometers, the zero should be redetermined at the recommended intervals and the necessary corrections applied. 2.3.4.2

Errors caused by the emergent stem

A thermometer used to measure air temperature is usually completely surrounded by air at an approximately uniform temperature, and is calibrated by immersing the thermometer either completely or only to the top of the liquid column (namely, calibrated by complete or partial

102

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

immersion). When such a thermometer is used to determine the temperature of a medium which does not surround the stem, so that the effective temperature of the stem is different from that of the bulb, an error will result. For meteorological applications, the most likely circumstance where this might be encountered is when checking the calibration of an ordinary thermometer in a vessel containing another liquid at a temperature significantly different from ambient temperature and only the bulb or lower part of the stem is immersed. 2.3.4.3

Parallax and gross reading errors

If the thermometer is not viewed on the plane that is perpendicular to the stem of the thermometer, parallax errors will arise. The error increases with the thickness of the thermometer stem and the angle between the actual and the correct line of sight. This error can be avoided only by taking great care when making an observation. With mercury-in-glass thermometers suspended vertically, as in an ordinary screen, the thermometer must be viewed at the horizontal level of the top of the mercury column. Errors can also occur because observers usually disturb the surroundings in some way when they approach to read the thermometer. It is, therefore, necessary for observers to take the readings to the nearest tenth of a degree as soon as possible. Gross reading errors are usually 1°, 5° or 10° in magnitude. Such errors will be avoided if observers recheck the tens and units figure after taking their initial reading. 2.3.4.4

Errors due to differential expansion

The coefficient of cubical expansion of mercury is 1.82·10 –4K–1, and that of most glass lies between 1.0·10 –5 and 3.0·10 –5K–1. The expansion coefficient of the glass is, thus, an important fraction of that of mercury and cannot be neglected. As neither the coefficients of cubical expansion of mercury and glass nor the cross-sectional area of the bore of the stem are strictly constant over the range of temperature and length of the stem being used, the scale value of unit length of the stem varies along the stem, and the thermometer has to be calibrated by the manufacturer against a standard thermometer before it can be used. 2.3.4.5

Errors associated with spirit thermometers

The expansion coefficients of the liquids used in spirit thermometers are very much larger than those of mercury, and their freezing points are much lower (ethyl alcohol freezes at –115°C). Spirit is used in minimum thermometers because it is colourless and because its larger expansion coefficient enables a larger bore to be used. Spirit thermometers are less accurate than mercury thermometers of similar cost and quality. In addition to having the general disadvantages of liquid-in-glass thermometers, spirit thermometers have some peculiarities to themselves: (a) Adhesion of the spirit to the glass: Unlike mercury, organic liquids generally wet the glass. Therefore, when the temperature falls rapidly, a certain amount of the liquid may remain on the walls of the bore, causing the thermometer to read low. The liquid gradually drains down the bore if the thermometer is suspended vertically; (b) Breaking of the liquid column: Drops of the liquid often form in the upper part of the thermometer stem by a process of evaporation and condensation. These can be reunited with the main column, but errors may be caused at the beginning of the process before it is noticed. The column is also often broken during transport. This error is reduced during manufacture by sealing off the thermometer at its lowest temperature so that it contains the maximum amount of air in the stem; (c) Slow changes in the liquid: The organic liquids used tend to polymerize with age and exposure to light, with a consequent gradual diminution in liquid volume. This effect is

CHAPTER 2. MEASUREMENT OF TEMPERATURE

103

speeded up by the presence of impurities; in particular, the presence of acetone in ethyl alcohol has been shown to be very deleterious. Great care has therefore to be taken over the preparation of the liquid for the thermometers. This effect may also be increased if dyes are used to colour the liquid to make it more visible. The reduction of errors caused by breakage in the liquid column and the general care of spirit thermometers are dealt with later in this chapter. 2.3.5

Comparison and calibration in the field and laboratory

2.3.5.1

Laboratory calibration

Laboratory calibrations of thermometers should be carried out by ISO/IEC17025-accredited calibration laboratories. For liquid-in-glass thermometers, a liquid bath should be employed, within which it should be possible to maintain the temperature at any desired values within the required range. The rate of temperature change within the liquid should not exceed the recommended limits, and the calibration apparatus should be provided with a means of stirring the liquid. The reference standard thermometers and thermometers being calibrated should be suspended independently of the container and fully immersed, and should not touch the sides. Sufficient measurements should be taken to ensure that the corrections to be applied represent the performance of the thermometer under normal conditions, with errors due to interpolation at any intermediate point not exceeding the non-systematic errors (see VolumeV, Chapter4 of the present Guide). 2.3.5.2

Field checks

All liquid-in-glass thermometers experience gradual changes of zero. For this reason, it is desirable to check them at regular intervals, usually about once every two years. The thermometers should be stored in an upright position at room temperature for at least 24h before the checking process begins. The ice point may be checked by a Dewar flask filled with crushed ice made from distilled water and moistened with more distilled water. The space between the ice pieces as well as the bottom of the vessel should be free from air. The water should remain 2cm beneath the ice surface. An ordinary thermos flask will accommodate the total immersion of most thermometers up to their ice point. The thermometers should be inserted so that as little of the mercury or spirit column as possible emerges from the ice. An interval of at least 15min should elapse to allow the thermometer to take up the temperature of the melting ice before a reading of the indicated temperature is taken. Each thermometer should be moved backwards and forwards through the mixture and immediately read to a tenth part of the scale interval. Further readings at 5min intervals should be taken and a mean value computed. Other points in the range can be covered by reference to a travelling standard or inspection thermometer. Comparison should be made by immersing the reference thermometer and the thermometer, or thermometers, to be checked in a deep vessel of water. It is generally better to work indoors, especially if the sun is shining, and the best results will be obtained if the water is at, or close to, ambient temperature. Each thermometer is compared with the reference thermometer; thermometers of the same type can be compared with each other. For each comparison, the thermometers are held with their bulbs close together, moved backwards and forwards through the water for about 1min, and then read. It must be possible to read both thermometers without changing the depth of immersion; subject to this, the bulbs should be as deep in the water as possible. Most meteorological thermometers are calibrated in the laboratory for total immersion; provided that the difference between the water and air temperature is not more than 5K, the emergent stem

104

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

correction should be negligible. Often, with the bulbs at the same depth, the tops of the columns of mercury (or other liquid) in the reference thermometer and the thermometer being checked will not be very close together. Particular care should therefore be taken to avoid parallax errors. These comparisons should be made at least three times for each pair of thermometers. For each set of comparisons, the mean of the differences between readings should not exceed the specified uncertainties given in the present volume, Chapter1, Annex1.A. Soil thermometers may be checked in this manner, but should be left in the water for at least 30min to allow the wax in which the bulbs are embedded to take up the temperature of the water. The large time constant of the soil thermometer makes it difficult to conduct a satisfactory check unless the temperature of the water can be kept very steady. If the check is carefully carried out in water whose temperature does not change by more than 1K in 30min, the difference from the corrected reading of the reference thermometer should not exceed 0.25K. 2.3.6

Corrections

When initially issued, thermometers identified by a serial number should be provided with either a dated certificate confirming compliance with the uncertainty requirement, or a dated calibration certificate giving the corrections that should be applied to the readings to achieve the required uncertainty. In general, if the errors at selected points in the range of a thermometer (for example, 0°C, 10°C, 20°C) are all within 0.05K, no corrections will be necessary and the thermometers can be used directly as ordinary thermometers in naturally ventilated screens and as maximum, minimum, soil or grass minimum thermometers. If the errors at these selected points are greater than 0.05K, a table of corrections should be available to the observer at the place of reading, together with unambiguous instructions on how these corrections should be applied. Thermometers for which certificates would normally be issued are those: (a) For use in ventilated psychrometers; (b) For use as travelling standards; (c) For laboratory calibration references; (d) For special purposes for which the application of corrections is justified. For psychrometric use, two identical thermometers should be selected. 2.3.7

Maintenance

2.3.7.1

Breakage in the liquid column

The most common fault encountered is the breaking of the liquid column, especially during transportation. This is most likely to occur in spirit (minimum) thermometers. Other problems associated with these thermometers are adhesion of the spirit to the glass and the formation by distillation of drops of spirit in the support part of the bore. A broken liquid column can usually be reunited by holding the thermometer bulb-end downward and tapping the thermometer lightly and rapidly against the fingers or something else which is elastic and not too hard. The tapping should be continued for some time (5min if necessary), and afterwards the thermometer should be hung, or stood, upright in a suitable container, bulb downward, for at least 1h to allow any spirit adhering to the glass to drain down to the main column. If such treatment is not successful, a more drastic method is to cool the bulb in a freezing mixture of ice and salt, while keeping the upper part of the stem warm; the liquid will slowly distil back to the main column. Alternatively, the thermometer may be held upright

CHAPTER 2. MEASUREMENT OF TEMPERATURE

105

with its bulb in a vessel of warm water, while the stem is tapped or shaken from the water as soon as the top of the spirit column reaches the safety chamber at the top of the stem. Great care must be taken when using this method as there is a risk of bursting the thermometer if the spirit expands into the safety chamber. 2.3.7.2

Scale illegibility

Another shortcoming of unsheathed liquid-in-glass thermometers is that with time their scale can become illegible. This can be corrected at the station by rubbing the scale with a dark crayon or black lead pencil. 2.3.8

Safety

Mercury, which has been the most commonly used liquid in liquid-in-glass thermometers, is poisonous if swallowed or if its vapour is inhaled. If a thermometer is broken and the droplets of mercury are not removed there is some danger to health, especially in confined spaces. More information on safety precautions for the use of mercury is given in the present volume, Chapter3, Annex3.A. There are also restrictions on the carriage of mercury-in-glass thermometers on aircraft, or special precautions that must be taken to prevent the escape of mercury in the event of a breakage. The advice of the appropriate authority or carrier should be sought.

2.4

MECHANICAL THERMOGRAPHS

2.4.1

General description

The types of mechanical thermographs still commonly used are supplied with bimetallic or Bourdon-tube sensors since these are relatively inexpensive, reliable and portable. However, they are not readily adapted for remote or electronic recording. Such thermographs incorporate a rotating chart mechanism common to the family of classic recording instruments. In general, thermographs should be capable of operating over a range of about 60K or even 80K if they are to be used in continental climates. A scale value is needed such that the temperature can be read to 0.2K without difficulty on a reasonably sized chart. To achieve this, provisions should be made for altering the zero setting of the instrument according to the season. The maximum error of a thermograph should not exceed 1K. 2.4.1.1

Bimetallic thermograph

In bimetallic thermographs, the movement of the recording pen is controlled by the change in curvature of a bimetallic strip or helix, one end of which is rigidly fixed to an arm attached to the frame. A means of finely adjusting this arm should be provided so that the zero of the instrument can be altered when necessary. In addition, the instrument should be provided with a means of altering the scale value by adjusting the length of the lever that transfers the movement of the bimetal to the pen; this adjustment is best left to authorized personnel. The bimetallic element should be adequately protected from corrosion; this is best done by heavy copper, nickel or chromium plating, although a coat of lacquer may be adequate in some climates. A typical time constant of about 25s is obtained at an air speed of5ms–1. 2.4.1.2

Bourdon-tube thermograph

The general arrangement is similar to that of the bimetallic type but its temperature-sensitive element is in the form of a curved metal tube of flat, elliptical section, filled with alcohol. The

106

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Bourdon tube is less sensitive than the bimetallic element and usually requires a multiplying level mechanism to give sufficient scale value. A typical time constant is about 6s at an air speedof5ms–1. 2.4.2

Measurement procedures

In order to improve the resolution of the reading, thermographs will often be set, in different seasons, to one of two different ranges with corresponding charts. The exact date for changing from one set of charts to the other will vary according to the locality. However, when the change is made the instrument will need to be adjusted. This should be done either in the screen on a cloudy, windy day at a time when the temperature is practically constant or in a room where the temperature is constant. The adjustment is made by loosening the screw holding the pen arm to the pen spindle, moving the pen arm to the correct position and retightening, the screws. The instrument should then be left as is before rechecking, and any further adjustments made as necessary. 2.4.3

Exposure and siting

These instruments should be exposed in a large thermometer screen (for example, Stevenson screen with an indoor measurement chamber of 450 x 700 x 400 mm). 2.4.4

Sources of error

In the thermograph mechanism itself, friction is the main source of error. One cause of this is bad alignment of the helix with respect to the spindle. Unless accurately placed, the helix acts as a powerful spring and, if rigidly anchored, pushes the main spindle against one side of the bearings. With modern instruments this should not be a serious problem. Friction between the pen and the chart can be kept to a minimum by suitably adjusting the gate suspension. 2.4.5

Comparison and calibration

2.4.5.1

Laboratory calibration

There are two basic methods for the laboratory calibration of bimetallic thermographs. They may be checked by fixing them in a position with the bimetallic element in a bath of water. Alternatively, the thermograph may be placed in a commercial calibration chamber equipped with an air temperature control mechanism, a fan and a reference thermometer. Comparisons should be made at two temperatures; from these, any necessary changes in the zero and magnification can be found. Scale adjustments should be performed by authorized personnel, and only after reference to the appropriate manufacturer’s instrument handbook. 2.4.5.2

Field comparison

The time constant of the instrument may be as low as one half that of the ordinary mercury thermometer, so that routine comparisons of the readings of the dry bulb and the thermograph at fixed hours will, in general, not produce exact agreement even if the instrument is working perfectly. A better procedure is to check the reading of the instrument on a suitable day at a time when the temperature is almost constant (usually a cloudy, windy day) or, alternatively, to compare the minimum readings of the thermograph trace with the reading of the minimum thermometer exposed in the same screen. Any necessary adjustment can then be made by means of the setting screw.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

2.4.6

107

Corrections

Thermographs would not normally be issued with correction certificates. If station checks show an instrument to have excessive errors, and if these cannot be adjusted locally, the instrument should be returned to an appropriate calibration laboratory for repair and recalibration. 2.4.7

Maintenance

Routine maintenance will involve an inspection of the general external condition, the play in the bearings, the inclination of the recording arm, the set of the pen, and the angle between the magnification arm and recording arm, and a check of the chart-drum clock timing. Such examinations should be performed in accordance with the recommendations of the manufacturer. In general, the helix should be handled carefully to avoid mechanical damage and should be kept clean. The bearings of the spindle should also be kept clean and oiled at intervals using a small amount of clock oil. The instrument is mechanically very simple and, provided that precautions are taken to keep the friction to a minimum and prevent corrosion, it should give good service.

2.5

RADIATION SHIELDS

A radiation shield or screen should be designed to provide an enclosure with an internal temperature that is both uniform and the same as that of the outside air. It should completely surround the thermometers and exclude radiant heat, precipitation and other phenomena that might influence the measurement. Screens with forced ventilation, in which air is drawn over the thermometer element by a fan, may help to reduce biases when the microclimate inside the screen deviates from the surrounding air mass. Such a deviation is most significant when the natural wind speed is very low (0°C) are constants which are found using the least-squares method on the data acquired during the calibration. Defining fixed points on the ITS-90 in the meteorological range Equilibrium state

Assigned value of ITS K

˚C

Equilibrium between the solid, liquid and vapour phases of argon (triple point of argon)

83.8058

–189.3442

Equilibrium between the solid, liquid and vapour phases of mercury (triple point of mercury)

234.3156

–38.8344

Equilibrium between the solid, liquid and vapour phases of water (triple point of water)

273.1600

0.01

Equilibrium between the solid and liquid phases of gallium (melting point of gallium)

302.9146

29.7646

Equilibrium between the solid and liquid phases of indium (freezing point of indium)

429.7485

156.5985

REFERENCES AND FURTHER READING

Andersson, T. and I. Mattison, 1991: A Field Test of Thermometer Screens. SMHI Report No.RMK62, Norrköping. ASTM, 1993: ASTM Manual on The Use of Thermocouples in Temperature Measurement. Philadelphia, ASTM. Bentley, R.E. (Ed.), 1998: Temperature and Humidity Measurement. Singapore, Springer-Verlag. Bugbee, B., O. Monje and B. Tanner, 1996: Quantifying energy and mass transfer in crop canopies. Advances in Space Research, 18:149–156. Bureau International des Poids et Mesures, 1989:Procès-Verbaux du Comité International des Poids et Mesures, 78th meeting, 1989, Paris (available from http://​www​.bipm​.org/​utils/​common/​pdf/​its​-90/​ITS​ -90​.pdf). Bureau International des Poids et Mesures/Comité Consultatif de Thermométrie, 1990: The International Temperature Scale of 1990 (ITS-90) (H. Preston-Thomas). Metrologia, 27:3–10 (amended version) (available from http://​www​.bipm​.org/​utils/​common/​pdf/​its​-90/​ITS ​-90 ​_metrologia​ .pdf). Çengal, Y.A. and A.J. Ghajar, 2014: Heat and Mass Transfer: Fundamentals and Application. Fifth edition. New York, McGraw-Hill Education. Diamond, H. J., T.R. Karl, M.A. Palecki, C.B. Baker, J.E. Bell, R.D. Leeper, D.R. Easterling, J.H. Lawrimore, T.P. Meyers, M.R. Helfert, G. Goodge and P.W. Thorne, 2013: U.S. Climate Reference Network after one decade of operations: status and assessment. Bulletin of the American Meteorological Society, 94:489–498, doi: 10.1175/BAMS-D-12-00170.1. Erell, E., V. Leal and E. Maldonaldo, 2005: Measurement of air temperature in the presence of a large radiant flux: an assessment of passively ventilated thermometer screens. Boundary Layer Meteorology, 114:205–231. Harrison, R.G., 2010: Natural ventilation effects on temperatures within Stevenson screens. Quarterly Journal of the Royal Meteorological Society, 136(646)A:253–259, DOI: 10.1002/qj.537. ———, 2015: Meteorological Measurements and Instrumentation. Chichester, Wiley. Harrison R.G. and M.A. Pedder, 2001: Fine wire thermometer for air measurement. Review of Scientific Instruments, 72(2):1539–1541. Harrison, R.G. and G.W Rogers, 2006: Fine wire thermometer amplifier for atmospheric measurements. Review of Scientific Instruments, 77(11):116112. Harvey, A.H., M.O. McLinden and W.L. Tew, 2013: Thermodynamic analysis and experimental study of the effect of atmospheric pressure on the ice point. AIP Conference Proceedings. 1552:221, doi: 10.1063/1.4819543. Her Majesty’s Stationery Office/Meteorological Office, 1980: Handbook of Meteorological Instruments, Volume2: Measurement of temperature. London. Incropera, F.P. and D.P. de Witt, 2011: Fundamentals of Heat and Mass Transfer. Chichester, Wiley. International Electrotechnical Commission, 2008: Industrial Platinum Resistance Thermometers and Platinum Temperature Sensors, IEC60751:2008. Geneva. International Organization for Standardization, 2007: Meteorology – Air Temperature Measurements – Test Methods for Comparing the Performance of Thermometer Shields/Screens and Defining Important Characteristics, ISO/DIS17714:2007. Geneva. Jones, E.B., 1992: Jones’ Instrument Technology. Volume 2: Measurement of temperature and chemical composition, Butterworths-Heinemann, Oxford. Middleton, W.E.K. and A.F. Spilhaus, 1960: Meteorological Instruments. University of Toronto Press. Ney, E.P., R.W. Maas and W.F. Huch, 1960: The measurement of atmospheric temperature. Journal of Meteorology, 18:60–80. Nicholas, J.V. and D.R. White, 1993: Traceable Temperatures. Second edition. Chichester, Wiley. Quinn, T.J., 1990: Temperature. Second edition. San Diego, Academic Press. Richiardone, R., M. Manfrin, S. Ferrarese, C. Francone, V. Fernicola, R. M. Gavioso and L. Mortarini, 2012: Influence of the Sonic Anemometer Calibration on Turbulent Heat-Flux Measurements. Boundary-Layer Meteorology, 142(3):425–442. Schotanus, P., F.T.M. Nieuwstadt and H.A.R e Bruin, 1983: Temperature measurement with a sonic anemometer and its application to heat and moisture fluxes. Boundary-Layer Meteorol. 26:81–93. Sparks, W.R., 1970: Current concepts of temperature measurement applicable to synoptic networks. Meteorological Monographs, 11(33):247–251. ———, 2001: Field trial of Metspec screens. Technical Report TR19, Met Office/OD, Wokingham, United Kingdom of Great Britain and Northern Ireland.

CHAPTER 2. MEASUREMENT OF TEMPERATURE

113

Underwood, R., M. de Podesta, G. Sutton, L. Stanger, R. Rusby, P Harris, P. Morantz and G. Machin, 2017: Further estimates of (T – T90) close to the triple point of water. International Journal of Thermophysics, 38:44. Underwood, R., T. Gardiner, A. Finlayson, S. Bell and M. de Podesta, 2017: An improved non-contact thermometer and hygrometer with rapid response. Metrologia, 54(1):S9. Underwood, R., T. Gardiner, A. Finlayson, J. Few, J. Wilkinson, S. Bell, J. Merrison, J.J. Iverson and M. de Podesta, 2015: A combined non-contact acoustic thermometer and infrared hygrometer for atmospheric measurements. Meteorological Applications, 22:830–835. White, D.R., 2015: Temperature errors in linearizing resistance networks for thermistors. International Journal of Thermophysics, 36:3404–3420, DOI 10.1007/s10765-015-1968-2. ———, 2017: Interpolation errors in thermistor calibration equations. International Journal of Thermophysics, 38(4):59, DOI 10.1007/s10765-017-2194-x. World Meteorological Organization, 1972: The Effect of Thermometer Screen Design on the Observed Temperature (W.R. Sparks). (WMO-No.315). Geneva. ———, 1992: Measurement of Temperature and Humidity: Specification, Construction, Properties and Use of the WMO Reference Psychrometer (R.G. Wylie and T.Lalas). Technical Note No.194 (WMO-No.759). Geneva. ———, 1998a: Recent Changes in Thermometer Screen Design and their Impact (A.Barnett, D.B.Hatton and D.W. Jones). Instruments and Observing Methods Report No.66 (WMO/TD-No.871). Geneva. ———, 1998b: An investigation of temperature screens and their impact on temperature measurements (J.Warne). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-98). Instruments and Observing Methods Report No.70 (WMO/TD-No.877). Geneva. ———, 1998c: A thermometer screen intercomparison (J.P. van der Meulen). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-98). Instruments and Observing Methods Report No.70 (WMO/TDNo.877). Geneva. ———, 1998d: Comparison of meteorological screens for temperature measurement (G.Lefebvre). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-98). Instruments and Observing Methods Report No.70 (WMO/TD-No.877). Geneva. ———, 2000a: A comparison of air temperature radiation screens by field experiments and computational fluid dynamics (CFD) simulations (A. Spetalen, C. Lofseik and P. Ø.Nordli). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2000). Instruments and Observing Methods Report No.74 (WMO/TD-No.1028). Geneva. ———, 2000b: Temperature measurements: Some considerations for the intercomparison of radiation screens (J.P. van der Meulen). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2000). Instruments and Observing Methods Report No.74 (WMO/TD-No.1028). Geneva. ———, 2002a: Measurement of temperature with wind sensors during severe winter conditions (M. Musa, S.Suter, R. Hyvönen, M. Leroy, J. Rast and B. Tammelin). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2002). Instruments and Observing Methods Report No.75 (WMO/TDNo.1123). Geneva. ———, 2002b: Norwegian national thermometer screen intercomparison (M.H.Larre and K.Hegg). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2002). Instruments and Observing Methods Report No.75 (WMO/TD-No.1123). Geneva. ———, 2002c: Results of an intercomparison of wooden and plastic thermometer screens (D.B.Hatton). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2002). Instruments and Observing Methods Report No.75 (WMO/TD-No.1123). Geneva. ———, 2002d: Temperature and humidity measurements during icing conditions (M.Leroy, B. Tammelin, R. Hyvönen, J. Rast and M. Musa). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2002). Instruments and Observing Methods Report No.75 (WMO/TD-No.1123). Geneva. ———, 2010: Guidance on Instrumentation for Calibration Laboratories, Including Regional Instrument Centres (D. Groselj) (WMO/TD-No. 1543). Instruments and Observing Methods Report No. 101. Geneva.

114

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

———, 2011: WMO Field Intercomparison of Thermometer Screens/Shields and Humidity Measuring Instruments (M. Lacombe, D. Bousri, M. Leroy and M. Mezred). Instruments and Observing Methods Report No.106 (WMO/TD-No.1579). Geneva. Zanghi, F., 1987: Comparaison des Abris Météorologiques. Technical Memorandum No.11, Météo-France/ SETIM, Trappes.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

3.1

GENERAL

3.1.1

Definition

The atmospheric pressure on a given surface is the force per unit area exerted by virtue of the weight of the atmosphere above. The pressure is thus equal to the weight of a vertical column of air above a horizontal projection of the surface, extending to the outer limit of the atmosphere. Apart from the actual pressure, pressure trend or tendency has to be determined as well. Pressure tendency is the character and amount of atmospheric pressure change for a three-hour or other specified period ending at the time of observation. Pressure tendency is composed of two parts, namely the pressure change and the pressure characteristic. The pressure change is the net difference between pressure readings at the beginning and end of a specified interval of time. The pressure characteristic is an indication of how the pressure has changed during that period of time, for example, decreasing then increasing, or increasing and then increasing more rapidly. 3.1.2

Units and scales

The basic unit for atmospheric pressure measurements is the pascal(Pa) (or newton per square metre,Nm-2). It is accepted practice to add the prefix “hecto” to this unit when reporting pressure for meteorological purposes, making the hectopascal(hPa), equal to 100Pa, the preferred terminology. This is largely because one hectopascal equals one millibar(mbar), the formerly used unit. Further details on the mandatory use of SI units are explained in the present volume, Chapter1. Note that units used for barometer readings such as "mmHg", "inHg" or "mbar" are not defined within SI and may not be used for the international exchange of data when reporting atmospheric pressure (see also Annex3.A of this chapter). In this chapter only the unit hPa is used. 3.1.3

Meteorological requirements

Analysed pressure fields are a fundamental requirement of the science of meteorology. It is imperative that these pressure fields be accurately defined as they form the basis for all subsequent predictions of the state of the atmosphere. Pressure measurements must be as accurate as technology allows, within realistic financial constraints, and there must be uniformity in the measurement and calibration procedures across national boundaries. The level of accuracy needed for pressure measurements to satisfy the requirements of various meteorological applications has been identified by the respective WMO commissions and is outlined in the present volume, Chapter1, Annex1.A, which is the primary reference for measurement specifications in the present Guide. These requirements should be considered achievable for new barometers in a strictly controlled environment, such as those available in a properly equipped laboratory. They provide an appropriate target uncertainty for barometers to meet before their installation in an operational environment. For barometers installed in an operational environment, practical constraints will require welldesigned equipment for an NMHS to maintain this target uncertainty. Not only the barometer itself, but the exposure also requires special attention. Nevertheless, the performance of the operational network station barometer should not be below the stated criteria.

116

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

3.1.4

Methods of measurement and observation

3.1.4.1

General measurement principles

For meteorological purposes, atmospheric pressure is generally measured with electronic barometers, aneroid barometers or hypsometers. The latter class of instruments, which depend on the relationship between the boiling point (temperature) of a liquid and the atmospheric pressure, has so far seen only limited application and will not be discussed in depth in this publication. Mercury barometers are still in use, but no longer recommended, taking into account the Minamata Convention on Mercury (see the present volume, Chapter1, 1.4.2). NMHSs are encouraged to urgently take appropriate measures to replace mercury barometers with modern alternatives (see3.1.4.5). Information on observation practices with mercury barometers is maintained in Annex3.A only to inform the reader on this obsolete practice. Most barometers with recent designs make use of transducers which transform the sensor response into pressure-related quantities. These are subsequently processed by using appropriate electrical integration circuits or data-acquisition systems with appropriate smoothing algorithms. A time constant of about 10s (and definitely no greater than 20s) is desirable for most synoptic barometer applications. There are several general methods for measuring atmospheric pressure and these are outlined in the following paragraphs. A membrane of elastic substance, held at the edges, is deformed if the pressure on one side is greater than on the other. In practice, this is achieved by using a completely or partially evacuated closed metal capsule containing a strong metal spring to prevent the capsule from collapsing due to external atmospheric pressure. Mechanical or electrical means are used to measure the deformation caused by the pressure differential between the inside and outside of the capsule. This is the principle of the well-known aneroid barometer. Pressure sensor elements comprising thin-walled nickel alloy cylinders, surrounded by a vacuum, have been developed. The natural resonant frequency of these cylinders varies as a function of the difference in pressure between the inside of the cylinder, which is at ambient atmospheric pressure, and the outside of the cylinder, which is maintained as a vacuum. In fact, these instruments measure the pressure by sensing the density of the gas (air) inside. Absolute pressure transducers, which use a crystalline quartz element, are also commonly used. Pressure exerted via flexible bellows on the crystal face causes a compressive force on the crystal. On account of the crystal’s piezoresistive properties, the application of pressure alters the balance of an active Wheatstone bridge. Balancing the bridge enables accurate determination of the pressure. These types of pressure transducers are virtually free of hysteresis effects. 3.1.4.2

General exposure requirements

It is important that the location of barometers at observation stations be selected with great care. The main requirements of the place of exposure are good light to read out (in case of manual readings), a draught-free environment, a solid, non-vibrating mounting, and protection against rough handling. Special effort in positioning is required to prevent any artificial wind impact. Such impact is typical for indoor measurement due to the build-up of pressure outside the building and generating errors which are sometime larger than 1hPa. For further details, see3.1.4.3.2.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

3.1.4.3

117

Sources of error: general comments

Errors in the measurement of pressure may be caused by an inappropriate placement of the instrument. The instrument must be placed in an environment where external effects will not lead to measurement errors. These effects include wind, radiation and temperature, shocks and vibrations, fluctuations in the electrical power supply, and pressure shocks. It is important that every meteorological observer or technical staff should fully understand these effects and be able to assess whether any of them are affecting the accuracy of the readings of the barometer in use. In case of manual readings, the instrument (or its display) should be easy to read. Instruments must be designed so that the resolution of their readings is better than the required measurement uncertainty, that is, rounding error does not increase significantly the uncertainty of the measurement results. 3.1.4.3.1

The effects of temperature

Instrument readings should not be affected by temperature variations. Instruments are suitable only if at least one of the following conditions is met: (a) The instrument is designed to be temperature independent or compensated for the whole temperature range, to be proven by adequate calibration and tests; (b) Procedures for correcting the readings for temperature effects are developed and implemented to ensure the required uncertainty; (c) The pressure sensing element is placed in an environment where the temperature is stabilized so that the required uncertainty is met. Most instruments measure the temperature of the pressure sensor to compensate for temperature effects. It is necessary to control and calibrate these temperature-compensating functions as part of the standard calibration activity. 3.1.4.3.2

The effects of wind

It should be noted that the effects of wind apply to all types of barometers. More information on wind effects is found in Liu and Darkow(1989). A barometer will not give a true reading of the static pressure if it is influenced by gusty wind. Its reading will fluctuate with the wind speed and direction and with the magnitude and sign of the fluctuations, depending also on the nature of the room’s openings and their position in relation to the direction of the wind. At sea, error is always present due to the ship’s motion. A similar problem will arise if the barometer is installed in an air-conditioned room. Wind can often cause dynamic changes of pressure in the room where the barometer is placed. These fluctuations are superimposed on the static pressure and, with strong and gusty wind, may amount to up to 2 or 3hPa. It is usually impractical to correct for such fluctuations because the “pumping” effect is dependent on both the direction and the force of the wind, as well as on the local circumstances of the barometer’s location. Thus, the “mean value” does not represent the true static pressure. When comparing barometers in different buildings, the possibility of a difference in readings due to the wind effect should be borne in mind. It is possible to overcome this effect to a very large extent by using a static head between the exterior atmosphere and the inlet port of the barometer. Details concerning the operating principles of static heads can be found in several publications (Miksad, 1976; United States Weather Bureau, 1963). Aneroid and electronic barometers usually have simple connections to allow for the use of a static head, which should be located in an open environment not

118

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

affected by the proximity of buildings. The design of such a head requires careful attention. Static pressure heads are commercially available, but there is limited published literature on intercomparisons to demonstrate their performance (WMO, 2012). 3.1.4.3.3

The effects of air conditioning

Air conditioning may create a significant pressure differential between the inside and outside of a room. Therefore, if a barometer is to be installed in an air-conditioned room, it is advisable to use a static head with the barometer which will couple it to the air outside the building. 3.1.4.3.4

The effects of hysteresis

Some barometers (in particular aneroid barometers) are affected by hysteresis, with an impact larger than 0.1 hPa. To demonstrate that any hysteresis is within the required measurement uncertainty, calibrations must be performed in both ascending and descending pressure steps. 3.1.4.3.5

Transport and use in a non-stabilized environment

Barometers may be sensitive to vibrations and shocks affecting the adjustment of the equipment. Special care must be taken to avoid any shock impact during transport and the instruments should be placed in a vibration-free environment. 3.1.4.4

Maintenance: general comments

The following maintenance procedures should be considered: (a) The instruments and especially the pressure inlet should be kept clean and free from obstruction; (b) The installation height of the sensing instrument and the mounting should be checked regularly; (c) The instruments must be calibrated (and adjusted if appropriate) regularly; the interval between two calibrations must be short enough to ensure that the total absolute measurement error will meet the uncertainty requirements; (d) Any variations in the uncertainty (long term and short term) must be much smaller than those outlined in the present volume, Chapter1, Annex1.A. If some instruments have a history of drift in calibration, they will be suitable operationally only if the period between calibrations is short enough to ensure the required measurement uncertainty at all times; (e) If the instrument has to be calibrated away from its operational location, the method of transportation employed must not affect the stability or accuracy of the instrument; effects that may alter the calibration of the instrument include mechanical shocks and vibrations, displacement from the vertical position, and large pressure variations that may be encountered during transportation by air. 3.1.4.5

Implications of the Minamata Convention for pressure measurement

The UNEP Minamata Convention on Mercury came into force globally in August 2017 and bans all production, import and export of mercury barometers (see the present volume, Chapter1, 1.4.2). Therefore, mercury barometers are no longer recommended and it is strongly encouraged to take appropriate measures to replace such barometers with modern alternatives. Electronic

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

119

barometers provide an economical, accurate and reliable alternative to these dangerous, mercury-based instruments and offer significant advantages in terms of data storage and realtime data display.

3.2

ELECTRONIC BAROMETERS

Most barometers with recent designs make use of transducers that transform the sensor response into a pressure-related electrical quantity in the form of either analogue signals (for example, voltage (DC or AC with a frequency related to the actual pressure)), or digital signals (for example, pulse frequency or with standard data communication protocols such as RS232, RS422, RS485 or IEEE488). Analogue signals can be displayed on a variety of electronic meters. Monitors and data-acquisition systems, such as those used in AWSs, are frequently used to display digital outputs or digitized analogue outputs. Current digital barometer technology employs various levels of redundancy to improve the longterm stability and accuracy of the measurements. One technique is to use three independently operating sensors under centralized microprocessor control. Even higher stability and reliability can be achieved by using three completely independent barometers, incorporating three sets of pressure transducers and microprocessors. Each configuration has automatic temperature compensation from internally mounted temperature sensors. Triple redundancy ensures excellent long-term stability and measurement accuracy, even in the most demanding applications. These approaches allow for continuous monitoring and verification of the individual sensor performances. 3.2.1

Integrated-circuit-based variable capacitive sensors

Capacitive pressure sensors use the electrical property of capacitance to measure the displacement of a diaphragm. The diaphragm is an elastic pressure sensor displaced in proportion to changes in pressure. It acts as one plate of a capacitor that detects strain due to applied pressure to become a variable capacitor. The change in value of the capacitance causes this electrical signal to vary. This is then conditioned and displayed on a device calibrated in terms of pressure. Common technologies use metal, ceramic and silicon diaphragms. Because this measurement is temperature dependent, sensor temperature is also measured for compensation to meet the accuracy requirements. Silicon-diaphragm sensors are popular in integrated circuit technology today (with a size of about 1μm). For this technique the absolute pressure is measured using a vacuum-based chamber (pressure smaller than 10 -3 hPa). 3.2.2

Digital piezoresistive barometers

Measurements of atmospheric pressure have become possible by utilizing the piezoelectric (piezoresistive) effect. A common configuration features four measuring resistors placed onto the flexible surface of a monolithic silicon substratum interconnected to form a Wheatstone bridge circuit. Axially loaded crystalline quartz elements are used in digital piezoresistive barometers and are a type of absolute pressure transducer. Crystalline quartz has been chosen because of its piezoelectric properties, stable frequency characteristics, small temperature effects and precisely reproducible frequency characteristics. Pressure applied to an inlet port causes an upward axial force by means of flexible bellows, thus resulting in a compressive force on the quartz crystal element. Since the crystal element is a substantially rigid membrane, the entire mechanical structure is constrained to minute deflections, thereby virtually eliminating mechanical hysteresis.

120

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The fully active Wheatstone bridge mentioned above may consist either of semiconductor strain gauges or piezoresistive gauges. The strain gauges are either bonded to a thin circular diaphragm, which is clamped along its circumference, or atomically diffused into a silicon diaphragm configuration. In the case of diffused devices, the silicon integrated chip itself is the diaphragm. Applied pressure presents a distributed load to the diaphragm which, in turn, provides bending stress and resultant strains to which the strain gauges react. This stress creates a strain that is proportional to the applied pressure and which results in a bridge imbalance. The bridge output is then proportional to the net difference in pressure acting upon the diaphragm. This mode of operation is based on the fact that the atmospheric pressure acts on the sensor element covering a small evacuated cell, through which the resistors are submitted to compressive and tensile stresses. By the piezoelectric effect, the values of resistance change proportionally with atmospheric pressure. To eliminate temperature errors, the instrument often incorporates a built-in thermostat. The output from the Wheatstone bridge, which is fed from a direct-current source, is transduced into a standard signal by an appropriate amplifier. A light-emitting diode (LED) or liquid crystal display usually presents the measured pressure values. In a modern version of the pressure transducer using a piezoelectric transducer, two resonance frequencies of the piezoelectric element are determined. By calculating a linear function of these frequencies and with an appropriate set of variables obtained after calibration, a pressure is calculated by a microprocessor which is independent of the temperature of the sensor. 3.2.3

Cylindrical resonator barometers

Cylindrical resonator barometers use a sensing element which is a thin-walled cylinder of nickel alloy. This is electromagnetically maintained in a “hoop” mode of vibration. The input pressure is sensed by the variation it produces in the natural resonant frequency of the vibrating mechanical system. Cylinder wall movement is sensed by a pick-up coil whose signal is amplified and fed back to a drive coil. The air pressure to be measured is admitted to the inside of the cylinder, with a vacuum reference maintained on the outside. The natural resonant frequency of vibration then varies precisely with the stress set up in the wall due to the pressure difference across it. An increase in pressure gives rise to an increase in frequency. The thin cylinder has sufficient rigidity and mass to cater for the pressure ranges over which it is designed to operate, and is mounted on a solid base. The cylinder is placed in a vacuum chamber and its inlet is connected to the free atmosphere for meteorological applications. Since there is a unique relationship between the natural resonant frequency of the cylinder and the pressure, the atmospheric pressure can be calculated from the measured resonant frequency. However, this relationship, determined during calibration, depends on the temperature and the density of the gas. Temperature compensation is therefore required and the air should be dried before it enters the inlet. 3.2.4

Aneroid displacement transducers

Contact-free measurement of the displacement of the aneroid capsule is a virtual necessity for precision pressure-measuring instruments for meteorological applications. A wide variety of such transducers are in use, including capacitive displacement detectors, potentiometric displacement detectors, strain gauges placed at strategic points on the sensor, and force-balanced servosystems which keep the sensor dimensions constant regardless of pressure. All sensitive components must be encased in a die-cast housing. Unless designed with an adequate temperature compensation, this housing must be kept at a constant temperature by an electronically controlled heater. Condensation of water vapour must be completely prevented. An effective technique is to put a hygroscopic agent, such as silica gel crystals, into the die-cast

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

121

housing and to prevent water vapour diffusion into the housing by connecting a long plastic tube (approximately 25m) with a bore of 2mm or less, between the pressure port and a static head (see3.1.4.3.2). The pressure-sensor housing must be airtight, allowing external connection to the compartment where the pressure is to be measured. 3.2.5

Exposure of electronic barometers

Details on general exposure requirements are provide in 3.1.4.2. Electronic barometers should be mounted away from electromagnetic sources; where this is not possible, the wires and casing should be shielded. 3.2.6

Reading electronic barometers

An electronic barometer measures the atmospheric pressure of the surrounding space or any space that is connected to it via a tube. In general, the barometer should be set to read the pressure at the level of the instrument. On board a ship or at low-level land stations, however, the instrument may be set to indicate the pressure at MSL, provided that the difference between the station pressure and the sea-level pressure can be regarded as constant. Electronic barometers give accurate readings on a digital read-out, normally scaled in hPa, but readily adaptable to other units if required. Provision can usually be made for digital recording. Trend in pressure changes can be presented if the unit is microprocessor-controlled. Circuits may be attached to primary transducers which correct the primary output for sensor non-linearities and temperature effects and which convert output to standard units. Standard modern barometer versions comprise the barometer sensor, the microcomputer unit (including the display) and an interface circuit to communicate with any data logger or AWS. Electronic barometers which have more than one transducer or sensing element generally calculate a weighted mean of the outputs from each of the sensors and establish the resultant pressure with a resolution of at least 0.1hPa. During calibration, each of the sensing elements can be checked with a resolution of at least 0.01hPa. 3.2.7

Sources of error

The accuracy of electronic barometers depends on the uncertainty of the barometer’s calibration, the effectiveness of the barometer’s temperature compensation (residual air method, temperature measurement and correction, use of a thermostat) and the drift with time. 3.2.7.1

Drift between calibrations

Drift between calibrations is one of the key sources of error with barometers. It is often greater when the barometer is new and decreases with the passage of time. Step jumps in calibration may occur. In order to maintain the acceptable performance of a barometer, the calibration corrections applied to the readings must be checked at relatively frequent intervals, for example, starting annually, for early detection and replacement of defective instruments. The need to check frequently the calibration of electronic barometers imposes an additional burden on NMHSs, particularly on those with extensive barometer networks. The ongoing cost of calibration must be taken into consideration when planning to replace mercury barometers with electronic barometers.

122 3.2.7.2

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Temperature

Most electronic barometers are adequately compensated for temperature, which can be proven during calibration or testing. In the case that an electronic barometer is not sufficiently compensated for temperature, it must be kept at a constant temperature if the calibration is to be maintained. The temperature should be near the calibration temperature. Electronic barometers that are not temperature-controlled are usually prone to greater error. Most depend on accurate temperature measurement of the sensing element and electronic correction of the pressure. This assumes that there are no thermal gradients within the sensing element of the barometer. In situations where the temperature changes reasonably quickly, this can result in short-term hysteresis errors in the measured pressure. The change in calibration may also be dependent on the thermal history of the barometer. Prolonged exposure to temperature changes may result in medium- to long-term calibration shifts. The electronics of the barometer can also introduce errors if it is not held at the same temperature as the sensing element. Electronic barometers are very often used in extreme climatic conditions, especially in AWSs. In these situations, the barometer can be exposed to temperatures well in excess of its manufacturer’s design and calibration specifications. 3.2.7.3

Electrical interference

As with all sensitive electronic measurement devices, electronic barometers should be shielded and kept away from sources of strong magnetic fields, such as transformers, computers, radar, and so forth. Although this is not often a problem, it can cause an increase in noise, with a resultant decrease in the precision of the device. 3.2.7.4

Nature of operation

Apparent changes in the calibration of an electronic barometer can be caused by differences in the way in which the barometer is operated during calibration, as compared with its operational use. A pressure read on a barometer that is run continuously and, therefore, warmed up will read differently from that read in a pulsed fashion every few seconds.

3.3

ANEROID BAROMETERS

3.3.1

Construction requirements

The principal components are a closed metal chamber, completely or partly evacuated, and a strong spring system that prevents the chamber from collapsing under the external atmospheric pressure. At any given pressure, there will be an equilibrium between the force caused by the spring and that of the external pressure. The aneroid chamber may be made of materials (steel or beryllium copper) that have elastic properties such that the chamber itself can act as a spring. A means is required to detect and display the changes in deflection which occur. This may be a system of levers that amplify the deflections and drive a pointer over a scale graduated to indicate the pressure. Alternatively, a ray of light may be deviated over the scale. Instead of these mechanical analogue techniques, certain barometers are provided with a manually operated micrometer whose counter indicates the pressure directly in tenths of a hectopascal. A reading is taken when a luminous indicator signals that the micrometer has just made contact with the aneroid. This type of aneroid is portable and robust.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

3.3.2

123

Achievable measurement uncertainty

The achievable measurement uncertainty of 0.3hPa is possible for a well-designed and constructed aneroid barometer. To achieve this uncertainty, apart from a regular, frequent calibration to reduce calibration drift (as already mentioned for electronic barometers in 3.2.7.1) the following rules should be considered: (a) It should be compensated for temperature so that the reading does not change by more than 0.3hPa for a change in temperature of 30K; (b) The scale errors at any point should not exceed 0.3hPa and should remain within this tolerance over periods of at least one year, when in normal use; (c) The hysteresis should be sufficiently small to ensure that the difference in reading before a change in pressure of 50hPa and after a return to the original value does not exceed 0.3hPa; (d) It should be capable of withstanding ordinary transit risks without introducing inaccuracies beyond the limits specified above. 3.3.3

Exposure of aneroid barometers

Details on general exposure requirements are provide in 3.1.4.2. The place selected for mounting the device should preferably have a fairly uniform temperature throughout the day. Therefore, a location is required where the barometer is shielded from the direct rays of the sun and from other sources of either heat or cold, which can cause abrupt and marked changes in its temperature. 3.3.4

Reading aneroid barometers

3.3.4.1

Accuracy of readings

An aneroid barometer should always be read in the same orientation (vertical or horizontal) as during calibration. It should be tapped lightly before being read. As far as possible, it should be read to the nearest 0.1hPa. Optical and digital devices are available to reduce the errors caused by mechanical levers. The readings should be corrected for instrumental errors, but the instrument is usually assumed to be sufficiently compensated for temperature, and it needs no correction for gravity. 3.3.4.2

Reductions applied to barometers

In general, aneroid barometers should be set to read the pressure at the level of the instrument. On board a ship or at low-lying land stations, however, the instrument may be set to indicate the pressure at MSL, provided that the difference between the station pressure and the sea-level pressure can be regarded as constant. 3.3.5

Sources of error

3.3.5.1

Incomplete compensation for temperature

In an aneroid barometer, if the spring is weakened by an increase in temperature, the pressure indicated by the instrument will be too high. This effect is generally compensated for in one of the following ways: (a) By means of a bimetallic link in the lever system; (b) By leaving a certain amount of gas inside the aneroid chamber.

124

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

In most ordinary aneroid barometers, the compensation obtained by these methods is complete only at one particular compensation pressure. It is desirable that all aneroid barometers and barographs used at meteorological stations should be properly compensated for temperatures over the full range of pressure. In digital read-out systems suitable for automation, such complete corrections can be applied as part of the electronic system. 3.3.5.2

Elasticity errors

An aneroid barometer may be subjected to a large and rapid change in pressure. For example, a strong gust of wind would cause an aneroid barometer to experience a rapid increase in pressure followed by a more gradual return to the original value. In such circumstances, the instrument will, owing to hysteresis, indicate a slightly different reading from the true pressure; a considerable time may elapse before this difference becomes negligible. However, since aneroids and barographs at surface stations are not usually directly exposed to such pressure changes, their hysteresis errors are not excessive. There is also a secular error caused by slow changes in the metal of the aneroid capsule. This effect can be allowed for only by comparison at regular intervals, for example, annually, with a standard barometer. A good aneroid barometer should retain an accuracy of 0.1hPa over a period of one year or more. In order to detect departures from this accuracy by individual barometers, a regular inspection procedure with calibration and adjustment as necessary should be instituted.

3.4

BAROGRAPHS

3.4.1

General requirements

Of the various types of barographs, only aneroid barographs are dealt with in detail here. For synoptic purposes, it is recommended that charts for barographs: (a) Be graduated in hPa; (b) Be readable to 0.1 hPa; (c) Have a scale factor of 10 hPa to 1.5 cm on the chart. In addition, the following requirements are desirable: (a) The barograph should employ a high quality aneroid unit (see 3.4.2); (b) The barograph should be compensated for temperature, so that the reading does not change by more than 1hPa for a 20K change in temperature; (c) Scale errors should not exceed 1.5 hPa at any point; (d) Hysteresis should be sufficiently small to ensure that the difference in reading before a change in pressure of 50hPa and after a return to the original value does not exceed 1hPa; (e) There should be a time-marking arrangement that allows the marks to be made without lifting the cover; (f) The pen arm should be pivoted in a “gate”, the axis of which should be inclined in such a way that the pen rests on the chart through the effects of gravity. A means of adjustment should be provided for setting the position of the pen. Marine barographs are subject to special requirements, which are considered in VolumeIII, Chapter4 of the present Guide.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

3.4.2

125

Construction of barographs

The principle of the aneroid barograph is similar to that of the aneroid barometer, except that a recording pen is used instead of a pointer. This involves some change in the design of the capsule stack, and usually means a decrease in the overall magnification and an increase in the number and size of the capsules used. The “control” of the barograph may be expressed as the force required to move the pointer over one unit of the scale (1hPa) and is, thus, equal to the force required to prevent the pen from moving when the pressure changes by 1hPa. It is a measure of the effect that friction is likely to have on the details of the record. The force required to overcome the movement of the capsule when the pressure changes by 1hPa is 100Anewtons, where A is the effective cross-sectional area of the capsule in square metres. If the magnification is X, the force necessary to keep the pen from moving is 100A/Xnewtons and varies as A/X. For a given type of capsule and scale value, the value of X will be largely independent ofA, so that the control of a barograph pen may be considered to vary approximately with the effective cross-sectional area of the capsule. 3.4.3

Exposure of barographs

Details on general exposure requirements are provide in 3.1.4.2. The barograph should be placed at a location where it is unlikely to be tampered with by unauthorized persons. Mounting the barograph on a sponge rubber cushion is a convenient means of reducing the effects of vibration. The site selected should be clean and dry. The air should also be relatively free of substances which would cause corrosion and fouling of the mechanism. It is important to place the instrument so that its face will be at a convenient height to be read at eye-level under normal operating conditions with a view to minimizing the effects of parallax. The exposure ought to be such that the barometer is uniformly illuminated, with artificial lighting being provided if necessary. 3.4.4

Sources of error

In addition to the sources of error mentioned for the aneroid (see3.3.5), the friction between the pen and the paper is important. The control of the pen depends largely on the effective crosssection of the aneroid. In a well-made barograph, the friction of the pen is appreciably greater than the total friction at all the pivots and bearings of the instrument; special attention should, therefore, be given to reduce such errors, for example, by having a sufficiently large aneroid capsule. A high quality barograph should be capable of an uncertainty of about 0.2hPa after corrections have been applied and should not alter for a period of one or two months. The barometric change read from such a barograph should usually be obtained within the same limits. 3.4.5

Reading a barograph

The barograph should be read without touching the instrument. The time mark and any inspection of the instrument involving lifting the cover, and so on, should always be made after the reading is completed. 3.4.5.1

Accuracy of readings

The chart should be read to the nearest 0.1hPa. The barometric change should be obtained within the same resolution limits.

126 3.4.5.2

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Corrections to be applied to barograph readings

The temperature compensation of each individual instrument should be tested before the instrument is used, and the scale factor should be adjusted by testing in a vacuum chamber. If the barograph is used only to find the barometric change, the corrections are not usually applied to the readings. In this case, the accurate setting of the pen position is not important. When absolute pressure values are required from the barograph, the record should be compared with the reading of an electronic barometer or a good aneroid barometer at least once every 24h and the desired values found by interpolation. 3.4.5

Transport

If a barograph has to be transported by air or transported at a high altitude, the pen arm should be disconnected and precautions should be taken to ensure that the mechanism is able to withstand the overload caused by exceeding the normal measuring range of the instrument.

3.5

BAROMETRIC CHANGE AND PRESSURE TENDENCY

3.5.1

Pressure tendency and pressure tendency characteristics

At surface synoptic observing stations, pressure tendency and the pressure tendency characteristic should be derived from pressure observations from the last 3h (over 24h in tropical regions). Typically, the pressure tendency characteristic can be expressed by the shape of the curve recorded during the 3 h period preceding an observation. In the case of hourly observations, the amount and characteristic can be based on only four observations, and misinterpretations may result. Therefore, it is recommended that the characteristic should be determined on a higher frequency of observations, for example with 10-min intervals (WMO, 1985). Nine types of pressure tendency characteristics are defined (see WMO, 2011). 3.5.2

Measurement of a barometric change

Several methods are available to stations making observations at least every 3h, as follows: (a) Digital electronic barometers usually display the pressure tendency together with the actual pressure; (b) The change can be read directly from a barograph; (c) The change can be obtained from appropriate readings of the barometer, corrected to station level. The error of a single barometric reading is mainly random, assuming that the barometer functions perfectly. Therefore, when two independent readings are subtracted to find the amount of change, the errors may be cumulative. Errors are partly systematic in nature, so that during the relatively short period of 3h, the errors are likely to have the same sign and would therefore be diminished by subtraction.

3.6

TRACEABILITY ASSURANCE AND CALIBRATION

3.6.1

General comments

In view of the importance of accurate pressure observations, especially for aeronautical and synoptic purposes, and of the various possible errors to which barometers are subject, traceability assurance and regular calibration of barometers has a very high importance. Starting

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

127

in the 1960s, a concept of barometer comparison, including designated regional standard barometers in each WMO regional association, had been used to ensure traceability of pressure measurements. This concept was discontinued by Decision36 (EC-69) made by the WMO Executive Council at its sixty-ninth session in2017. Currently, the traceability of atmospheric pressure measurements to SI units can be provided more efficiently and economically through an unbroken traceability chain and a new “strategy for traceability assurance” is implemented instead (see the present volume, ChapterI, Annex1.B). Some guidance is given in the following sections regarding the equipment to be used for laboratory or mobile calibration and for field checks. Definitions and general comments on calibration can be found in VolumeV, Chapter4 of the present Guide; while guidance on the computation of calibration uncertainties can be found in WMO (2015). 3.6.2

Laboratory calibration

Laboratory calibration of barometers should be carried out regularly by calibration laboratories with ISO/IEC 17025 accreditation or by an NMI service covered by a CIPM MRA. If a suitable laboratory is not available, traceability to SI should be assured according to the strategy for traceability assurance as described in the present volume, Chapter 1, Annex 1.B. In general, calibrations can be performed at different locations. To achieve lower uncertainties the calibration should be performed at a permanent calibration laboratory situated at a fixed location. Under such circumstances more sensitive primary standards can be used, the environmental conditions (for example, temperature and humidity) can be controlled very well and a vibration-proof set-up can be realized. If the instruments to be calibrated cannot be moved to a permanent calibration laboratory regularly, the calibrations can be performed with mobile calibration equipment on-site in a building at the observation site or in a specially equipped vehicle. As the environmental conditions cannot be controlled so precisely as in a permanent calibration laboratory, the achievable uncertainties are usually larger. 3.6.2.1

General equipment set-up

In most cases calibration equipment includes a pressure controller in combination with the reference barometer that is traceable to SI. Pressure controllers regulate the pressure in a hose with the connected instrument to be calibrated. A vacuum pump and pressure supply are connected to the pressure controller. It is highly recommended to use a pressurized gas cylinder with dry, clean air with very high purity as the pressure source. The container must be equipped with a pressure-reducing valve. A micro-filter has to be attached between the pressure-reducing valve and the hose to the pressure controller. Data from the reference barometer are used as the reference data, not the data from the controller. Purified nitrogen may also be used for some barometers. However, for barometers using a technology based on the measurement on air density (such as cylindrical resonator barometers) nitrogen may not be used because the density of air differs from the density of nitrogen. The following aspects based on European Association of National Metrology Institutes (EURAMET) guidelines (EURAMET, 2017) should also be taken into account: (a) The whole equipment must be protected from direct sunlight and any source of heat. (b) The instruments to be calibrated should be placed as close as possible to the reference instrument and at the same height. (c) The pressure reference levels of both instruments should be as close as possible. If there are differences they have to be taken in account for corrections and uncertainties.

128

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(d) The equipment needs time for warming and acclimatization to reach thermal equilibrium in the whole system. (e) All barometers measure pressure using techniques that are sensitive to temperature. Therefore, these instruments are temperature compensated (mechanically or by appropriate software). When barometers are used within a wider temperature range than at normal indoor temperatures, the barometers must be calibrated or tested at a number of temperatures to be representative for that specific range. (f) The calibration should be performed at an ambient temperature stable to within ±1°C. This temperature should be representative for the range used in operational conditions, typically lying between 18°C and 28°C. Temperature should be recorded. (g) Normally the calibration of meteorological pressure instruments is performed in absolute pressure mode so the air density has no effect. If the air density has an effect on the calibration result, not only the ambient temperature, but also the atmospheric pressure and the relative humidity are to be recorded. (h) The workplace should be kept clean and well organized. 3.6.2.2

Laboratory standards

The reference instrument must be traceable to national or international standards and the uncertainty should be better than that of the instrument to be calibrated. The ratio of the uncertainty of the instrument to be calibrated to that of the reference should be, if practicable, at least two. 3.6.2.2.1

Pressure controller with internal reference

Pressure controllers can be used as working standards, but only if the measurement uncertainty is within the required limits and traceable to SI (WMO, 2010).These controllers work in absolute pressure mode. The preselected pressure is generated by gas supply, vacuum pump and valves. The internal pressure gauge is used as reference and for regulation of the pressure. The devices under test are connected directly or via pressure hose. A slight drift may occur so the pressure controller must be recalibrated in regular intervals. Either the whole pressure controller should be sent to the calibration laboratory or only the internal pressure reference, which can be uninstalled. An uncertainty better than 0.1hPa is possible. 3.6.2.2.2

Pressure controller with an external reference

In this case the internal pressure controller has a reduced precision or cannot be calibrated to be traceable to SI. An external precision pressure gauge is used as working standard. It is connected in parallel to the device under test. Maintenance and calibration of the external reference is easier than with an internal reference. An uncertainty better than 0.05hPa can be achieved. Examples of such external references, with high stability (less than 0.1hPa in 10years), excellent temperature compensation (better than 0.001hPa K-1) and without hysteresis are typically high precision electrical digital barometers that use the technology explained in3.2. These types of reference barometers are highly efficient because they can be used in an automatic calibration environment requiring limited human resources. Despite high stability, it is recommended to calibrate this reference with SI-traceable equipment every year.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

129

3.6.2.2.3 Piston gauges A piston gauge is a primary standard and offers the lowest possible uncertainties and the highest stability. Due to its ultra-low drift, a recalibration interval of five years is recommended. The uncertainty is about 0.05hPa or less. Although they are primary standards, they are often used also as working standards. There are two principles based on a piston–cylinder system made of tungsten carbide. The effective area of the piston has been determined by an accredited calibration laboratory or an NMI. The temperature is measured with a PRT and the change of the effective area due to the change of temperature is calculated permanently by the piston gauge controller. The piston rotates in a cylinder driven by a motor. The surface of the piston and the cylinder is ultra-smooth, cleaned and there is no lubrication except the molecules of the used gas. An additional pressure controller is needed in any case, so the investment is by far the highest. In absolute pressure mode built-in vacuum gauges are needed for both systems. Due to the relatively complicated calibration of these vacuum gauges, external vacuum gauges are recommended. In most cases vacuum-gauges suffer from the problem of drift so the calibration intervals are shorter than the calibration interval of the piston gauge itself. The uncertainty of the vacuum-gauge must be taken into account. Piston gauges with a dynamometer gauge The preselected pressure is generated by the pressure controller. The piston gauge and the devices under test are connected in parallel via a pressure hose. The generated pressure acts on the piston that is connected to a dynamometer which measures the force. The area around the dynamometer is evacuated so there is only a very low force due to the residual gas. With known temperature-corrected effective area and the measured force, the pressure is calculated. The vacuum is measured with a vacuum gauge. The residual pressure must be taken into account by the piston gauge controller. Regular adjustments of the dynamometer zero point and the gradient are performed with precision weights that are calibrated by an accredited calibration laboratory or an NMI. Piston gauges with loaded piston This kind of primary standard does not measure the pressure. Its piston is loaded with weights that are calibrated by an accredited calibration laboratory or an NMI. Due to the absence of a dynamometer, this kind of pressure gauge is a fundamental gauge with the lowest possible uncertainty. It is directly traceable to SI units of mass, length, temperature and time. The pressure is generated by, and its value derived from, the known mass of the piston and the weights, the local gravity and the temperature-corrected effective area (A in Figure 3.1). To determine the measurement uncertainty, among other contributions to the uncertainty budget, the uncertainty of these three components must be known. Special attention must be given to the local gravity and its uncertainty. It is necessary that this local gravity (at the location of the standard) is determined by qualified personnel or accredited services. Note that the building in which the standard operates will affect the local gravity. See also Annex 3.B on the use of gravimeters. A pressure controller is needed to raise the weights. At a certain height the piston is accelerated by a temporarily connected belt which is driven by a motor. At a certain rotation speed the belt is disconnected and the motor stops. Due to the extremely low friction of the nitrogen molecules the rotation speed will decelerate very slow. Depending on the amount of weights the rotation can persist up to a half an hour.

130

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Pressure p

Effective area

A

Force F p=

Vacuum

F A

Figure 3.1. Piston gauge with dynamometer If the piston rotates, the height of the piston vary slightly. To bring the piston back to a specific height, the pressure controller regulates the pressure below the piston area. Then, the pressure controller becomes inactive and its valves close. The pressure in the area below the piston and the connected pressure hose is generated by the rotating and very slowly sinking piston (Figure3.2). The area above the piston–cylinder system is covered with a glass bell. The bell is evacuated using a strong vacuum pump. The vacuum is measured with a vacuum gauge. The residual pressure has to be taken into account by the piston gauge controller.

Vacuum Mxg

PISTON

CYLINDER

A

P

Figure 3.2. Loaded piston gauge

131

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

A disadvantage of piston gauges is the exchange of the weights. The evacuated area must be pressurized and the glass bell must be removed to change the weights. After reinstalling the glass bell the area must be evacuated again. The work with piston gauges is very timeconsuming, but an automatic mass handling system is available for some types of piston gauges. Note that this technique requires well-trained personnel. Method of calibration

3.6.2.3

To achieve the required expanded measurement uncertainty, a comprehensive calibration procedure should be performed. Several guidelines are available. The following describes a proven procedure that is commonly used by accredited laboratories. It allows the evaluation of linearity, repeatability and reversibility. The pressure range for calibration can be chosen either from 0% to 100% of the full scale of the instrument, or the interval can be reduced based on a client’s requirements (for example, the range to be expected in operational use, such as 850–1050 hPa). Figure3.3 shows the general calibration process. The calibration process starts with generating maximum and minimum calibration points, sequentially, three times. The preloading time at the highest value and the time between two preloadings should be at least 30seconds. The change of the pressure should be realized in 30 seconds and at least 120seconds of holding time is needed. The calibration should then be carried out at calibration points uniformly distributed over the calibration range. A cycle of measurements, each consisting of a series of increasing pressure and a series of decreasing pressure, must be taken. The number of points a series consists of should not be less than nine. The time between two successive load steps should be the same and not shorter than 30seconds. At each calibration point, the waiting time, during which steady-state conditions are achieved, should be at least 120seconds. The mounting and connections should stay unchanged during the whole process. The determination of the zero point deviation is usually omitted in the case of absolute pressure gauges, such as barometers, and consequently a zero-point adjustment is not performed.

Procedure A P

M1

M2

M3

M4

t Preloading

M1 .... M4: Measuring series

Figure 3.3. Calibration procedure

132

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

3.6.2.3.1

Calculation of repeatability

The repeatability is calculated from the difference between the deviations measured in the corresponding measurement series. The index j represents the nominal pressure point:

bup , j =

bdown , j =

 p3, j − p1, j  p4, j − p2, j

bmean , j = max {bup , j ,� bdown , j }

The repeatability must be considered for the calculation of the uncertainty. Example: Reference (hPa)

Series 1 Δp (hPa)

Series 2 Δp (hPa)

Series 3 Δp (hPa)

Series 4 Δp (hPa)

996.371

-0.002

0.008

0.001

0.007

bup , j = 0.001 hPa − ( −0.002 hPa ) = 0.003 hPa

bdown , j = 0.007 hPa − 0.008 hPa = 0.001 hPa

bmean , j = max {0.003 hPa, 0.001 hPa} = 0.003 hPa

3.6.2.3.2

Calculation of reversibility (hysteresis)

The reversibility (hysteresis) is calculated from the differences between the corresponding deviations of the output values measured at increasing and decreasing pressure:

hmean , j =

{

1 ∆p2, j − ∆p1, j + (∆p4, j − ∆p3, j 4

}

The reversibility must be considered for the calculation of the uncertainty. Example:

3.6.3

Reference (hPa)

Series 1 Δp (hPa)

Series 2 Δp (hPa)

Series 3 Δp (hPa)

Series 4 Δp (hPa)

996.371

-0.002

0.008

0.001

0.007

hmean , j =

{

}

1 0.008 hPa − ( −0.002 hPa ) + ( 0.007 hPa − 0.001 hPa ) = 0.004 hPa . 4

Field inspections

During field inspection, a comparison with a travelling standard should be carried out. This comparison is not a calibration, as in most cases just a one-point comparison at actual atmospheric pressure is performed. These checks can therefore only indicate the plausibility of the readings of the instrument on-site. For field inspections, a mobile electronic pressure gauge, preferably with more than one pressure transducer, should be used as a travelling standard (see3.2). With an appropriate temperature compensation, an uncertainty of 0.1hPa or less can be achieved. Instruments with rechargeable

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

133

batteries are available and the values from the internal transducers can be displayed separately or as a mean value. Before comparison, the instrument should always be acclimated to ambient conditions. Field inspections should be performed in low gradient weather conditions with stable atmospheric pressure and low wind speeds. Field inspection equipment should be calibrated by an accredited calibration laboratory, preferably before and after field use, or at appropriate calibration intervals, depending on the drift of the equipment.

3.7

ADJUSTMENT OF BAROMETER READINGS TO STANDARD AND OTHER LEVELS

To compare barometer readings taken at stations at different altitudes, it is necessary to reduce them to the same level. Whereas various methods are in use for carrying out this reduction, WMO has recommended a standard method described in the following paragraphs. The recommended method is described in more detail in WMO (1954, 1964,1968). WMO (1966) contains a comprehensive set of formulae that may be used for calculations involving pressure. 3.7.1

Standard levels

The observed atmospheric pressure should be reduced to MSL (see the present volume, Chapter1) for all stations where this can be done with reasonable accuracy. Where this is not possible, a station should, by regional agreement, report either the geopotential of an agreed “constant pressure level” or the pressure reduced to an agreed datum for the station. The level chosen for each station should be reported to the WMO Secretariat for promulgation (that is, the WMO Observing Systems Capability Analysis and Review Tool (OSCAR)/Surface, https:/​/​oscar​​.wmo​​.int/​surface). 3.7.2

General reduction formula

Reduction formula for sea-level pressure feasible for stations below 750m (from WMO, 1964, p.22, equation2):

log10

Kp ⋅ Hp p0 = = Tmv ps

Kp ⋅ Hp

Ts +

a ⋅ Hp 2

(3.1)

+ es ⋅ Ch

where p 0 is the pressure reduced to sea level in hPa; pSis the station pressure in hPa; Kpis the constant =0.0148275K gpm-1; Hpis the station elevation in gpm; Tmvis the mean virtual temperature of the fictitious air column below station level inK, (Tmv=TS+(a·Hp)/2+eS·Ch); TSis the station temperature inK, TS=273.15+t; tis the station temperature in°C; ais the assumed lapse-rate in the fictitious air column extending from sea level to the station elevation level =0.0065Kgpm-1; eSis the vapour pressure at the station inhPa; and Chis the coefficient =0.12KhPa-1. The same formula is often used in the exponential form:

  gn ⋅ Hp   R  (3.2) p0 = ps ⋅ exp  a ⋅ Hp   T e C + + ⋅  s s h 2  

where gn is the standard acceleration of gravity =9.80665ms–2 and Ris the gas constant of dry air =287.05Jkg-1K-1.

134 3.7.3

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Reduction formula for low-level stations

At low-level stations (namely, those at a height of less than 50m above MSL), pressure readings should be reduced to MSL by adding to the station pressure a reduction constant C given by the following expression:

C = p ⋅ Hp

(29.27 Tv ) (3.3)

where p is the observed station pressure in hectopascals; Hp is the station elevation in metres; and Tvis the mean annual normal value of virtual temperature at the station inK. Note: The virtual temperature of damp air is the temperature at which dry air of the same pressure would have the same density as the damp air. WMO (1966) contains virtual temperature increments of saturated moist air for various pressures and temperatures.

This procedure should be employed only at stations of such low elevation that when the absolute extreme values of virtual temperature are substituted for Tv in the equation, the deviation of the result due to the other approximations of the equation (used for height rather than standard geopotential, and with C to be small compared withp) is negligible in comparison.

ANNEX 3.A. METHODS OF MEASUREMENT WITH MERCURY BAROMETERS

As outlined in 3.1.4.5, the use of mercury barometers is not recommended anymore. The reasons to move away from their use are: mercury vapour is highly toxic; free mercury is corrosive to the aluminium alloys used in air; special lead glass is required for the tube; the barometer is very delicate and difficult to transport; it is difficult to maintain the instrument and to clean the mercury; the instrument must be read and corrections applied manually; and other barometers of equivalent accuracy and stability with electronic read-out are now commonly available. This annex is kept for information only.

1.

UNITS AND SCALES

Some barometers are graduated in “millimetres or inches of mercury under standard conditions”, (mmHg)n and (inHg)n, respectively. When it is clear from the context that standard conditions are implied, the briefer terms “millimetre of mercury” or “inch of mercury” may be used. Under these standard conditions, a column of mercury having a true scale height of 760(mmHg)n exerts a pressure of 1013.250hPa. The following conversion factors will then apply:

1 hPa = 0.750062 (mm Hg)n 1 (mm Hg)n = 1.333224 hPa

In the case where the conventional engineering relationship between the inch and the millimetre is assumed, namely 1in = 25.4mm, the following conversion factors are obtained:

1 hPa = 0.029530 (in Hg)n 1 (in Hg)n = 33.8639 hPa 1 (mm Hg)n = 0.03937008 (in Hg)n

Scales on mercury barometers for meteorological purposes should be so graduated that they yield true pressure readings directly in standard units when the entire instrument is maintained at a standard temperature of 0°C and the standard value of gravity is 9.80665ms–2. Barometers may have more than one scale engraved on them, for example, hPa and mm Hg, or hPa and in Hg, provided that the barometer is correctly calibrated, adjusted and compensated for use under standard conditions.

2.

REQUIREMENTS FOR MERCURY BAROMETERS

2.1

Construction requirements

The basic principle of a mercury barometer is that the pressure of the atmosphere is balanced against the weight of a column of mercury. In some barometers, the mercury column is weighed on a balance, but, for normal meteorological purposes, the length of the mercury column is measured against a scale graduated in units of pressure. There are several types of mercury barometers in use at meteorological stations, with the fixed cistern and the Fortin types being the most common. The length to be measured is the distance between the top of the mercury column and the upper surface of the mercury in the cistern. Any change in the length of the mercury column is, of course, accompanied by a change in the level

136

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

of the mercury in the cistern. In the Fortin barometer, the level of the mercury in the cistern can be adjusted to bring it into contact with an ivory pointer, the tip of which is at the zero of the barometer scale. In the fixed-cistern barometer, often called the Kew-pattern barometer, the mercury in the cistern does not need to be adjusted as the scale engraved on the barometer is constructed to allow for changes in the level of the mercury in the cistern. 2.2

General requirements

The main requirements of a good mercury station barometer include the following: (a) Its accuracy should not vary over long periods. In particular, its hysteresis effects should remain small; (b) It should be quick and easy to read, and readings should be corrected for all known effects. The observers employing these corrections must understand their significance to ensure that the corrections applied are correct and not, in fact, causing a deterioration in the accuracy of the readings; (c) It should be transportable without a loss of accuracy; (d) The bore of the tube should not be less than 7mm and should preferably be 9mm; (e) The tube should be prepared and filled under vacuum. The purity of the mercury is of considerable significance. It should be double-distilled, degreased, repeatedly washed, and filtered; (f) The actual temperature, for which the scale is assumed to give correct readings, at standard gravity, should be engraved upon the barometer. The scale should preferably be calibrated to give correct readings at 0°C; (g) The meniscus should not be flat unless the bore of the tube is large (greater than 20mm); (h) For a marine barometer, the error at any point should not exceed 0.5hPa. The response time for mercury barometers at land stations is usually very small compared with that of marine barometers and instruments for measuring temperature, humidity and wind. 2.3

Exposure of mercury barometers

The general exposure requirements of mercury barometers have been outlined in the preceding sections. Mercury barometers have additional exposure requirements above those already mentioned. It is always preferable to hang the mercury barometer on an inside wall. For very accurate work, the best position would be in an unheated basement room with no windows and with a small electric fan to prevent any stratification of temperature. In order to obtain uniform lighting conditions for reading the barometer, it is advisable to use artificial lighting for all observations. For this purpose, some sort of illuminator – which can provide a white and slightly luminous background for the mercury meniscus and, if necessary, for the fiducial point – may be provided. If no illuminator is used, care should be taken to provide the meniscus and the fiducial point with a light background, by such means as pieces of milk glass, white celluloid, or a sheet of white paper. Artificial light should also be provided for reading the barometer scale and the attached thermometer. Care should, however, be taken to guard against heating the barometer with artificial light during a barometer reading. The barometer should be mounted in a place where it is not subject to vibration, preferably on a solid wall. The instrument must be mounted with the mercury column in a vertical position. Errors due to departure from verticality are more critical for asymmetric barometers. Such

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

137

barometers should be mounted with their longest axis vertical in order that a true setting of the mercury surface to the fiducial point remains correct even when the instruments are tilted from the vertical. To protect the barometer from rough handling, dust and air currents, it is recommended that the instrument be placed in a box furnished with a hinged door with provisions for sufficient ventilation to prevent stratification of the air inside. Great care should be taken when transporting a mercury barometer. The safest method is to carry the barometer upside down in a wooden case furnished with a sling. If the barometer cannot be accompanied by a responsible person, it ought to be transported in a suitable sprung crate with the cistern uppermost. The barometer should not be subject to violent movements and must always be turned over very slowly. Special precautions must be taken for some individual types of barometers before the instrument is turned over.

3.

MEASUREMENTS USING MERCURY BAROMETERS

3.1

Standard conditions

Given that the length of the mercury column of a barometer depends on other factors, especially on temperature and gravity, in addition to the atmospheric pressure, it is necessary to specify the standard conditions under which the barometer should theoretically yield true pressure readings. The following standards are laid down in the international barometer conventions. 3.1.1

Standard temperature and density of mercury

The standard temperature to which mercury barometer readings are reduced to remove errors associated with the temperature-induced change in the density of mercury is 0°C. The standard density of mercury at 0°C is taken to be 1.35951·10 4kgm–3 and, for the purpose of calculating absolute pressure using the hydrostatic equation, the mercury in the column of a barometer is treated as an incompressible fluid. The density of impure mercury is different from that of pure mercury. Hence, a barometer containing impure mercury will produce reading errors as the indicated pressure is proportional to the density of mercury. 3.1.2

Standard gravity

Barometric readings have to be reduced from the local acceleration of gravity to standard (normal) gravity. The value of standard gravity(gn) is regarded as a conventional constant, gn=9.80665ms–2. Note: The need to adopt an arbitrary reference value for the acceleration of gravity is explained in WMO (1966). This value cannot be precisely related to the measured or theoretical value of the acceleration of gravity in specified conditions, for example, sea level at latitude45°, because such values are likely to change as new experimental data become available.

3.2

Reading mercury barometers

When making an observation with a mercury barometer, the attached thermometer should be read first. This reading should be taken as quickly as possible, as the temperature of the thermometer may rise owing to the presence of the observer. The barometer should be tapped a few times with the finger in two places, one adjacent to the meniscus and the other near the

138

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

cistern, so as to stabilize the mercury surfaces. If the barometer is not of a fixed-cistern type, the necessary adjustment should be made to bring the mercury in the cistern into contact with the fiducial pointer. Lastly, the vernier should be set to the meniscus and the reading taken. The vernier is correctly adjusted when its horizontal lower edge appears to be touching the highest part of the meniscus; with a magnifying glass it should be possible to see an exceedingly narrow strip of light between the vernier and the top of the mercury surface. Under no circumstances should the vernier “cut off” the top of the meniscus. The observer’s eye should be in such a position that both front and back lower edges of the vernier are in the line of vision. 3.2.1

Accuracy of readings

The reading should be taken to the nearest 0.1hPa. Usually it is not possible to read the vernier to any greater accuracy. Optical and digital systems have been developed to improve the reading of mercury barometers. Although they normally ease the observations, such systems may also introduce new sources of error, unless they have been carefully designed and calibrated. 3.2.2

Changes in index correction

Any change in the index correction shown during an inspection should be considered on its merits, keeping in mind the following: (a) The history of the barometer; (b) The experience of the inspector in comparison work; (c) The magnitude of the observed change; (d) The standard deviation of the differences; (e) The availability of a spare barometer at the station, the correction of which is known with accuracy; (f) The behaviour of travelling standards during the tour; (g) The agreement, or otherwise, of the pressure readings of the station with those of neighbouring stations on the daily synoptic chart if the change is accepted; (h) Whether or not the instrument was cleaned before comparison. Changes in index errors of station barometers, referred to as drift, are caused by: (a) Variations in the capillary depression of the mercury surfaces due to contamination of the mercury. In areas of severe atmospheric pollution from industrial sources, mercury contamination may constitute a serious problem and may require relatively frequent cleaning of the mercury and the barometer cistern; (b) The rise of air bubbles through the mercury column to the space above. These changes may be erratic, or consistently positive or negative, depending on the cause.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

139

Changes in index correction are also caused by: (a) Observer error resulting from failure to tap the barometer before taking the reading and improper setting of the vernier and fiducial point; (b) Lack of temperature equilibrium in either the station barometer or the travelling standard; (c) Non-simultaneity of readings when the pressure is changing rapidly. Such changes can be caused by accidental displacement of the adjustable scale and the shrinkage or loosening of fiducial points in Fortin-type barometers. 3.2.3

Permissible changes in index correction

Changes in index correction should be treated as follows: (a) A change in correction within 0.1hPa may be neglected unless persistent; (b) A change in correction exceeding 0.1hPa but not exceeding 0.3hPa may be provisionally accepted unless confirmed by at least one subsequent inspection; (c) A change in correction exceeding 0.3hPa may be accepted provisionally only if the barometer is cleaned and a spare barometer with known correction is not available. This barometer should be replaced as soon as a correctly calibrated barometer becomes available. Barometers with changes in index correction identified in(b) and(c) above warrant close attention. They should be recalibrated or replaced as soon as practicable. The same criteria apply to changes in the index corrections of the travelling standards as those applied as to station barometers. A change in correction of less than 0.1hPa may be neglected unless persistent. A larger change in correction should be confirmed and accepted only after repeated comparisons. The “before” and “after” tour index corrections of the travelling standard should not differ by more than 0.1hPa. Only barometers with a long history of consistent corrections should, therefore, be used as travelling standards. 3.3

Correction of barometer readings to standard conditions

In order to transform barometer readings taken at different times and different places into usable atmospheric pressure values, the following corrections should be made: (a) Correction for index error; (b) Correction for gravity; (c) Correction for temperature. For a large number of operational meteorological applications, it is possible to obtain acceptable results by following the barometer manufacturer’s instructions, provided that it is clear that these procedures give pressure readings of the required uncertainty. However, if these results are not satisfactory or if higher precision is required, detailed procedures should be followed to correct for the above factors; these procedures are described in Annex3.B.

140

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

3.4

Errors and faults with mercury barometers

3.4.1

Uncertainties as to the temperature of the instrument

The temperature indicated by the attached thermometer will not usually be identical to the mean temperature of the mercury, the scale and the cistern. The resultant error can be reduced by favourable exposure and by using a suitable observation procedure. Attention is drawn to the frequent existence of a large, stable vertical temperature gradient in a room, which may cause a considerable difference between the temperature of the upper and lower parts of the barometer. An electric fan can prevent such a temperature distribution but may cause local pressure variations and should be switched off before an observation is made. Under normal conditions, the error associated with the temperature reduction will not exceed 0.1hPa if such precautions are taken. 3.4.2

Defective vacuum space

It is usually assumed that there is a perfect vacuum, or only a negligible amount of gas, above the mercury column when the instrument is calibrated. Any change in this respect will cause an error in pressure readings. A rough test for the presence of gas in the barometer tube can be made by tilting the tube and listening for the click when the mercury reaches the top, or by examining the closed end for the presence of a bubble, which should not exceed 1.5mm in diameter when the barometer is inclined. The existence of water vapour cannot be detected in this way, as it is condensed when the volume decreases. According to Boyle’s Law, the error caused by air and unsaturated water vapour in the space will be inversely proportional to the volume above the mercury. The only satisfactory way to overcome this error is by conducting a recalibration over the entire scale; if the error is large, the barometer tube should be refilled or replaced. 3.4.3

The capillary depression of the mercury surfaces

The height of the meniscus and the capillary depression1, for a given tube, may change with the ageing of the glass tube, mercury contamination, pressure tendency, and the position of the mercury in the tube. As far as is practicable, the mean height of the meniscus should be observed during the original calibration and noted on the barometer certificate. No corrections should be made for departures from the original meniscus height, and the information should be used only as an indication of the need, or otherwise, to overhaul or recalibrate the barometer. A 1mm change in the height of the meniscus (from 1.8 to 0.8mm) for an 8mm tube may cause an error of about 0.5hPa in the pressure readings. It should be noted that large variations in the angle of contact between the mercury and the wall of the cistern in a fixed-cistern barometer may cause small but appreciable errors in the observed pressure. 3.4.4

Lack of verticality

If the bottom of a symmetrical barometer of normal length (about 90cm), which hangs freely, is displaced by about 6mm from the vertical position, the indicated pressure will be about 0.02hPa too high. Such barometers generally hang more truly vertical than this. In the case of an asymmetrical barometer, however, this source of error is more critical. For example, if the fiducial pointer in the cistern is about 12mm from the axis, the cistern needs to be displaced by only about 1mm from the vertical to cause an error of 0.02hPa.

1

Capillary depression is a reduction in height of the meniscus of a liquid contained in a tube where the liquid (such as mercury) does not wet the walls of the tube. The meniscus is shaped convex upward.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

3.4.5

141

General accuracy of the corrected pressure readings

The standard deviation of a single, corrected barometer reading at an ordinary meteorological station should be within 0.1hPa. This error will mainly be the result of the unavoidable uncertainty in the instrument correction, the uncertainty concerning the temperature of the instrument, and the error caused by the pumping effect of wind gusts on the mercury surface.

4.

SAFETY PRECAUTIONS FOR THE USE OF MERCURY

Mercury is used in relatively large quantities in barometers and, because it is poisonous, must be handled with care. Elemental mercury is a liquid at temperatures and pressures experienced at the Earth’s surface. Mercury vapour forms in the air whenever liquid mercury is present. Mercury can be absorbed through the skin in both liquid and gaseous states and can be inhaled as a vapour. The properties of mercury are described by Sax (1975). In many countries, precautions for its use are prescribed by regulations governing the handling of hazardous goods. The UNEP Minamata Convention on Mercury entered into force in August 2017 and has a significant impact on the use of mercury for meteorological applications. A large dose of mercury may cause acute poisoning. It can also accumulate in the body’s hard and soft tissues and prolonged exposure to even a low dose can cause long-term damage to organs, or even death. Mercury mainly affects the central nervous system, and the mouth and gums, with symptoms that include pain, loosening of teeth, allergic reactions, tremors and psychological disturbance. For barometric applications, the main risks occur in laboratories where barometers are frequently emptied or filled. There may also be problems in meteorological stations if quantities of mercury, for example from a broken barometer, are allowed to remain in places where it may continuously vaporize into an enclosed room where people work. A danger exists even if the mercury is properly contained and if it is cleaned up after an accident. The following points must be considered when using mercury: (a) Vessels containing mercury must be well sealed and not likely to leak or easily break, and must be regularly inspected; (b) The floor of a room where mercury is stored or used in large quantities should have a sealed, impervious and crack-free floor covering, such as PVC. Small cracks in the floor, such as those between floor tiles, will trap mercury droplets. It is preferable to have the flooring material curving up the walls by approximately 10cm, leaving no joint between the floor and the walls at floor level; (c) Mercury must not be stored in a metal container as it reacts with almost all metals, except iron, forming an amalgam which may also be hazardous. Mercury should not come into contact with any other metallic object; (d) Mercury must not be stored with other chemicals, especially amines, ammonia or acetylene; (e) Large quantities of mercury should always be stored and handled in a well-ventilated room. The raw material should be handled in a good-quality fume cupboard; (f) Mercury should never be stored near a heat source of any kind as it has a relatively low boiling point (357°C) and may produce hazardous concentrations of toxic vapour, especially during a fire; (g) If mercury is handled, the room where it is used and the personnel using it should be regularly tested to determine if hazardous quantities of mercury are being encountered.

142

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Under the Minamata Convention, imports and exports of mercury will no longer be allowed. In this context, the production, import and export of mercury-added products such as thermometers will be stopped by 2020. The Convention (Article 4) states that “Each party shall not allow, by taking appropriate measures, the manufacture, import or export of mercury-added products listed in PartI of AnnexA [of the Convention] after the phase-out date specified for those products” (UNEP, 2013). More specifically, this list includes (citation): The following non-electronic measuring devices except non-electronic measuring devices installed in large-scale equipment or those used for high precision measurement, where no suitable mercury-free alternative is available: (a) barometers; (b) hygrometers; (c) manometers; (d) thermometers; (e) sphygmomanometers.

4.1

Spillages and disposal

The two common methods of cleaning up mercury spillages are either with a suitable aspirated pick-up system, as outlined below, or by adsorption/amalgamation of the mercury onto a powder. Mercury should be cleaned up immediately. The operator should wear PVC gloves or gauntlets, safety goggles and, for significant spills, a respirator fitted with a mercury vapour cartridge. Depending upon how large the spillage is, the mercury will be picked up by using a vacuum system; an adsorption kit should then be used to clean up the small droplets. The use of an adsorption kit is imperative because, during a spillage, dozens of small droplets of less than 0.02mm in diameter will adhere to surfaces and cannot be efficiently removed with a vacuum system. In an aspirated pick-up system, the mercury is drawn through a small-diameter plastic tube into a glass flask with approximately 3cm of water in the bottom, with the tube opening being below the water line in the flask. One end of a larger diameter plastic tube is connected to the air space above the water in the flask, and the other end is connected to a vacuum cleaner or vacuum pump. The water prevents the mercury vapour or droplets from being drawn into the vacuum cleaner or pump. The slurry is then placed in a clearly labelled plastic container for disposal. By using adsorption material, a variety of compounds can be used to adsorb or amalgamate mercury. These include zinc powder, sulphur flour or activated carbon. Commercial kits are available for cleaning up mercury spills. The powder is sprinkled on the spill and allowed to adsorb or amalgamate the mercury. The resulting powder is swept up and placed in a clearly labelled plastic container for disposal. The collected mercury can be either disposed of or recovered. Details on how to dispose of mercury can be obtained from local authorities and/or the supplier. The supplier can also advise on recovery and purification. 4.2

Fire

Mercury will not burn but does give off significant concentrations of toxic fumes. After a fire, the mercury vapour will condense on the nearest cool surfaces, contaminating large areas and being adsorbed onto open surfaces, such as carbonized timber. During a fire, evacuate the area and remain upwind of any fumes. Advise the fire authorities of the location and quantity of mercury involved.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

4.3

143

Transportation

The transportation by air of mercury or instruments containing mercury is regulated by the International Air Transport Association. Airlines will provide the specific conditions for such transport upon request. Transportation by rail or road is usually governed by the hazardous material regulations in each country. In general, metallic mercury must be packed in glass or plastic containers. The containers should be packed with sufficient cushioning to prevent breakage and should be clearly labelled. Mercury-containing instruments should be packed in a strong cushioned case which is leak-proof and impervious to mercury.

ANNEX 3.B. CORRECTION OF MERCURY BAROMETER READINGS TO STANDARD CONDITIONS

Correction for index error The residual errors in the graduation of the scale of a barometer should be determined by comparison with a standard instrument. They may include errors due to inaccurate positioning or subdividing of the scale, capillarity and imperfect vacuum. Certificates of comparison with the standard should state the corrections to be applied for index error at no fewer than four points of the scale, for example, at every 50hPa. In a good barometer, these corrections should not exceed a few tenths of a hectopascal. Corrections for gravity The reading of a mercury barometer at a given pressure and temperature depends upon the value of gravity, which in turn varies with latitude and altitude. Barometers for meteorological applications are calibrated to yield true pressure readings at the standard gravity of 9.80665ms–2 and their readings at any other value of gravity must be corrected. The following method is recommended for reducing such barometer readings to standard gravity. Let B be the observed reading of the mercury barometer, Bt the barometer reading reduced to standard temperature but not to standard gravity, and corrected for instrumental errors, Bn be the barometer reading reduced to standard gravity and standard temperature, and corrected for instrumental errors, Bca be the climatological average of Bt at the station, gφH the local acceleration of gravity (inms–2) at a station at latitude φ and elevation H above sea level, and gn the standard acceleration of gravity, 9.80665ms–2. The following relations are appropriate:

(

)

(

)

Bn = Bt gϕ H g n (3.A.1)

or:

Bn = Bt + Bt  gϕ H g n − 1 (3.A.2)  

The approximate equation 3.A.3 may be used, provided that the results obtained do not differ by more than 0.1hPa from the results that would be obtained with the aid of equation3.A.2:

(

)

Bn = Bt + Bca  gϕ H g n − 1 (3.A.3)  

The local acceleration of gravity gφH should be determined by the procedure outlined in the following section. The values so derived should be referred to as being on the International Gravity Standardization Net 1971 (IGSN71). Determining local acceleration of gravity In order to determine the local value of the acceleration of gravity at a station to a satisfactory degree of precision, one of two techniques should be used. These techniques involve, in the first case, the use of a gravimeter (an instrument for measuring the difference between the values of the acceleration of gravity at two points) and, in the second case, the use of the so-called Bouguer anomalies. Preference should be given to the gravimeter method. If neither of these methods can be applied, the local acceleration of gravity may be calculated using a simple model of the Earth.

145

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

Use of a gravimeter Suppose g1 represents the known local acceleration of gravity at a certain pointO, usually a gravity base station established by a geodetic organization, where g1 is on the IGSN71, and suppose further that g represents the unknown local acceleration of gravity on the meteorological gravity system at some other point X for which the value g is desired. Let Δg denote the difference in gravity acceleration at the two places, as observed by means of a gravimeter. That is, Δg is the value at point X minus the value at point O on a consistent system. Then, g is given by equation 3.A.4:

g = g1 + ∆ g (3.A.4)

Use of Bouguer anomalies If a gravimeter is not available, interpolated Bouguer anomalies (AB) may be used to obtain g at a given point. It is necessary that a contour chart of these anomalies be available from a geodetic organization or from a network of gravity stations spaced at a density of at least one station per 10000km2 (no more than a 100km distance between stations) in the vicinity of the point. Gravity networks of somewhat less density can be used as a basis provided that a geodetic organization considers that this method is expected to yield more reliable results than those that could be obtained by using a gravimeter. The definition of the Bouguer anomaly (AB) is derivable from equation 3.A.5:

(

)

g s = gϕ ,0 s − C ⋅ H + AB (3.A.5)

where (gφ,0)s is the theoretical value of the acceleration of gravity at latitude φ at sea level, as given by the formula actually used in computing the Bouguer anomaly. This formula expresses the value as a function of latitude in some systems. H is the elevation of the station (in metres) above sea level at which gs is measured, gsis the observed value of the acceleration of gravity (inms–2); AB is the Bouguer anomaly (in ms–2); and C is the elevation correction factor used in computing the Bouguer anomaly (for example, using a crustal specific gravity of2.67, this factor is 0.000001968ms–2). When g is desired for a given station and has not been measured, the value of gs should be computed by means of equation3.A.5, provided that the appropriate value ofAB for the locality of the station can be interpolated from the aforementioned contour charts or from data representing the Bouguer anomalies supplied by a suitable network of gravity stations, as defined. Calculating local acceleration of gravity If neither of the preceding methods can be applied, the local value may be calculated less accurately according to a simple model. According to the Geodetic Reference System 1980, the theoretical value (gφ,0) of the acceleration of gravity at MSL at geographic latitude, φ, is computed by means of equation3.A.6:

(

)

gϕ ,0 = 9.806 20 1 − 0.002 644 2 cos 2ϕ + 0.000 005 8 cos2 2ϕ (3.A.6)

The local value of the acceleration of gravity at a given point on the surface of the ground at a land station is computed by means of equation3.A.7:

g = gϕ ,0 − 0.000 003 086 H + 0.000 001 118 ( H − H ') (3.A.7)

where g is the calculated local value of the acceleration of gravity, in ms–2, at a given point; gφ,0is the theoretical value of the acceleration of gravity in ms–2 at MSL at geographic latitudeφ, computed according to equation3.A.6 above; His the actual elevation of the given point, in metres above MSL; and H’is the absolute value in metres of the difference between the height of the given point and the mean height of the actual surface of the terrain included within a circle whose radius is about 150km, centred at the given point.

146

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The local value of the acceleration of gravity at a given point within height H above MSL of not more than about 10km, and where that point lies over the sea water surface, is computed by means of equation3.A.8:

g = gϕ ,0 − 0.000 003 086 H − 0.000 006 88 ( D − D ') (3.A.8)

where D is the depth of water in metres below the given point; and D’is the mean depth of water, in metres, included within a circle whose radius is about 150km centred at the given point. At stations or points on or near the coast, the local value of acceleration of gravity should be calculated, so far as practicable, through the use of equations3.A.7 and 3.A.8 on a pro rata basis, weighting the last term of equation3.A.7 according to the relative area of land included within the specified circle, and weighting the last term of equation3.A.8 according to the relative area of the sea included within the circle. The values thus obtained are then combined algebraically to obtain a correction which is applied to the final term in the right-hand side of both equations, as shown in equation3.A.9:

g = gϕ ,0 − 0.000 003 086 H + 0.000 001 118 α

( H − H ') − 0.000 006 88 (1 − α ) ( D − D ')

(3.A.9)

where α is the fraction of land area in the specified area, and H’ and D’ refer to the actual land and water areas, respectively. Corrections for temperature Barometer readings must be corrected to the values that would have been obtained if the mercury and the scale had been at their standard temperatures. The standard temperature for mercury barometers is 0°C. With reference to scales, some barometers have scales which read accurately at this same temperature, but some read accurately at 20°C. The temperature correction necessary for adjustable cistern barometers (Fortin-type barometers) is different from that required for fixed-cistern barometers, though the principle reasons leading to the necessity for temperature corrections are the same for both types, namely, the fact that the coefficient of cubic thermal expansion of mercury is different from the coefficient of linear thermal expansion of the scale. Thus, a certain correction term is required for both types of mercury barometer. A fixed-cistern barometer requires an additional correction. The reason for this is that an increase in temperature of the instrument causes an increase both in the volume of the mercury and in the cross-sectional areas of the (iron) cistern and the (glass) tube. Owing to these area changes, the apparent rise of the mercury resulting from a temperature increase is less than would be the case if the areas remained constant. This is because some of the mercury from the barometer goes to occupy the capacity increment produced by the expansion of the cistern and tube. The scale of a fixed-cistern barometer must, for a variety of reasons, undergo a calibration check against a primary standard barometer of the adjustable-cistern type. Some manufacturers decrease the volume of mercury by such an amount that the readings of the test barometer agree with the readings of the standard barometer at 20°C. Correction tables can be generated for fixed-cistern barometers using the readings from a primary standard barometer whose scales are accurate when 20°C is used as the reference temperature.

CHAPTER 3. MEASUREMENT OF ATMOSPHERIC PRESSURE

147

Temperature corrections for mercury barometers Researchers have conducted exhaustive studies for temperature corrections for mercury barometers, the results of which are summarized below: 1 (a) Scale correct at 0 °C and additionally (b) Hg volume correct at 0 °

Ct

= –B (α – β) · t

Ct,V

= –B (α – β) · t – (α – 3η) · t ·4V/3A

2 Scale correct at 0 °C and Hg volume correct at 20 °C

Ct,V

= –B (α – β) · t – (α – 3η) · (t – 20) · 4V/3A

3 (a) Scale correct at 20 °C (b) Hg volume correct at 0 °C (c) Hg volume decreasing by an amount equivalent to 0.36hPa

Ct Ct,V Ct,V

= –B [α · t – β · (t – 20)] = –B [α · t – β · (t – 20)] – (α – 3η) · t · (4V/3A) = –B (α – β) · t – (α – 3η) · t · (4V/3A)

Ct,V Ct,V

= –B [α · t – β (t – 20)] – (α – 3η) · (t – 20) · (4V/3A) = –B (α – β) ·t – (α – 3η) · (t – 20) · (4V/3A)

4 Scale correct at 20 °C and (a) Hg volume correct at 20°C (b) Hg volume decreasing by an amount equivalent to 0.36hPa where: Ct

= temperature correction;

Ct,V = additional correction for fixed-cistern barometers; B

= observed barometer reading;

V

= total volume of mercury in the fixed-cistern barometer;

A

= effective cross-sectional area of the cistern;

t

= temperature;

α

= cubic thermal expansion of mercury;

β

= coefficient of linear thermal expansion of the scale;

η

= coefficient of linear thermal expansion of the cistern.

REFERENCES AND FURTHER READING

Brock, F.V. and S.J. Richardson, 2001: Meteorological Measurement Systems. New York, Oxford University Press. European Association of National Metrology Institutes, 2017: Guidelines on the Calibration of Electromechanical and Mechanical Manometers. Calibration Guide No. 17 (Version 3.0). Braunschweig, Germany, EURAMET. Liu, H. and G. Darkow, 1989: Wind effect on measured atmospheric pressure. Journal of Atmospheric and Oceanic Technology,6(1):5–12. Miksad, R., 1976: An omni-directional static pressure probe. Journal of Applied Meteorology,15:1215–1225. Sax, N.I., 1975: Dangerous Properties of Industrial Materials. Van Nostrand Reinhold Co., New York. United Nations Environment Programme, 2013: Minamata Convention on Mercury. Geneva, United Nations. United States Weather Bureau, 1963: Manual of Barometry (WBAN). 1, US Government Printing Office, Washington DC. World Meteorological Organization, 1954: Reduction of Atmospheric Pressure: Preliminary Report on Problems Involved. Technical Note No.7 (WMO-No.36,TP.12). Geneva. ———, 1964: Note on the Standardization of Pressure Reduction Methods in the International Network of Synoptic Stations: Report of a Working Group of the Commission for Synoptic Meteorology. Technical Note No.61 (WMO-No.154,TP.74). Geneva. ———, 1966: International Meteorological Tables (S. Letestu, ed.) (1973amendment).(WMO-No.188, TP.94). Geneva. ———, 1968: Methods in Use for the Reduction of Atmospheric Pressure. Technical Note No.91 (WMONo.226,TP.120). Geneva. ———, 1985: “Pressure tendency” and “discontinuity in wind” – discussion of two algorithms used in Swedish automatic weather stations (L. Bergman, T.Hovberg and H. Wibeck). Paper presented at the Third WMO Technical Conference on Instruments and Methods of Observation (TECIMO-III). Instruments and Observing Methods Report No.22 (WMO/TD-No.50). Geneva. ———, 1992: The WMO Automatic Digital Barometer Intercomparison (J.P. van der Meulen). Instruments and Observing Methods Report No.46 (WMO/TD-No.474). Geneva. ———, 2010: Guidance on Instrumentation for Calibration Laboratories, Including Regional Instrument Centres (D. Groselj) (WMO/TD-No. 1543). Instruments and Observing Methods Report No.101. Geneva. ———, 2011(updated in 2018): Manual on Codes. (WMO-No.306). VolumeI.1. Geneva. ———, 2012: A Laboratory Intercomparison of Static Pressure Heads (E. Lanzinger and K. Schubotz). In: Papers and Posters presented at the WMO Technical Conference on Instruments and Methods of Observation (TECO 2012), Brussels, 16–18 October, poster P1(15). Instruments and Observing Methods Report No.109. Geneva ———, 2014: Guide to Meteorological Observing and Information Distribution Systems for Aviation Weather Services (WMO-No.731). Geneva. ———, 2015: Guidance on the Computation of Calibration Uncertainties (J. Duvernoy). Instruments and Observing Methods Report No.119. Geneva.

CHAPTER 4. MEASUREMENT OF HUMIDITY

4.1

GENERAL

The measurement of atmospheric humidity, and often its continuous recording, is an important requirement in most areas of meteorological activity. This chapter deals with the measurement of humidity at or near the Earth’s surface. There are many different methods in use, and there is extensive literature on the subject. Accounts of techniques are given in Burt(2012), Harrison(2014) and Sonntag(1994). An older but still useful wide-ranging account of many measurement principles is given in Wexler(1965). 4.1.1

Definitions

Definitions of the most frequently used quantities in humidity measurements are as follows. Further definitions are found in Annex4.A. Mixing ratio r.  Ratio between the mass of water vapour and the mass of dry air; Specific humidity q.  Ratio between the mass of water vapour and the mass of moist air; Dew-point temperature or dew point td.  The temperature at which moist air saturated with respect to water at a given pressure has a saturation mixing ratio equal to the given mixing ratio; or more simply, the temperature at which moist air is saturated with water vapour; Relative humidity U.  Ratio in per cent of the observed vapour pressure to the saturation vapour pressure with respect to water at the same temperature and pressure; the term “relative humidity” is often abbreviated toRH; Vapour pressure e’.  The partial pressure of water vapour in air; Saturation vapour pressures e’w and e’i.  Vapour pressures in air in equilibrium with the surface of water and ice, respectively. Annex4.B provides the formulae for the computation of various measures of humidity. These versions of the formulae and coefficients were adopted by WMO in1990.1 They are convenient for computation and sufficiently accurate for all normal meteorological applications, strictly within temperature limitation with T>-45°C for liquid water and T>-65°C for ice (WMO,1989a). More accurate, extended in range and detailed formulations of these and other quantities may be found in Sonntag (1990,1994). Other detailed formulations2 are presented in WMO (1966, introductions to tables4.8–10). 4.1.2

Units and scales

The following units and symbols are normally used for expressing the most commonly used quantities associated with water vapour in the atmosphere: (a) Mixing ratio r and specific humidity q (dimensionless quotient of masses, in kilogrammes per kilogramme,kgkg–1); (b) Vapour pressure in air e’, e’w, e’i and pressure p (in units of pressure, suchas hPa);3

1 2 3

Adopted by the Executive Council at its forty-second session through Resolution6 (EC‑XLII). Adopted by the Fourth Congress through Resolution19 (Cg‑IV). 1 hPa = 1 mbar.

150

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(c) Temperature t, wet-bulb temperature tw, dew-point temperature td, and frost-point temperaturetf (in degrees Celsius,°C); (d) Temperature T, wet-bulb temperature Tw, dew-point temperature Td, and frost-point temperatureTf (in K, as used for certain humidity calculations, and for expressing differences, rather than for general expression of humidity values); (e) Relative humidity U (in per cent; the alternative symbol, %RH, is also often used to avoid confusion with other percentages; it is used throughout this chapter). 4.1.3

Meteorological requirements

Humidity measurements at the Earth’s surface are required for meteorological analysis and forecasting, for climate studies, and for many special applications in hydrology, agriculture, aeronautical services and environmental studies, in general. They are particularly important because of their relevance to the changes of state of water in the atmosphere. General requirements for the range, resolution and accuracy of humidity measurements are given in the present volume, Chapter1, Annex1.A. The uncertainties listed in the table are requirements, not performances of any particular instruments. In practice, these uncertainties are not easy to achieve, even using good quality instruments that are well operated and maintained. In particular, the psychrometer in a thermometer shelter without forced ventilation, still in use, may have significantly worse performance. Even modern electronic humidity instruments can suffer drift that is significant relative to the requirements. For most purposes, time constants of the order of 1min are appropriate for humidity measurements. The response times readily available with operational instruments are discussed in4.8.1. 4.1.4

Methods of measurement and observation

General overviews of humidity instruments for meteorology, and their usage, are given by Burt(2012), Harrison(2014) and Sonntag(1994). Wexler(1965) gives somewhat dated, but still useful, details of many hygrometer principles. 4.1.4.1

Overview of general measurement principles

Any instrument for measuring humidity is known as a hygrometer. The physical principles most widely used for humidity measurement in meteorology are given in the following subsections. Reports of WMO international comparisons of various hygrometers are given in WMO (2011a,1989b). The main methods and types of instruments used in meteorology for measuring relative humidity are reviewed here in4.1.4. Some outdated or no longer used methods and instruments are shortly described in Annex4.C. 4.1.4.1.1

Electronic sensing

Electronic relative humidity instruments exploit the change in electrical properties of a material on taking up a variable amount water vapour from the air. For relative humidity measurement, the material is commonly a specialized polymer film coated with electrodes. The measured change in electrical impedance (capacitance or resistance) is scaled to indicate relative humidity. Usually, a compact temperature sensor is also incorporated in the same probe housing.

CHAPTER 4. MEASUREMENT OF HUMIDITY

151

Relative humidity sensor-based hygrometers are increasingly the preferred method for remotereading applications, particularly where a direct reading of relative humidity is required and where data are to be automatically logged. It is essential to have temperature information alongside humidity observations because relative humidity is strongly affected by temperature, and because temperature values are needed to calculate other humidity quantities (such as dew point) from relative humidity. Meteorological observations do not commonly use the integral temperature sensor in an electronic relative humidity instrument; it is normal to use a separate temperature measurement. Capacitive polymer hygrometers are the most convenient and leading technology for meteorological applications, as they are easier to produce, maintain and calibrate. More detail about electrical capacitance hygrometers is given in4.2. Electrical resistance hygrometers, while not commonly in use in meteorology, are nevertheless described in Annex4.C.4. 4.1.4.1.2

Psychrometric method

A psychrometer measures evaporative cooling of a wet surface. The steady-state cooling can be related to the partial pressure of water vapour, and to the relative humidity. A psychrometer consists essentially of two thermometers exposed side by side, with the surface of the sensing element of one being covered with a sleeve maintaining a thin film of water or ice and termed the wet or ice bulb, as appropriate. The sensing element of the second thermometer is simply exposed to the air and is termed the dry bulb. The measurement is either aspirated or under natural ventilation. Owing to evaporation of water from the wet bulb, the temperature measured by the wet-bulb thermometer is generally lower than that measured by the dry bulb. The difference in the temperatures measured by the pair of thermometers is a measure of the humidity of the air; the lower the ambient humidity, the greater the rate of evaporation and, consequently, the greater the depression of the wet-bulb temperature below the dry-bulb temperature. The size of the wetbulb depression is related to the ambient humidity by a psychrometer formula. Psychrometers remain in use for observational purposes, although they are increasingly being replaced by electronic sensor based hygrometers. Psychrometers are also sometimes used as working standards. More detail about this instrument type is given in 4.3. 4.1.4.1.3

Condensation method

The temperature of condensation of water vapour (dew point or frost point) is related to the partial pressure of water and can be measured using a chilled-mirror hygrometer (condensation hygrometer). When moist air is cooled, it eventually reaches its saturation point with respect to water (or to ice) and condensation can form as dew or frost. The temperature of this saturation point is the dew point or frost point. A typical chilled-mirror hygrometer uses a small mirrored surface, cooled using a Peltier-effect device, to obtain a film of water or ice. Usually, optical detection of the condensed film is used in a feedback loop to control the temperature at the threshold of constant condensation. This temperature is measured using an embedded temperature sensor. Air to be measured is typically sampled through tubing and flowed through the instrument.

152

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Condensation hygrometers are not widely used for meteorological observations but are commonly used as laboratory reference instruments. More detail about this instrument type is given in4.4. 4.1.4.1.4

Water vapour spectrometers

The water molecule absorbs EMR in a range of wavebands and discrete wavelengths; this property can be exploited to obtain a measure of the molecular concentration of water vapour in a gas. This principle is used in a variety of instruments, using absorption lines of different strengths for different ranges of measurement (stronger absorption for lower concentrations). In simplest form, an instrument measures the transmission (or absorption) of narrowband IR radiation from a fixed-intensity source to a calibrated detector, sometimes compared to a reference wavelength. Certain instruments based on this principle can measure ranges of humidity observed at ground level. For the trace water vapour range, absorption spectrometers measure the absorption of IR light in multiple reflections through the gas within the measurement cell, giving a long optical path length to extend the range downwards. A particular instrument type is the tunable diode laser spectrometer. The amplitude of light absorption is related to the concentration of water vapour. Cavity ring-down spectroscopy also uses IR absorption through a long path for measuring trace concentrations. A pulse of light is multiply reflected through the gas in a measurement cell. The time taken for the light intensity to decay is measured and is related to the concentration of water vapour. Lyman alpha hygrometers operate in the ultraviolet(UV) range. UV light from an instrument source is absorbed by water molecules in proportion to the concentration of water vapour. The so-called “Lyman alpha line” corresponds to radiation emitted or absorbed during an energy transition of atomic hydrogen. Absorption spectrometers and Lyman alpha instrument are used for some aircraft-borne observations, including measurement of trace levels of water at high flight altitudes. These applications benefit from the relatively fast response time of these instruments. More detail about this instrument type is given in4.5. 4.1.4.1.5

Mechanical methods

Historically, hygrometers have used the dimensional change of organic materials to indicate relative humidity. Water sorption processes of materials are related to relative humidity because the driving force is chemical potential. Sensing elements have included hair and, more recently, synthetic fibres. The change in length with humidity of the sensing element is amplified using a lever system, moving a pointer to indicate relative humidity on a scale, a chart (as a record for the hygrograph), or less commonly via a transducer to an electrical output. Only the hair hygrograph is still in use in meteorology, though phasing out. More detail about this instrument is given in4.6.

CHAPTER 4. MEASUREMENT OF HUMIDITY

4.1.4.2

153

Exposure: general comments

The general requirements for the exposure of humidity instruments are similar to those for temperature sensors, and a suitably positioned thermometer screen can be used for the purpose. Particular requirements include: (a) Protection from direct solar radiation, atmospheric contaminants, rain and wind; (b) Avoidance of the creation of a local microclimate within the instrument housing structure or sampling device. Note that wood and many synthetic materials will adsorb or desorb water vapour according to the atmospheric humidity. Exposures appropriate to particular instruments are described in4.2 to4.6. The siting classification for surface observing stations on land (see the present volume, Chapter1, Annex1.D) provides additional guidance on the selection of a site and the location of a hygrometer within a site to optimize representativeness. 4.1.4.3

Sources of error: general comments

Errors in the measurement of humidity can be caused by any of the following: (a) Modification of the air sample: for example, by a heat or water-vapour source or sink; (b) Contamination of the sensor: for example, by dirt, sea spray, chemical exposure or other pollution; (c) Calibration error, including pressure correction, temperature coefficient of sensor, and electrical interface; (d) Inappropriate treatment of water/ice phase; (e) Intrinsic design weaknesses of instruments: for example, stem heat conduction in the wetbulb thermometer; (f) Slow response time of instrument, or failure to achieve stable equilibrium in operation; (g) Inappropriate sampling and/or averaging intervals; (h) Hysteresis: Many humidity-measuring instruments indicate differently depending on whether they approach the condition after having previously been wetter, or dryer; (i) Long-term drift between calibrations, particularly for electronic humidity measuring instruments in high relative humidity environments; (j)

Radiant heating of the humidity sensor to above the air temperature: for example, due to heating from a radiation screen that is itself warmed by solar radiation;

(k) Error of any kind in temperature measurement, if the temperature value is used in calculating other humidity quantities (for example, calculating dew point from relative humidity). The time constant of the sensor (see4.8.1), the time averaging of the output and the data requirement should be consistent. The different types of humidity-measuring instruments vary in their susceptibility to, and the significance of, each of the above; further discussion will be found in the appropriate sections of this Chapter.

154 4.1.4.4

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Maintenance: general comments

The vast majority of commercially available hygrometers have operating manuals freely available online. These are generally a good source of guidance for maintenance of instruments, and manufacturers are generally willing and able to advise about particular questions. The following maintenance procedures should be considered: (a) Cleanliness: Instruments and housings should be kept clean. Some humidity-measuring instruments, for example, chilled-mirror and hair hygrometers, may be cleaned with distilled water and this should be carried out regularly. Others, notably those having some form of electrolyte coating, but also some with a polymeric substrate, should never be cleaned. The provision of clear instructions for observers and maintenance staff is vital. (b) Calibration of field instruments: Regular calibration is required for all humidity-measuring instruments installed in the field. A calibration identifies any errors in readings by comparison against a reference. Such errors are ideally addressed by applying corrections (for example by adjustment, for an electronic hygrometer). Any uncorrected errors need to be considered as part of the uncertainty of the measurement. Calibrations should be made using a reference with metrological traceability (JCGM,2012) to a national standard wherever possible (see the present volume, Chapter1, Annex1.B). (c) Checking of field instruments is useful in between calibrations. A check against another instrument can be used to assess consistent operation. Results of checks are usually assessed according to a tolerance or criterion based on the uncertainties of the two instruments being compared. Field hygrometers can be checked conveniently using a calibrated electronic hygrometer. An instrument used for such checks should be equilibrated to the local ambient temperature, and should have a response time well within the period allowed for the check. Saturated salt solution systems are commercially available to be used for either checking or calibration. However, they must be equilibrated to the ambient temperature, and the salt mixture itself may need additional equilibration time to generate the correct humidity. It is difficult to be confident about their use in the field, unless used together with a transfer standard (calibrated hygrometer). The use of a standard type of aspirated psychrometer, such as the Assmann, as a field reference has the advantage that some degree of self-checking can be made by comparing the dry- and (unsheathed) wet-bulb thermometers, and that adequate aspiration may be expected from a healthy sounding fan. However, psychrometers emit water vapour in operation, and this can affect the humidity conditions in the surrounding atmosphere, possibly limiting the accuracy of the check if it is close to the instrument being compared. For any calibration or check, the reference instrument should itself be calibrated at intervals that are appropriate to its type. It is important to check the calibration of electrical interfaces regularly and throughout their operational range. A simulator may be used in place of the sensor for this purpose. However, it will still be necessary to calibrate the ensemble at selected points, since the combination of calibration errors for sensor and interface that are individually within specification may be outside the specification for the ensemble. Detailed maintenance requirements specific to each class of hygrometer described in this chapter are included in the appropriate section below.

CHAPTER 4. MEASUREMENT OF HUMIDITY

4.1.5

155

Implications of the Minamata Convention for humidity measurement

The UNEP Minamata Convention on Mercury came into force globally in August 2017 and bans all production, import and export of mercury thermometers (see the present volume, Chapter1, 1.4.2). Therefore, humidity instruments based on mercury thermometers are no longer recommended and it is strongly encouraged to take appropriate measures to replace them with modern alternatives as soon as possible.

4.2

ELECTRICAL CAPACITANCE HYGROMETERS

4.2.1

General considerations

Electronic relative humidity instruments exploit the change in electrical properties of a material on taking up a variable amount water vapour from the air. Water sorption processes of materials are related to relative humidity because the driving force is chemical potential. For relative humidity measurement, the material is commonly a specialized polymer film, coated with electrodes. The measured change in electrical impedance is scaled to indicate relative humidity. Usually, a compact temperature sensor is also incorporated in the same probe housing. The humidity sensor is typically housed in a probe, and this usually incorporates a compact temperature sensor. The sensor region is normally protected by a cage or a filter. In addition, the humidity sensor itself is often directly encased in a protective porous material. The instruments typically incorporate linearizing electronics, with temperature compensation if needed, to optimize accurate response to relative humidity. Manufacturers variously supply display, data-processing, or data-logging systems. In some cases this is integral to the instrument; in others a cable connects to the supporting electronics unit. Hygrometers using electrical relative humidity sensors are increasingly used for remote-reading applications, particularly where a direct display of relative humidity is required. 4.2.2

Electrical capacitance hygrometer

The method is based upon the variation of the dielectric properties of a solid, hygroscopic material in relation to the ambient relative humidity. Sensing dielectric materials are chosen or deliberately developed for humidity sensor purposes. Polymers are most widely used for their stability, selectivity and water sorption, but also because adequate capacitor properties are achieved with such materials. The water bound in the polymer alters its dielectric properties owing to the large dipole moment of the water molecule. Typically, the humidity sensor is built on ceramic or glass substrate. It is a parallel thin film stack with layer thicknesses from a few nanometers to one micrometer. The active part of the humidity sensor consists of a polymer film sandwiched between two electrodes to form a capacitor. The upper electrode is permeable for water molecules and the polymer absorbs water proportional to the relative humidity. The upper electrode may also be covered with a protective layer to improve stability in harsh environments. The capacitance provides a measure of relative humidity. The nominal value of capacitance may be only a few hundred picofarads, depending upon the size of the electrodes and the thickness of the dielectric. This will, in turn, influence the range of excitation frequency used to measure the impedance of the device, which is normally at least several kilohertz and, thus, requires that short connections be made between the sensor and the signal processing electronics to minimize the effect of stray capacitance. Therefore, capacitance sensors often have the signal processing built into the instrument. Typical sensitivity for a 200pF device is 0.5pF per%RH. To prevent condensation when the condition approaches 100 %RH, instrument manufacturers provide different heating options. The sensor may be heated by an integrated heater or the

156

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

whole probe itself is warmed. The heating is controlled using either temperature difference between ambient temperature and internal temperature or a relative humidity threshold. For good measurement results, it is crucial to measure both the temperature of the humidity sensor and the temperature of the ambient air accurately. By using the relative humidity measured by the sensor, the sensor temperature and the temperature of the air, it is possible to calculate the relative humidity of the air. Even without the known air temperature, heated measurement can be used to determine dew-point temperature. Chemical exposure-related drift can be reduced by using an integrated heater to heat the humidity sensor, at repeatable intervals, during a short time at high temperature. A drawback is the dead time during heating. 4.2.3

Observation procedure

Hygrometers using electronic relative humidity sensors are frequently used in AWSs, and wherever unattended or data-logged humidity measurements are needed. Temperature observations are essential alongside humidity observations, since temperature values are used to calculate other humidity quantities (such as dew point) from relative humidity. This normally involves a separate thermometer, not the integral temperature sensor in an electronic relative humidity instrument. 4.2.4

Exposure and siting

Hygrometer probes should be mounted inside a thermometer screen. The manufacturer’s advice regarding the mounting of the actual instrument should be followed. The use of a protective filter is essential to minimize contamination which can cause progressive error. Instruments using hygroscopic electrolyte as a sensing element will be damaged by direct contact with liquid. Capacitive sensors that have been wetted can often recover at least partially after drying. However, exposure to high or condensing humidity is associated with long-term drift of some capacitive sensors. 4.2.5

Sources of error

Measurements using relative humidity sensors can be particularly affected by any of the following causes of error: –

Calibration error can be present, such that the initial adjustment of the instrument leaves residual uncorrected errors. This error can have the character of non-linearity, or some other form. This can also appear to be temperature dependent, since it is typically not possible to calibrate at multiple temperatures, or to implement temperature-dependent calibrations.

Sensors can suffer contamination; for example, by dirt, sea spray, chemical exposure or other pollution. This type of error can take the form of reduced sensitivity across the whole range, with over-reading at low humidity and under-reading at high humidity, or it can follow some other pattern.

Hysteresis can affect electronic humidity instruments, so that they read differently depending on whether they approach the condition after having previously been wetter, or dryer. Response time can also differ for rising and falling changes in condition.

Long-term drift between calibrations can be significant, particularly for instruments exposed to high or condensing relative humidity (dew, fog or other wetting). Such drift is most typically upwards at high humidity although it can be downwards, and varies greatly (Burt,2012; Bell etal., 2017). Upwards drift leads to over-reading at high humidity values, for example indicating 100 %RH at a condition of 95 %RH. Heated sensors are potentially less prone to such drift.

CHAPTER 4. MEASUREMENT OF HUMIDITY

157

Radiant heating of the humidity sensor to above air temperature can mean that the sensor is warmer than the air. This can happen even within a screen if the screen itself is warmed by solar radiation. This can potentially give a falsely low reading of relative humidity.

Error in the measurement of temperature of any kind is significant with relative humidity if both values are used in calculating other humidity quantities (for example, calculating dew point from relative humidity). In such calculations, an error of 0.1 °C near 20°C has the same magnitude of effect as an error of 0.6 %RH. WMO(2011a) details this effect at other temperatures. 4.2.6

Calibration and field inspection

A calibration identifies any errors in readings by comparison against a reference. Calibration of relative humidity instruments is normally a laboratory process that involves comparison against reference for relative humidity, often in a climatic chamber. Calibrations should be made using a reference with metrological traceability to a national standard wherever possible (see the present volume, Chapter1, Annex1.B). Further details are given in4.7 and VolumeV, Chapter4 of the present Guide. Calibrations are ideally implemented by applying corrections (commonly, for an electronic hygrometer, by applying instrument adjustments). For some electronic hygrometers, adjustments can be applied using manufacturer software at the time of calibration. In other cases, adjustments can be made by adjusting potentiometers corresponding to the “range” and “zero” of the hygrometer indication. While calibration corrections can be applied arithmetically, this is more useful in a laboratory application than in meteorology settings. Any uncorrected calibration errors need to be considered as part of the uncertainty of the measurement. Field inspections of electronic relative humidity instruments involve viewing the condition and functioning of the instruments. In particular, the condition of the sensor filter is inspected, and this is cleaned or replaced if it is dirty. Field checks of hygrometers can conveniently be made using another calibrated electronic hygrometer. An instrument used for such checks should be equilibrated to the local ambient temperature. It should either be calibrated at the temperature of use, or allowance should be made for the different temperatures of operation. The hygrometer used for any field check should have a response time well within the time period allowed for the check. A check will normally have a defined criterion for acceptance. In principle, field checks of relative humidity instruments can be made using salt-based systems, which are supplied by some instrument manufacturers. These are only reliable after they are fully equilibrated to the local ambient temperature. Therefore it is difficult to be confident about their use in the field. In principle, a field humidity generator can be used for checking on site, but these are not widely available. Further details are given at4.7.6.3. The use of a standard type of aspirated psychrometer, such as the Assmann, as a field reference has been advocated. However, psychrometers emit water vapour in operation, and this can affect the surrounding conditions of humidity, possibly affecting the accuracy of the check if it is close to the instrument being compared. For any calibration or check, the reference instrument should itself be calibrated at intervals that are appropriate to its type. Where relevant, a check of an electronic hygrometer should include checking the data-logging interfaces. A simulator can possibly be used in place of the sensor for this purpose. Depending on the configuration of the system, whole-system checks (hygrometer plus interface) may be needed. For example, on older systems the combination of calibration errors for sensor and interface that are individually within specification could be outside the specification for the ensemble.

158 4.2.7

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Maintenance

Observers should be encouraged to keep the hygrometer clean (see4.1.4.4). If it is fitted with an interchangeable protective filter-cap, this should be visually inspected for evidence of contamination and replaced if necessary. The body of a hygrometer can be cleaned if necessary using a damp cloth, taking care not to wet the sensor. Electronic elements must not be cleaned in the field, as this would alter their calibration. Time intervals for field servicing and calibration of relative humidity instruments will generally depend on the level of long-term stability expected and required, on the location, and on the availability of facilities and personnel. The lifetime before failure, for electronic relative humidity instruments in service in weather stations in damp climates, is commonly between six months and two or more years. There is often significant sensor drift on shorter timescales. The cause of failure (especially early failures) is commonly the sensor element. Usually, this can be replaced, and the hygrometer recalibrated before being used again. In some cases, field servicing of an electronic hygrometer will mean the replacement of a failed instrument. In other cases, field hygrometers are replaced (perhaps annually) with a newly calibrated instrument, and the one taken out of use is sent for servicing, recalibration and (if satisfactory) re-deployment. If a hygrometer has failed, often this can be remedied by the replacement of just the sensor element, followed by recalibration. In order to address the tendency of sensors to drift, a more intensive management approach can be adopted where resources allow. Sensor drift can be evaluated on return from the field, by comparison against a reference in a calibration facility. Those instruments showing minor drift can be adjusted and then recalibrated for redeployment. However, these can be expected to have worse ongoing reliability than new instruments. Those instruments that are found to have more extreme in-field drift can be refurbished (by buying a new sensing element, changing it in the laboratory and calibrating the renewed instrument). However, after a number of deployments, performance can be expected to worsen, and a policy of routinely replacing these hygrometers after a defined period can lead to improved overall reliability of the observations. The vast majority of commercially available hygrometers have operating manuals freely available online. These are generally a good source of guidance for maintenance of instruments, and manufacturers are generally willing and able to advise about particular questions.

4.3

THE PSYCHROMETER

4.3.1

General considerations

4.3.1.1

Psychrometric formulae

The usual practice is to derive the vapour pressure e’ under the conditions of observation from the following semi-empirical psychrometric formulae:

e′ = e′w ( p, tw ) − Ap ( t − tw ) (4.1)

and:

e′ = ei′ ( p, ti ) − Ap ( t − ti ) (4.2)

where e’wis the saturation vapour pressure with respect to water at temperature t w and pressurep of the wet bulb; e’iis the saturation vapour pressure with respect to ice at temperature ti and pressure p of the ice bulb; pis the pressure of the air; tis the temperature of the dry bulb; and Ais the psychrometer coefficient (the latter is preferred to the term “psychrometer constant”, which is a misnomer). The formulae and coefficients appropriate for the various forms of psychrometer are discussed in the following sections.

CHAPTER 4. MEASUREMENT OF HUMIDITY

4.3.1.2

159

The specification of a psychrometer

The equipment used for psychrometric observations should, as far as practicable, conform to the following recommendations: (a) At sea level, and in the case where the thermometers are of the types ordinarily used at meteorological stations, air should be drawn past the thermometer bulbs at a rate of no less than 2.2ms–1 and no greater than 10ms–1. For appreciably different altitudes, these air speed limits should be adjusted in inverse proportion to the density of the atmosphere; (b) The wet and dry bulbs must be protected from radiation, preferably by a minimum of two shields. In a psychrometer with forced ventilation, such as the Assmann, the shields may be of polished, unpainted metal, separated from the rest of the apparatus by insulating material. Thermally insulating material is preferable in principle and must be used in psychrometers which rely on natural ventilation; (c) If the psychrometer is exposed in a louvred screen with forced ventilation, separate ventilation ducts should be provided for the two thermometers. The entrance to the ducts should be located so as to yield a measurement of the true ambient temperature, and the air should be exhausted above the screen in such a way as to prevent recirculation; (d) The greatest care should be taken to prevent the transfer of significant amounts of heat from an aspirating motor to the thermometers; (e) The water reservoir and wick should be arranged in such a way that the water will reach the bulb with sensibly the wet-bulb temperature, so as not to affect the temperature of the dry bulb. 4.3.1.3

The wet-bulb sleeve

The wet bulb usually has a cotton wick, or similar fabric, fitting closely around the sensing element in order to maintain an even covering of water, which is either applied directly or by some form of capillary feed from a reservoir. The wick commonly takes the form of a sleeve that has a good fit around the bulb and extends at least 2cm up the stem of the thermometer to give extended cooling, to reduce stem conduction. Distilled water should be used for the wet bulb. The fabric used to cover the wet bulb should be thin and closely woven. Where the supplier offers a wick designed for the size of the thermometers, this should be used. Before installation, it should be washed thoroughly in an aqueous solution of sodium bicarbonate (NaHCO3) at a dilution of5g per litre, and rinsed several times in distilled water. Alternatively, boiling in a dilute solution of pure detergent in water may be performed, followed by boiling in distilled water. Great care should be exercised in handling the clean sleeve or wick to prevent contamination from hands, for example by using tweezers that have been cleaned, or clean plastic residue-free gloves. The proper management of the wet bulb is particularly important. Any visible contamination of the wick or the wet-bulb sleeve should be considered an absolute indication of the necessity for its immediate replacement. Otherwise, observers should be encouraged to change the wetbulb sleeve and wick at least once a week for all psychrometers that are continuously exposed. At places near the sea and industrialized districts it may be necessary to replace these items more frequently. The water supply should be checked frequently and replaced or replenished as required. Under hot, dry conditions, it can be an advantage to wet the covering with water from a porous vessel. This will cause the water to be pre-cooled by evaporation from the porous surface. The vessel should be kept in the shade, but not in the immediate vicinity of the psychrometer.

160 4.3.1.4

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Operation of the wet bulb below freezing

The psychrometer is difficult to operate at temperatures below freezing, but it is still used in climates where such temperatures occur. A wick cannot be used to convey water from a reservoir to the wet-bulb sleeve by capillary action when the wick is frozen. Under these conditions, care should be taken to allow the formation of only a thin layer of ice on the sleeve. It is an absolute necessity that the thermometers be artificially ventilated; if they are not, the management of the wet bulb will be extremely difficult. The wet bulb of the aspirated and sling psychrometers should be moistened immediately before use. The water should, as far as possible, have a temperature close to freezing point. If a button of ice forms at the lowest part of the bulb, it should be immersed in water long enough to melt the ice. The time required for the wet bulb to reach a steady reading after the sleeve is wetted depends on the ventilation rate and the actual wet-bulb temperature. An unventilated thermometer usually requires from15 to 45min, while an aspirated thermometer will require a much shorter period. It is essential that the formation of a new ice film on the bulb be made at an appropriate time. If hourly observations are being made with a simple psychrometer, it will usually be preferable to form a new coating of ice just after each observation. If the observations are made at longer intervals, the observer should visit the screen sufficiently in advance of each observation to form a new ice film on the bulb. The evaporation of an ice film between readings can be prevented or slowed by enclosing the wet bulb in a small glass tube, or by stopping the ventilation inlet of the wet bulb between periods of measurement. If this is done the wet-bulb temperature will not be accurate during these interventions. (Note that the latter course should not be taken if the circumstances are such that the ventilating fan would overheat.) The effect of supercooled water on the wet bulb may be dealt with in two ways: (a) By using different formulae or tables when the wet bulb is coated with ice and with supercooled water, respectively. To find out which table should be used, the wet bulb should be touched with a snow crystal, a pencil, needle, or other object, just after each observation is completed. The degree of gloss on the surface of the wet-bulb is also useful to check if the wet bulb is frozen. If the temperature rises towards 0°C, and then commences to fall again, it can be assumed that the water on the wet bulb was supercooled at the time of the observation; (b) By using a formula or table appropriate for an ice-covered wet bulb, and inducing the freezing of supercooled water in the same way as for method(a). In order to save time and to ensure that the wet bulb is ice-covered, the observer should make a point of initiating the freezing of the water at each observation as soon as possible after moistening the bulb. From the behaviour of the wetted thermometer at the freezing point it may usually be determined whether the bulb is covered by ice or by supercooled water. The recommended procedure, however, is to initiate the freezing of the water at each observation when the wet-bulb temperature is assumed to be below 0°C, regardless of whether the behaviour of the thermometer after moistening has been observed or not. Although the first method is usually the quickest, it requires two tables and this may cause some confusion.

CHAPTER 4. MEASUREMENT OF HUMIDITY

4.3.1.5

161

General procedure for making observations

The procedures outlined in the present volume, Chapter2, for the measurement of temperature should be followed, in addition to the following procedures: (a) If the wet-bulb sleeve, wick or water has to be changed, this should be done sufficiently in advance of the observation. The period required for the correct wet-bulb temperature to be attained will depend upon the type of psychrometer; (b) The thermometers should be read to the nearest 0.1degree; (c) When making an observation, the readings of the two thermometers should, as far as possible, be taken simultaneously (reading first the dry thermometer, then the wet one, and finally the dry one again is a reasonable solution) and it should be ascertained that the wet bulb is receiving a sufficient water supply. 4.3.1.6

Use of electrical resistance thermometers

Precision platinum electrical resistance thermometers are widely used in place of liquid-in-glass thermometers, in particular where remote reading and continuous measurements are required. It is necessary to ensure that the devices, and the related electronics, meet the performance requirements. These are detailed in the present volume, Chapter2. Particular care should always be taken with regard to self-heating effects in electrical thermometers. The psychrometric formulae in Annex4.B used for Assmann aspiration psychrometers are also valid if PRTs are used in place of the mercury-in-glass instruments, with different configurations of elements and thermometers. The formula for water on the wet bulb is also valid for some transversely ventilated psychrometers (WMO,1989a). 4.3.1.7

Psychrometric formulae and tables

The following paragraphs summarize some existing principles and practice in drawing up psychrometric tables. The wet-bulb thermometer temperature Tw for most instruments is not identical to the theoretical thermodynamic wet-bulb temperature, defined in Annex4.A, which depends only upon p, T andr(the humidity mixing ratio). The temperature measured by a practical wet-bulb thermometer depends also upon a number of variables that are influenced by the dynamics of heat transfer across a liquid/gas interface (in which the gas must be characterized in terms of its component laminar and turbulent layers). The description of a satisfactory thermodynamic model is beyond the scope of this publication. The inequality of the thermodynamic and measured wet-bulb temperatures is resolved in practice through the empirical determination of the psychrometer coefficientA (WMO,1992). In general, coefficient A depends upon the design of the psychrometer (in particular the wetbulb system), the diameter of the thermometers, the rate of airflow past the wet bulb (termed the ventilation rate), and the air temperature and its humidity. At low rates of ventilation, A depends markedly upon the ventilation rate. However, at ventilation rates of3 to 5ms–1 (for thermometers of conventional dimensions) or higher, the value of A becomes substantially independent of the ventilation rate and is practically the same for all well-designed psychrometers. The value ofA does not, then, depend very much on temperature or humidity and its dependence on these variables is usually ignored. Ais smaller when the wet bulb is coated with ice than when it is covered with water.

162

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

4.3.1.8

Sources of error in psychrometry

The following main sources of error must be considered: (a) Errors of the thermometers: It is very important in psychrometric measurements that the errors of the thermometers be known over the actual temperature range and that corrections for these errors be applied to the readings before the humidity formulae tables are used. In general, thermometers should be pre-selected to have minimum errors.

Any other errors in the wet-bulb or ice-bulb temperature caused by other influences will appear in the same way as thermometer errors.

Table4.1 shows the error in relative humidityε(U), derived from wet- and ice-bulb measurements having errorsε(tx), where xis water for t>0°C and ice for t 20

0.10

5

High crops; scattered obstacles, 15 < x/H < 20

0.25

6

Parkland, bushes; numerous obstacles, x/H≈10

0.5

7

Regular large obstacle coverage (suburb, forest)

1.0

8

City centre with high- and low-rise buildings

≥2

Note: Here x is a typical upwind obstacle distance and H is the height of the corresponding major obstacles. For more detailed and updated terrain class descriptions see Davenport et al. (2000) (see also VolumeIII, Chapter9, Table9.2 of the present Guide).

REFERENCES AND FURTHER READING

Ackermann, G.R., 1983: Means and standard deviations of horizontal wind components. Journal of Climate and Applied Meteorology, 22:959–961. Albers, A., H. Klug and D. Westermann, 2000: Outdoor comparison of cup anemometers. DEWI Magazin, No.17. Beljaars, A.C.M., 1987: The influence of sampling and filtering on measured wind gusts. Journal of Atmospheric and Oceanic Technology, 4:613–626. Busch, N.E. and L. Kristensen, 1976: Cup anemometer overspeeding. Journal of Applied Meteorology, 15:1328–1332. Coppin, P.A., 1982: An examination of cup anemometer overspeeding. Meteorologische Rundschau, 35:1–11. Curran, J.C., G.E. Peckham, D. Smith, A.S. Thom, J.S.G. McCulloch and I.C. Strangeways, 1977: Cairngorm summit automatic weather station. Weather, 32:60–63. Davenport, A.G., 1960: Rationale for determining design wind velocities. Journal of the Structural Division, American Society of Civil Engineers, 86:39–68. Davenport, A.G., C.S.B. Grimmond, T.R. Oke and J. Wieringa, 2000: Estimating the roughness of cities and sheltered country. Preprints of the Twelfth American Meteorological Society Conference on Applied Climatology (Asheville, NC, United States), pp. 96–99. Evans, R.A. and B.E. Lee, 1981: The problems of anemometer exposure in urban areas: a wind-tunnel study. Meteorological Magazine, 110:188–189. Frenkiel, F.N., 1951: Frequency distributions of velocities in turbulent flow. Journal of Meteorology, 8:316–320. Gill, G.C., L.E. Olsson, J. Sela and M. Suda, 1967: Accuracy of wind measurements on towers or stacks. Bulletin of the American Meteorological Society, 48:665–674. Gold, E., 1936: Wind in Britain: The Dines anemometer and some notable records during the last 40years. Quarterly Journal of the Royal Meteorological Society, 62:167–206. Grimmond, C.S.B., T.S. King, M. Roth and T.R. Oke, 1998: Aerodynamic roughness of urban areas derived from wind observations. Boundary-Layer Meteorology, 89:1–24. International Organization for Standardization, 2002: Meteorology – Sonic Anemometers/Thermometers – Acceptance Test Methods for Mean Wind Measurements, ISO16622:2002. Geneva. ———, 2007: Meteorology – Wind Measurements – PartI: Wind Tunnel Test Methods for Rotating Anemometer Performance, ISO17713-1:2007. Geneva. Kaimal, J.C., 1980: Sonic anemometers. In: Air-sea Interaction: Instruments and Methods (F. Dobson, L.Hasse and R. Davis, eds.). Plenum Press, New York, pp. 81–96. Lenschow, D.H. (ed.), 1986: Probing the Atmospheric Boundary Layer. American Meteorological Society, Boston. MacCready, P.B., 1966: Mean wind speed measurements in turbulence. Journal of Applied Meteorology, 5:219–225. MacCready, P.B. and H.R. Jex, 1964: Response characteristics and meteorological utilization of propeller and vane wind sensors. Journal of Applied Meteorology, 3:182–193. Makinwa, K.A.A., J.H. Huijsing and A. Hagedoorn, 2001: Industrial design of a solid-state wind sensor. Proceedings of the First ISA/IEEE Conference (Houston, November2001), pp.68–71. Mazzarella, D.A., 1972: An inventory of specifications for wind-measuring instruments. Bulletin of the American Meteorological Society, 53:860–871. Mollo-Christensen, E. and J.R. Seesholtz, 1967: Wind tunnel measurements of the wind disturbance field of a model of the Buzzards Bay Entrance Light Tower. Journal of Geophysical Research, 72:3549–3556. Patterson, J., 1926: The cup anemometer. Transactions of the Royal Society of Canada, 20(III):1–54. Smith, S.D., 1980: Dynamic anemometers. In: Air-sea Interaction: Instruments and Methods (F. Dobson, L.Hasse and R. Davis, eds.). Plenum Press, New York, pp.65–80. Taylor, P.A. and R.J. Lee, 1984: Simple guidelines for estimating wind speed variations due to small scale topographic features. Climatological Bulletin, Canadian Meteorological and Oceanographic Society, 18:3–22. van Oudheusden, B.W. and J.H. Huijsing, 1991: Microelectronic thermal anemometer for the measurement of surface wind. Journal of Atmospheric and Oceanic Technology, 8:374–384. Verkaik, J.W., 2000: Evaluation of two gustiness models for exposure correction calculations. Journal of Applied Meteorology, 39:1613–1626. Walmsley, J.L., I.B. Troen, D.P. Lalas and P.J. Mason, 1990: Surface-layer flow in complex terrain: Comparison of models and full-scale observations. Boundary-Layer Meteorology, 52:259–281. Wieringa, J., 1967: Evaluation and design of wind vanes. Journal of Applied Meteorology, 6:1114–1122.

CHAPTER 5. MEASUREMENT OF SURFACE WIND

213

———, 1980a: A revaluation of the Kansas mast influence on measurements of stress and cup anemometer overspeeding. Boundary-Layer Meteorology, 18:411–430. ———, 1980b: Representativeness of wind observations at airports. Bulletin of the American Meteorological Society, 61:962–971. ———, 1983: Description requirements for assessment of non-ideal wind stations, for example Aachen. Journal of Wind Engineering and Industrial Aerodynamics,11:121–131. ———, 1986: Roughness-dependent geographical interpolation of surface wind speed averages. Quarterly Journal of the Royal Meteorological Society, 112:867–889. ———, 1996: Does representative wind information exist? Journal of Wind Engineering and Industrial Aerodynamics, 65:1–12. Wieringa, J. and F.X.C.M. van Lindert, 1971: Application limits of double-pin and coupled wind vanes. Journal of Applied Meteorology, 10:137–145. World Meteorological Organization, 1981: Review of Reference Height for and Averaging Time of Surface Wind Measurements at Sea (F.W. Dobson). Marine Meteorology and Related Oceanographic Activities Report No.3. Geneva. ———, 1984a: Compendium of Lecture Notes for Training Class IV Meteorological Personnel (B.J. Retallack). (WMO-No.266), VolumeII. Geneva. ———, 1984b: Distortion of the wind field by the Cabauw Meteorological Tower (H.R.A. Wessels). Paper presented at the WMO Technical Conference on Instruments and Cost-effective Meteorological Observations (TECEMO). Instruments and Observing Methods Report No.15. Geneva. ———, 1987: The Measurement of Gustiness at Routine Wind Stations: A Review (A.C.M. Beljaars). Instruments and Observing Methods Report No.31. Geneva. ———, 1989: Wind Measurements Reduction to a Standard Level (R.J. Shearman and A.A. Zelenko). Marine Meteorology and Related Oceanographic Activities Report No.22 (WMO/TD-No.311). Geneva. ———, 1990: Abridged Final Report of the Tenth Session of the Commission for Instruments and Methods of Observation (WMO-No.727). Geneva. ———, 1991: Guidance on the Establishment of Algorithms for Use in Synoptic Automatic Weather Stations: Processing of Surface Wind Data (D. Painting). Report of the CIMO Working Group on Surface Measurements, Instruments and Observing Methods Report No.47 (WMO/TD-No.452). Geneva. ———, 2000: Wind measurements: Potential wind speed derived from wind speed fluctuations measurements, and the representativity of wind stations (J.P. van der Meulen). Paper presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2000). Instruments and Observing Methods Report No.74 (WMO/TD-No.1028). Geneva. ———, 2001: Lecture Notes for Training Agricultural Meteorological Personnel (J. Wieringa and J.Lomas) (WMONo.551). Geneva. ———, 2011 (updated in 2017): Manual on Codes (WMO-No.306), VolumeI.1. Geneva. Wyngaard, J.C., 1981: The effects of probe-induced flow distortion on atmospheric turbulence measurements. Journal of Applied Meteorology, 20:784–794.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

6.1

GENERAL

This chapter describes the well-known methods of precipitation measurements at ground stations. It also addresses precipitation intensity measurements (in particular the rate of rainfall or rainfall intensity) due to the rapidly increasing need for such measurements for the interpretation of rainfall patterns, rainfall event modelling and forecasts. While this chapter does include measurement of precipitation in the form of snow and other solid products of the condensation of water vapour, measurement of snow on the ground and new snow are discussed in detail in VolumeII, Chapter2 of the present Guide. This chapter does not discuss measurements which attempt to define the structure and character of precipitation, or which require specialized instrumentation, which are not standard meteorological observations (such as drop size distribution). Marine and radar measurements are discussed in VolumeIII, Chapters4 and 7respectively of the present Guide, while space‑based observations are discussed in VolumeIV. The general problem of representativeness is particularly acute in the measurement of precipitation. Precipitation measurements are particularly sensitive to exposure, wind and topography, and metadata describing the circumstances of the measurements are particularly important for users of the data. The analysis of precipitation data is much easier and more reliable if the same gauges and siting criteria are used throughout the networks. This should be a major consideration in designing networks. 6.1.1

Definitions

Precipitation is defined as the liquid or solid products of the condensation of water vapour falling from clouds, in the form of rain, drizzle, snow, snow grains, snow pellets, hail and ice pellets; or falling from clear air in the form of diamond dust. Solid precipitation is less dense than liquid precipitation and more variable in terms of structure (for example, different ice crystal shapes, or “habits”) and related aerodynamics. Moisture can also be transferred to the ground through dew, rime, hoar frost, or fog, but these forms of deposited particles are not included in the definition of precipitation. Nevertheless, they are described in 6.6. The total amount of precipitation which reaches the ground in a stated period is expressed in terms of the vertical depth of water (or water equivalent in the case of solid forms) to which it would cover a horizontal projection of the Earth’s surface. Precipitation intensity is defined as the amount of precipitation collected per unit time interval. According to this definition, precipitation intensity data can be derived from the measurement of precipitation amount using an ordinary precipitation gauge. In that sense, precipitation intensity is a secondary parameter, derived from the primary parameter precipitation amount. However, precipitation intensity can also be measured directly (see 6.1.4.1). 6.1.2

Units and scales

The unit of precipitation is linear depth, usually in millimetres (volume/area), or kgm–2 (mass/area) for liquid precipitation. Daily amounts of precipitation should be read to the nearest

CHAPTER 6. MEASUREMENT OF PRECIPITATION

215

0.2mm and, if feasible, to the nearest 0.1mm; weekly or monthly amounts should be read to the nearest 1mm (at least). Daily measurements of precipitation should be taken at fixed times common to the entire network or networks of interest. Less than 0.1mm (or 0.2mm depending on the resolution used) is generally referred to as a trace. The measurement unit of rainfall intensity is linear depth per hour, usually in millimetres per hour (mmh–1). Rainfall intensity is normally measured or derived over one-minute time intervals due to the high variability of intensity from minute to minute. 6.1.3

Meteorological and hydrological requirements

Chapter 1, Annex 1.A of the present volume gives a broad statement of the requirements for uncertainty, range and resolution for precipitation measurements. The common observation times are hourly, three-hourly and daily, for synoptic, climatological and hydrological purposes. For some purposes, such as the design and management of urban drainage systems, forecasting and mitigation of flash floods, transport safety measures, and in general most of the applications where rainfall data are sought in real time, a much greater time resolution is required to measure very high rainfall rates over very short periods (typically 1min for rainfall intensity). For some other applications, storage gauges are used with observation intervals of weeks or months or even a year in mountains and deserts. 6.1.4

Measurement methods

6.1.4.1

Instruments

Precipitation gauges (or raingauges if only liquid precipitation can be measured) are the most common instruments used to measure precipitation. Generally, an open receptacle with vertical sides is used, usually in the form of a right cylinder, with a funnel if its main purpose is to measure rain. Since various sizes and shapes of orifice and gauge heights are used in different countries, the measurements are not strictly comparable (WMO, 1989a). The volume or weight of the catch is measured, the latter in particular for solid precipitation. The gauge orifice may be at one of many specified heights above the ground or at the same level as the surrounding ground. The orifice must be placed above the maximum expected depth of snow cover, and above the height of significant potential in-splashing from the ground. The most commonly used elevation height in more than 100countries varies between 0.5 and 1.5m (WMO, 1989a). The measurement of precipitation is very sensitive to exposure, and in particular to wind. For solid precipitation measurement, which is more susceptible to wind effect than liquid precipitation measurement due to the lower density of hydrometeors, an artificial shield should be placed around the gauge orifice. Section6.2 discusses exposure, while section6.4 discusses at some length the errors to which precipitation gauges are prone, and the corrections that may be applied. Rainfall intensity can be either derived from the measurement of precipitation amount using a recording raingauge (see6.5) or measured directly. The latter can be done, for example, by using a gauge and measuring the flow of the captured water, measuring the accretion of collected water as a function of time, or using some optical principles of measurement. A number of techniques for determining precipitation amount are based on these direct intensity measurements by integrating the measured intensity over a certain time interval. This chapter also refers to some other special techniques for measuring solid precipitation, and other types of precipitation (dew, and the like). Some techniques that are in operational use are not described here; for example, the optical raingauge, which makes use of optical scattering. Useful sources of information on new methods under development are the reports of recurrent conferences, such as the Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO), the international workshops on precipitation

216

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

measurement (for example, Slovak Hydrometeorological Institute and Swiss Federal Institute of Technology, 1993; WMO, 1989b), and the instrument intercomparisons organized by CIMO (for example, WMO, 1998). Point measurements of precipitation serve as the primary source of data for areal analysis. However, even the best measurement of precipitation at one point is only representative of a limited area, the size of which is a function of the length of the accumulation period, the physiographic homogeneity of the region, local topography and the precipitation-producing process. Radar and satellites are used to define and quantify the spatial distribution of precipitation. In principle, a suitable integration of all three sources of areal precipitation data into national precipitation networks (automatic gauges, radar, and satellite) can be expected to provide sufficiently accurate areal precipitation estimates on an operational basis for a wide range of precipitation data users. Instruments that detect and identify precipitation, as distinct from measuring it, may be used as present weather sensors, and are referred to in the present volume, Chapter14. 6.1.4.2

Reference gauges and intercomparisons

Several types of gauges have been used as reference gauges. The main feature of their design is that of reducing or controlling the effect of wind on the catch, which is the main reason for the different behaviours of gauges. They are chosen also to reduce the other errors discussed in 6.4. Ground-level gauges are used as reference gauges for liquid precipitation measurement. Because of the near absence of wind-induced error, they generally show more precipitation than any elevated gauge (WMO, 1984, 2009). The gauge is placed in a pit with the gauge rim at ground level, sufficiently distant from the nearest edge of the pit to avoid in-splashing. A strong plastic or metal anti-splash grid with a central opening for the gauge should span the pit. Provision should be made for draining the pit. A description and drawings of a standard pit gauge are given in Annex6.A and more details are provided in WMO (2009) and the EN13798:2010 standard (European Committee for Standardization (CEN), 2010). The reference gauge for solid precipitation is the gauge known as the Double Fence Intercomparison Reference (DFIR). It has octagonal vertical double fences surrounding a Tretyakov gauge, which itself has a particular form of wind-deflecting shield. Drawings and a description are given by Goodison et al. (1989) and in WMO (1985, 1998). Recommendations for comparisons of precipitation gauges against the reference gauges are given in Annex6.B. 6.1.4.3

Documentation

The measurement of precipitation is particularly sensitive to gauge exposure, so metadata about the measurements must be recorded meticulously to compile a comprehensive station history, in order to be available for climate and other studies and QA. Section 6.2 discusses the site information that must be kept, namely detailed site descriptions, including vertical angles to significant obstacles around the gauge, gauge configuration, height of the gauge orifice above ground and height of the wind speed measuring instrument above ground. Changes in observational techniques for precipitation, mainly the use of a different type of precipitation gauge and/or a change of gauge site or configuration (for example, installation height, wind shield) can cause temporal inhomogeneities in precipitation time series (seeVolumeV, Chapter2 of the present Guide). The use of differing types of gauges and site exposures causes spatial inhomogeneities. This is due to the systematic errors of precipitation measurement, mainly the wind-induced error. Since adjustment techniques based on statistics

CHAPTER 6. MEASUREMENT OF PRECIPITATION

217

can remove the inhomogeneities relative to the measurements of surrounding gauges, the correction of precipitation measurements for the wind-induced error can reduce the bias of measured values. The following sections (especially6.4) on the various instrument types discuss the corrections that may be applied to precipitation measurements. Such corrections have uncertainties, and the original records and the correction formulae should be kept. Any changes in the observation methods should also be documented.

6.2

SITING AND EXPOSURE

All methods for measuring precipitation should aim to obtain a sample that is representative of the true amount falling over the area which the measurement is intended to represent, whether on the synoptic scale, mesoscale or microscale. The choice of site, as well as the systematic measurement error, is, therefore, important. For a discussion of the effects of the site, see Sevruk and Zahlavova (1994). The location of precipitation stations within the area of interest is important, because the number and locations of the gauge sites determine how well the measurements represent the actual amount of precipitation falling in the area. Areal representativeness is discussed at length in WMO (1992a), for rain and snow. WMO (2008) gives an introduction to the literature on the calculation of areal precipitation and corrections for topography. The effects on the wind field of the immediate surroundings of the site can give rise to local excesses and deficiencies in precipitation. In general, objects should not be closer to the gauge than a distance of twice their height above the gauge orifice. For each site, the average vertical angle of obstacles should be estimated, and a site plan should be made. Sites on a slope or the roof of a building should be avoided. Sites selected for measuring snowfall and/or snow cover should be in areas sheltered as much as possible from the wind. The best sites are often found in clearings within forests or orchards, among trees, in scrub or shrub forests, or where other objects act as an effective wind-break for winds from all directions. Preferably, however, the effects of the wind, and of the site on the wind, can be reduced by using a ground-level gauge for liquid precipitation or by making the airflow horizontal above the gauge orifice using the following techniques (listed in order of decreasing effectiveness): (a) In areas with homogeneous dense vegetation; the height of such vegetation should be kept at the same level as the gauge orifice by regular clipping; (b) In other areas, by simulating the effect in (a) through the use of appropriate fence structures, such as that used for the DFIR; (c) By using windshields around the gauge. The surface surrounding the precipitation gauge can be covered with short grass, gravel or shingle, but hard, flat surfaces, such as concrete, should be avoided to prevent excessive insplashing. A classification of measurement sites has been developed in order to quantify and document the influence of the surrounding environment (see the present volume, Chapter1, Annex1.D). This classification uses a relatively simple description of the (land-based) sites.

218

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

6.3

NON-RECORDING PRECIPITATION GAUGES

6.3.1

Ordinary gauges

6.3.1.1

Instruments

The commonly used precipitation gauge consists of a collector placed above a funnel leading into a container where the accumulated water and melted snow are stored between observation times. Different gauge shapes are in use worldwide as shown in Figure6.1. Where solid precipitation is common and substantial, a number of special modifications are used to improve the accuracy of measurements. Such modifications include the removal of the raingauge funnel at the beginning of the snow season or the provision of a special snow fence (see WMO, 1998) to protect the catch from blowing out. Windshields around the gauge reduce the error caused by deformation of the wind field above the gauge and by snow drifting into the gauge. They are advisable for rain and essential for snow. A wide variety of gauges are in use (see WMO, 1989a). The stored water is either collected in a measure or poured from the container into a measure, or its level in the container is measured directly with a graduated stick. The size of the collector orifice is not critical for liquid precipitation, but an area of at least 200cm2 is required if solid forms of precipitation are expected in significant quantity. An area of 200 to 500cm2 will probably be found most convenient. The most important requirements of a gauge are as follows: (a) The rim of the collector should have a sharp edge and should fall away vertically on the inside, and be steeply bevelled on the outside; the design of gauges used for measuring snow should be such that any narrowing of the orifice caused by accumulated wet snow about the rim is small; (b) The area of the orifice should be known to the nearest 0.5%, and the construction should be such that this area remains constant while the gauge is in normal use;

1

4

2

3

5

6

Figure 6.1. Different shapes of standard precipitation gauges. The solid lines show streamlines and the dashed lines show the trajectories of precipitation particles. The first gauge shows the largest wind field deformation above the gauge orifice, and the last gauge the smallest. Consequently, the wind-induced error for the first gauge is larger than for the last gauge. Source: Sevruk and Nespor (1994)

CHAPTER 6. MEASUREMENT OF PRECIPITATION

≥ 90°

219

These lines must intersect the vertical wall below the rim of the gauge.

≥ 90°

Figure 6.2. Suitable collectors for raingauges (c) The collector should be designed to prevent rain from splashing in and out. This can be achieved if the vertical wall is sufficiently deep and the slope of the funnel is sufficiently steep (at least 45%). Suitable arrangements are shown in Figure6.2; (d) The construction should be such as to minimize wetting errors. This can be done by choosing the proper material and minimizing the total inner surface of the collector; (e) The container should have a narrow entrance and be sufficiently protected from radiation to minimize the loss of water by evaporation. Precipitation gauges used in locations where only weekly or monthly readings are practicable should be similar in design to the type used for daily measurements, but with a container of larger capacity and stronger construction. The measuring cylinder should be made of clear glass or plastic which has a suitable coefficient of thermal expansion and should be clearly marked to show the size or type of gauge with which it is to be used. Its diameter should be less than 33% of that of the rim of the gauge; the smaller the relative diameter, the greater the precision of measurement. The graduations should be finely engraved; in general, there should be marks at 0.2mm intervals and clearly figured lines at each whole millimetre. It is also desirable that the line corresponding to 0.1mm be marked. The maximum error of the graduations should not exceed ±0.05mm at or above the 2mm graduation mark and ±0.02mm below this mark. To measure small precipitation amounts with adequate precision, the inside diameter of the measuring cylinder should taper off at its base. In all measurements, the bottom of the water meniscus should define the water level, and the cylinder should be kept vertical when reading, to avoid parallax errors. Repetition of the main graduation lines on the back of the measure is also helpful for reducing such errors. Dip-rods should be made of cedar wood, or another suitable material that does not absorb water appreciably and possesses only a small capillary effect. Wooden dip-rods are unsuitable if oil has been added to the collector to suppress evaporation. When this is the case, rods made of metal or other materials from which oil can be readily cleaned must be used. Non-metallic rods should be provided with a brass foot to avoid wear and be graduated according to the relative areas of cross-section of the gauge orifice and the collector; graduations should be marked at least every 10mm and include an allowance for the displacement caused by the rod itself. The maximum error in the dip-rod graduation should not exceed ±0.5mm at any point. A dip-rod measurement should be checked using a volumetric measure, wherever possible. 6.3.1.2

Operation

The measuring cylinder must be kept vertical when it is being read, and the observer must be aware of parallax errors. Snow collected in non-recording precipitation gauges should be either weighed or melted immediately after each observation and then measured using a standard

220

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

graduated measuring cylinder. It is also possible to measure precipitation catch by accurate weighing, a procedure which has several advantages. The total weight of the can and contents is measured and the known weight of the can is subtracted. There is little likelihood of spilling the water and any water adhering to the can is included in the weight. The commonly used (volumetric) methods are, however, simpler and cheaper. 6.3.1.3

Calibration and maintenance

The graduation of the measuring cylinder or stick must, of course, be consistent with the chosen size of the collector. The calibration of the gauge, therefore, includes checking the diameter of the gauge orifice and ensuring that it is within allowable tolerances. It also includes volumetric checks of the measuring cylinder or stick. For measurements based on weight, regular calibration of the weighing balance is required. Routine maintenance should include, at all times, keeping the gauge level in order to prevent an out-of-level gauge (see Rinehart, 1983; Sevruk, 1984). As required, the outer container of the gauge and the graduate should be kept clean at all times both inside and outside by using a long-handled brush, soapy water and a clean water rinse. Worn, damaged or broken parts should be replaced, as required. The vegetation around the gauge should be kept trimmed to 5cm (where applicable). The exposure should be checked and recorded. 6.3.2

Storage gauges

Storage gauges are used to measure total seasonal precipitation in remote and sparsely inhabited areas. Such gauges consist of a collector above a funnel, leading into a container that is large enough to store the seasonal catch (or the monthly catch in wet areas). A layer of no less than 5mm of a suitable oil or other evaporation suppressant should be placed in the container to reduce evaporation (WMO, 1972). This layer should allow the free passage of precipitation into the solution below it. An antifreeze solution may be placed in the container to convert any snow that falls into the gauge into a liquid state. It is important that the antifreeze solution remain dispersed. A mixture of 37.5% by weight of commercial calcium chloride (78% purity) and 62.5% water makes a satisfactory antifreeze solution. Alternatively, aqueous solutions of ethylene glycol or of 1,2-propylene glycol are used. Not recommended are antifreeze components with dangerous properties (considered to be hazardous goods for transport or hazardous material while handling), such as those containing methanol, a dangerous material classified (highly) toxic. Thorough reading of the safety data sheet, also called the material safety data sheet, is highly recommended. These documents are provided by the manufacturer and detail all relevant information on the composition, properties, potential danger, safety measures, handling and storage of the material. While more expensive, the latter solutions are less corrosive than calcium chloride and give antifreeze protection over a much wider range of dilution resulting from subsequent precipitation. The volume of the solution initially placed in the container should not exceed 33% of the total volume of the gauge. In some countries, this antifreeze and oil solution is considered toxic waste and, therefore, harmful to the environment. Guidelines for the disposal of toxic substances should be obtained from local environmental protection authorities. The seasonal precipitation catch is determined by weighing or measuring the volume of the contents of the container (as with ordinary gauges; see 6.3.1). The amount of oil and antifreeze solution placed in the container at the beginning of the season and any contraction in the case of volumetric measurements must be carefully taken into account. Corrections may be applied as with ordinary gauges.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

221

The operation and maintenance of storage gauges in remote areas pose several problems, such as the capping of the gauge by snow or difficulty in locating the gauge for recording the measurement, and so on, which require specific monitoring. Particular attention should be paid to assessing the quality of data from such gauges.

6.4

PRECIPITATION GAUGE ERRORS AND CORRECTIONS

It is convenient to discuss at this point the errors and corrections that apply in some degree to most precipitation gauges, whether they are recording or non-recording gauges. The particular cases of recording gauges are discussed in 6.5. Comprehensive accounts of errors and corrections can be found in WMO (1982, 1984, 1986; specifically for snow, 1998; and specifically for rainfall intensity, 2006, 2009). Details of the models used for adjusting raw precipitation data in Canada, Denmark, Finland, the Russian Federation, Switzerland and the United States are given in WMO (1982). WMO (1989a) gives a description of how the errors occur. There are collected conference papers on the topic in WMO (1986, 1989b). Details on the improvement of the reliability of rainfall intensity measurements as obtained by traditional tipping-bucket gauges, weighing gauges and other types of gauges (optical, floating/siphoning, and so forth) are given in WMO (2006, 2009). The amount of precipitation measured by commonly used gauges may be less than the actual precipitation reaching the ground by up to 30% or more. Systematic losses will vary by type of precipitation (snow, mixed snow and rain, and rain) and wind speed. The systematic error of solid precipitation measurements is commonly large and may be of an order of magnitude greater than that normally associated with liquid precipitation measurements. For many hydrological purposes it is necessary first to make adjustments to the data in order to allow for the error before making the calculations. The adjustments cannot, of course, be exact (and may even increase the error). Thus, the original data should always be kept as the basic archives both to maintain continuity and to serve as the best base for future improved adjustments if, and when, they become possible. The traditional assessment of errors in precipitation gauges refers to so-called weather-related errors. It is well recognized that the measurement of liquid precipitation at the ground is affected by different sources of systematic and random errors, mainly due to wind-, wetting- and evaporation-induced losses (see WMO, 1982) which make the measurement of light to moderate rainfall scarcely reliable in the absence of an accurate calibration. Wind-induced errors still have an influence on rainfall intensities of the order of 20–50mmh–1 with an incidence of about 5% observed in some intercomparison stations in central Europe (WMO, 1984). Sampling errors due to the discrete nature of the rain measurement are also recognized to be dependent on the bucket size (for tipping-bucket gauges) and sampling interval or instrument response time, though not on precipitation intensity, and can be analytically evaluated (Colli et al., 2013a). The true amount of precipitation may be estimated by correcting for some or all of the various error terms listed below: (a) Error due to systematic wind field deformation above the gauge orifice: typically 2% to 10% for rain and 10% to 50% for snow; (b) Error due to the wetting loss on the internal walls of the collector; (c) Error due to the wetting loss in the container when it is emptied: typically 2% to 15% in summer and 1% to 8% in winter, for (b) and (c) together; (d) Error due to evaporation from the container (most important in hot climates): 0% to 4%; (e) Error due to blowing and drifting snow;

222

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(f) Error due to the in- and out-splashing of water: 1% to 2%; (g) Systematic mechanical and sampling errors, and dynamic effects errors (i.e. systematic delay due to instrument response time): typically 5% to 15% for rainfall intensity, or even more in high-rate events (see WMO, 2009); (h) Random observational and instrumental errors, including incorrect gauge reading times. The first seven error components are systematic and are listed in order of general importance. The net error due to blowing and drifting snow and to in- and out-splashing of water can be either negative or positive, while net systematic errors due to the wind field and other factors are negative. Since the errors listed as (e) and (f) above are generally difficult to quantify, the general model for adjusting data from most gauges, originally proposed by WMO (1982) and later modified by Legates and Willmott (1990), can be written as:

(

)

(

)

Pk = kr Pcr + k s Pcs = kr Pgr + ∆ P1r + ∆ P2r + ∆ P3r + ∆ P4r + k s Pgs + ∆ P1s + ∆ P2 s + ∆ P3s + ∆ P4 s (6.1) where subscripts r and s refer to liquid (rain) and solid (snow) precipitation, respectively; Pk is the adjusted precipitation amount; k (see Figure6.3) is the adjustment factor for the effects of wind field deformation; Pc is the amount of precipitation caught by the gauge collector; Pg is the measured amount of precipitation in the gauge; ΔP1 is the adjustment for the wetting loss on the internal walls of the collector; ΔP2 is the adjustment for wetting loss in the container after emptying; ΔP3 is the adjustment for evaporation from the container; and ΔP4 is the adjustment for systematic mechanical errors.

1.15

uhp = 3.0 m s-1 uhp = 3.0 m s-1 uhp = 1.0 m s-1 uhp = 1.0 m s-1

k 1.10

uhp = 3.0 m s-1 uhp = 3.0 m s-1 uhp = 1.0 m s-1 uhp = 1.0 m s-1

1.05

1.00

1

2 3 i (mm h-1)

2.0

k

4

5

1

1.6

4

5

uhp = 3.0 m s-1 uhp = 2.0 m s-1 uhp = 1.5 m s-1 uhp = 1.0 m s-1

uhp = 3.0 m s-1 uhp = 2.0 m s-1 uhp = 1.5 m s-1 uhp = 1.0 m s-1

1.8

3 2 i (mm h-1)

1.4 1.2 1.0

1

2

3 i (mm h-1)

4

5

1

2 3 i (mm h-1)

4

5

Figure 6.3. Conversion factor k defined as the ratio of “correct” to measured precipitation for rain (top) and snow (bottom) for two unshielded gauges in dependency of wind speed uhp, intensity i and type of weather situation according to Nespor and Sevruk (1999). On the left is the German Hellmann manual standard gauge, and on the right the recording, tipping‑bucket gauge by Lambrecht. Void symbols in the top diagrams refer to orographic rain, and black ones to showers. Note the different scales for rain and snow. For shielded gauges, k can be reduced to 50% and 70% for snow and mixed precipitation, respectively (WMO, 1998). The heat losses are not considered in the diagrams (in Switzerland they vary with altitude between 10% and 50% of the measured values of fresh snow).

CHAPTER 6. MEASUREMENT OF PRECIPITATION

223

Errors due to the weather conditions at the collector, as well as those related to wetting, splashing and evaporation, are typically referred to as catching errors. They indicate the ability of the instrument to collect the exact amount of water according to the definition of precipitation at the ground, that is, the total water falling over the projection of the collector's area over the ground. Systematic mechanical and sampling errors, typically referred to as quantification errors, are related to the ability of the instrument to sense correctly the amount of water collected by the instrument. The WMO laboratory and field intercomparisons on rainfall intensity gauges (WMO 2006, 2009) both contributed to the assessment of quantification errors and documented laboratory and field calibration methods for identifying and/or correcting quantification errors in rainfall intensity measurements. Obviously, these errors may derive from very different aspects of the sensing phase since the instruments may differ in the measuring principle applied, construction details, operational solutions and so forth. The corrections of precipitation measurement errors are applied to daily or monthly totals or, in some practices, to individual precipitation events. When dealing with precipitation intensity measurements, systematic mechanical errors can be properly corrected through a standardized laboratory calibration referred to as a dynamic calibration in steady-state conditions of the reference flow rate (Niemczynowicz, 1986; WMO, 2009). For more details, see Annex6.C. In general, the supplementary data needed to make adjustments related to weather conditions include the wind speed at the gauge orifice during precipitation, drop size, precipitation intensity, air temperature and humidity, and the characteristics of the gauge site. Although temperature has some effect on gauge undercatch, the effect is significantly less than the effects of wind speed at gauge height (Yang et al., 1993; Yang et al., 1995). Wind speed and precipitation type or intensity may be sufficient variables to determine the corrections. Wind speed alone is sometimes used. At sites where such observations are not made, interpolation between the observations made at adjacent sites may be used for making such adjustments, but with caution, and for monthly rainfall data only. For most precipitation gauges, wind speed is the most important environmental factor contributing to the under-measurement of solid precipitation. These data must be derived from standard meteorological observations at the site in order to provide daily adjustments. In particular, if wind speed is not measured at gauge orifice height, it can be derived by using a mean wind speed reduction procedure after having knowledge of the roughness of the surrounding surface and the angular height of surrounding obstacles. A suggested scheme is shown in Annex6.D.1 This scheme is very site-dependent, and estimation requires a good knowledge of the station and gauge location. Shielded gauges catch more precipitation than their unshielded counterparts, especially for solid precipitation. Therefore, gauges should be shielded either naturally (for example, forest clearing) or artificially (for example, Alter, Canadian Nipher type, Tretyakov windshield) to minimize the adverse effect of wind speed on measurements of solid precipitation (for some information on shield design, refer to WMO, 1998, 2008). The type of windshield configuration, as well as gauge type, will alter the relationship between wind speeds and catch efficiency and have implications on data homogeneity. Wetting loss (Sevruk, 1974a) is another cumulative systematic loss from manual gauges which varies with precipitation and gauge type; its magnitude is also a function of the number of times the gauge is emptied. Average wetting loss can be up to 0.2mm per observation. At synoptic stations where precipitation is measured every 6h, this can become a very significant loss. In some countries, wetting loss has been calculated to be 15% to 20% of the measured winter precipitation. Correction for wetting loss at the time of observation is a feasible alternative. Wetting loss can be kept low in a well-designed gauge. The methodology to determine the wetting loss of manual gauges (WMO, 1998) would suffice. It is recommended that the wetting loss for manual gauges be re-examined periodically (for example, every 5years) as it tends to change with the age of the collector. The internal surfaces should be of a material which can be kept smooth and clean; paint, for example, is unsuitable, but baked enamel is satisfactory. Seams in the construction should be kept to a minimum. 1

A wind reduction scheme recommended by CIMO at its eleventh session (1994).

224

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Evaporation losses (Sevruk, 1974b) vary by gauge type, climatic zone and time of year (seasons mentioned below correspond to the northern hemisphere). Evaporation loss is a problem with gauges that do not have a funnel device in the bucket, especially in late spring at mid-latitudes. Losses of over 0.8mm per day have been reported. Losses during winter are much less than during comparable summer months, ranging from 0.1 to 0.2mm per day. These losses, however, are cumulative. In a well-designed gauge, only a small water surface is exposed, its ventilation is minimized, and the water temperature is kept low by a reflective outer surface. In storage and accumulating recording gauges, errors associated with evaporation can be virtually eliminated through the use of oil in the collector. It is clear that, in order to achieve data compatibility when using different gauge types and shielding during all weather conditions, corrections to the actual measurements are necessary. In all cases where precipitation measurements are adjusted in an attempt to reduce errors, it is strongly recommended that both the measured and adjusted values be published.

6.5

RECORDING PRECIPITATION GAUGES

Recording precipitation automatically has the advantage that it can provide better time resolution than manual measurements, and it is possible to reduce the evaporation and wetting losses. These readings are of course subject to the wind effects discussed in 6.4. Three types of automatic precipitation recorders are in general use, namely the weighing‑recording type, the tilting or tipping-bucket type, and the float type. Only the weighing type is satisfactory for measuring all kinds of precipitation, while the use of the other two types being for the most part limited to the measurement of rainfall. Some other automatic gauges that measure precipitation without using moving parts are also available. These gauges use devices such as capacitance sensors, pressure transducers, acoustic and optical sensors, or small radar devices to provide an electronic signal that is proportional to the precipitation equivalent. The clock device that times intervals and that dates the time record is a very important component of the recorder. Because of the high variability of precipitation intensity over a 1min timescale, a single 1min rainfall intensity value is not representative of a longer time period. Therefore, 1min rainfall intensity should not be used in a temporal sampling scheme, such as one synoptic measurement every one or three hours. Very good time synchronization, better than 10s, is required between the reference time and the different instruments of the observing station. 6.5.1

Weighing-recording gauge

6.5.1.1

Instruments

In these instruments, the weight of a container, together with the precipitation accumulated therein, is recorded continuously using a spring mechanism, a system of balance weights, or vibrating wire transducers and load cells. All precipitation, both liquid and solid, is recorded as it falls. This type of gauge normally has no provision for emptying itself; the capacity (namely, the maximum accumulation between recharge) ranges from 250 to 1500mm depending on the model. Low-capacity models should be avoided in areas where the maximum accumulation could occur over short periods of time. The gauges must be maintained to minimize evaporation losses, which can be accomplished by adding sufficient oil or other evaporation suppressants inside the container to form a film over the water surface. Any difficulties arising from oscillation of the balance in strong winds can be reduced by suitably programming a microprocessor to eliminate this effect on the readings. Such weighing gauges are particularly useful for recording snow, hail, and mixtures of snow and rain, since the solid precipitation does not need to be melted before it can be recorded. For winter operation, the catchment container is charged with an antifreeze solution (see 6.3.2) to dissolve the solid contents. The amount of antifreeze depends on the expected amount of precipitation and the minimum temperature expected at

CHAPTER 6. MEASUREMENT OF PRECIPITATION

225

the time of minimum dilution. These instruments do not use any moving mechanical parts in the weighing mechanism; only elastic deformation occurs. Therefore, mechanical degradation and the resulting need for maintenance are significantly reduced. The digitized output signal is generally averaged and filtered. Precipitation intensity can also be calculated from the differences between two or more consecutive weight measurements. The accuracy of these types of gauges is related directly to their measuring and/or recording characteristics, which can vary with manufacturer. Many instruments have data output that contain diagnostic parameters which are very useful for further evaluations of measured data and for data QC. Weighing technology combined with a self-emptying tipping-bucket enables high resolution and high precision measurements with a very small construction volume. This type of instrument measures the weight of water in a tipping-bucket with a volume of up to 20 ml and can determine smaller amounts of precipitation compared to “classic” tipping-bucket gauges (see 6.5.2). 6.5.1.2

Errors and corrections

Except for error due to the wetting loss in the container when it is emptied, weighing-recording gauges are susceptible to all of the other sources of error discussed in 6.4. It should also be noted that automatic recording gauges alone cannot identify the type of precipitation. A significant problem with this type of gauge is that precipitation, particularly freezing rain or wet snow, can stick to the inside of the gauge orifice and not fall into the bucket until later. This severely limits the ability of weighing-recording gauges to provide accurate timing of precipitation events. Another common fault with weighing-type gauges is wind pumping. This usually occurs during high winds when turbulent air currents passing over and around the catchment container cause oscillations in the weighing mechanism. Errors associated with such anomalous recordings can be minimized by averaging readings over short time intervals usually ranging from 1 to 5min. Timing errors in the instrument clock may assign the catch to the wrong period or date. Some weighing-recording gauges may also exhibit some temperature sensitivity in the weighing mechanism that adds a component to the output which is proportional to the diurnal temperature cycle. Some potential errors in manual methods of precipitation measurement can be eliminated or at least minimized by using weighing-recording gauges. Random measurement errors associated with human observer error and certain systematic errors, particularly evaporation and wetting loss, are minimized. In some countries, trace observations are officially given a value of zero, thus resulting in a biased underestimate of the seasonal precipitation total. This problem is minimized with weighing-type gauges, since even very small amounts of precipitation will accumulate over time. A fundamental characteristic of weighing-recording gauges when measuring precipitation intensity is the response time (filtering process included), which leads to measurement errors (systematic delay). The response times, available in operation manuals or evaluated during a previous WMO intercomparison (WMO, 2009), are of the order of six seconds to a few minutes depending on the gauge's design and model. The 1min precipitation intensity resolution of weighing-recording gauges can be very different from gauge to gauge and depends on the transducer resolution. Such gauges may also exhibit a limit or discrimination threshold for precipitation intensity. The correction of weighing gauge data on an hourly or daily basis may be more difficult than on longer time periods, such as monthly climatological summaries. Ancillary data from AWSs, such as wind at gauge height, air temperature, present weather or snow depth, will be useful in interpreting and correcting accurately the precipitation measurements from automatic gauges.

226 6.5.1.3

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Calibration and maintenance

Weighing-recording gauges usually have few moving parts and, therefore, should seldom require calibration. Calibration commonly involves the use of a series of weights which, when placed in the bucket or catchment container, provide a predetermined value equivalent to an amount of precipitation. Calibrations should normally be done in a laboratory setting and should follow the manufacturer’s instructions. An alternative procedure for calibrating weighing-recording gauges when dealing with precipitation intensity measurements is given in Annex6.C. This calibration, referred to as a dynamic calibration in steady-state conditions of the reference flow rates, is performed to evaluate the measurement errors of the weighing gauge. This procedure can also be used to assess the dynamic response of the weighing gauge by performing the classic step-response test, that is, by providing the instrument with a reference flow rate showing a single abrupt change from zero to a given equivalent rainfall rate. Moreover, the repeating of the dynamic calibration in unsteady conditions (time-varying reference flow rates as a simulation of real-world events) permits a finer calibration of weighing gauges (especially for systematic delays due to the instrument’s response time) and could lead to improved dynamic performance and accuracy in real-world events (Colli et al., 2013b). Routine maintenance should be conducted every three to four months, depending on precipitation conditions at the site. Both the exterior and interior of the gauge should be inspected for loose or broken parts and to ensure that the gauge is level. Any manual read‑out should be checked against the removable data record to ensure consistency before removing and annotating the record. The bucket or catchment container should be emptied, inspected, cleaned, if required, and recharged with oil for rainfall-only operation or with antifreeze and oil if solid precipitation is expected (see 6.3.2). The recording device should be set to zero in order to make maximum use of the gauge range. The digital memory as well as the power supply should be checked and replaced, if required. Timing intervals and dates of record must be checked. A proper field calibration, and field calibration check or field inspection should also be conducted on a regular basis as part of the routine maintenance and check, taking into account site and operational constraints. For rainfall intensity gauges, a recommended procedure by means of a portable device for reference flow rates is given in Annex6.E. 6.5.2

Tipping-bucket gauge

The tipping-bucket raingauge is used for measuring accumulated totals and the rate of rainfall. Suitable intensity-dependent corrections (see 6.5.2.2) should be applied to improve the accuracy of the intensity measurements and to overcome the underestimation of intensity for high rainfall rates and the overestimation of intensity for low rainfall rates, both of which are typical in non‑corrected tipping‑bucket gauges. 6.5.2.1

Instruments

The principle behind the operation of this instrument is simple. A tipping-bucket raingauge uses a metallic or plastic twin bucket balance to measure the incoming water in portions of equal weight. When one bucket is full, its centre of mass is outside the pivot and the balance tips, dumping the collected water and bringing the other bucket into position to collect. The bucket compartments are shaped in such a way that the water is emptied from the lower one. The water mass content of the bucket is constant (m (g)). Therefore, by using the density of water (ρ=1g/cm3), the corresponding volume (V (cm3)) is derived from the weight of the water and, consequently, the corresponding accumulation height (h (mm)) is retrieved by using the area of the collector (S(cm2)). The equation is:

V = m ρ = h ⋅ S (6.2)

CHAPTER 6. MEASUREMENT OF PRECIPITATION

227

Thus, by using the density of water, h is calculated, where 1mm corresponds to 1g of water over an area of 10cm2. To have detailed records of precipitation, the amount of rain should not exceed 0.2mm. For a gauge area of 1000cm2, this corresponds to a bucket content of 20g of water. Tipping-bucket gauges employ a contact closure (reed switch or relay contact), such that each tip produces an electrical impulse as a signal output. This output must be recorded by a data logger or an ADC (data acquisition system equipped with reed switch reading ports). This mechanism provides a continuous measurement without manual interaction. The rainfall intensity of non-corrected tipping-bucket gauges is calculated based on the number of tips in a periodic sampling rate (typically 6 or 10s) and averaged over a chosen time interval (for example, 1min). In this way, an intensity value is available every minute that represents the intensity of the past minute or minutes. This sampling scheme reduces the uncertainty of the average. In addition, the rainfall intensity resolution depends on the size of the bucket and the chosen time interval. For example, a tip equivalent to 0.2mm leads to a 1min rainfall intensity resolution of 12mmh–1 which is constant over the measurement range of the gauge if no intensity‑dependent corrections are applied. The bucket takes a small but finite time to tip and, during the first half of its motion, additional rain may enter the compartment that already contains the calculated amount of rainfall. The water losses during the tipping movement indicate a systematic mechanical error that is rather a function of the intensity itself and can be appreciable during heavy rainfall (>100mmh–1). However, this can be corrected by using a calibration procedure as given in Annex6.C and applying a correction curve or algorithm (see 6.4). An alternative simple method is to use a device like a siphon at the foot of the funnel to direct the water to the buckets at a controlled rate. This smoothes out the intensity peaks of very short-period rainfall. Alternatively, a device can be added to accelerate the tipping action; essentially, a small blade is impacted by the water falling from the collector and is used to apply an additional force to the bucket, varying with rainfall intensity. The tipping-bucket gauge is particularly convenient for AWSs because it lends itself to digital methods. The pulse generated by a contact closure can be monitored by a data logger, preferably including the time the tips occurred, to calculate a corrected rainfall intensity, which can then be used to retrieve the precipitation amount over selected periods. It may also be used with a chart recorder. 6.5.2.2

Errors and corrections

Since the tipping-bucket raingauge has sources of error which differ somewhat from those of other gauges, special precautions and corrections are advisable. Some sources of error include the following: (a) The loss of water during the tipping action in heavy rain; this can be considerably reduced by conducting a dynamic calibration (see Annex6.C) and applying an intensity-dependent correction; (b) With the usual bucket design, the exposed water surface is large in relation to its volume, meaning that appreciable evaporation losses can occur, especially in hot regions. This error may be significant in light rain; (c) The discontinuous nature of the record may not provide satisfactory data during light drizzle or very light rain. In particular, the time of onset and cessation of precipitation cannot be accurately determined; (d) Water may adhere to both the walls and the lip of the bucket, resulting in rain residue in the bucket and additional weight to be overcome by the tipping action. Tests on waxed buckets produced a 4% reduction in the volume required to tip the balance compared

228

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

with non‑waxed buckets. Volumetric calibration can change, without adjustment of the calibration screws, by variation of bucket wettability through surface oxidation or contamination by impurities and variations in surface tension; (e) The stream of water falling from the funnel onto the exposed bucket may cause overreading, depending on the size, shape and position of the nozzle; (f) The instrument is particularly prone to bearing friction and to having an improperly balanced bucket because the gauge is not level; (g) The limited repeatability at various rainfall intensities of the inter-tip time interval due to low stability of the mechanics of the buckets (that is, bucket movement) degrades the measurements; this systematic mechanical effect can be investigated by means of specific tests recording a series of inter-tip time intervals that make it possible to estimate the mechanical precision of the bucket (see Colli et al., 2013b); such errors may be reduced by improving the construction quality of the gauges; (h) The sampling errors of tipping-bucket gauges (Habib et al., 2001) have an additional strong impact on field performance under light precipitation regimes; these errors consist in a delay of the tipping-bucket mechanism in assigning the collected amount of water to the corresponding time interval; different calculation techniques exist for reducing the impact of sampling errors and providing rainfall intensity measurements at a higher resolution than the tipping‑bucket gauges' sensitivity would allow (see Colli et al., 2013a; Stagnaro et al., 2016). Careful calibration can provide corrections for the systematic parts of these errors. Effective corrections for improving the measurement of rainfall intensity (WMO, 2009), and consequently the corresponding accumulated amount, consist in performing a dynamic calibration and applying correction curves (see 6.4), for example, by applying a software correction or an algorithm in the data acquisition system. Alternatively, they can involve conducting a linearization procedure in the instrument’s electronics circuit (generating an intensitydependent emission of extra pulses) or through a mechanism (for example, small deflectors that induce a dynamic pressure which increases with intensity, allowing the tip to occur before the bucket is full). In WMO (2009), it is shown that linearization by extra electronic pulses is well suited for measuring precipitation amount but less so for measuring intensity. On the other hand, mechanical linearization compensates for the loss of water during the movement of the balance and greatly minimizes the intensity underestimation during high-rate events. The software correction (correction curve or algorithm) resulted in being the most effective method for correcting systematic mechanical errors. The field performances of three different types of tipping-bucket raingauges: without correction, with software correction and with extra pulse correction, in a tropical environment can be found in Chan et al. (2015). The measurements from tipping‑bucket raingauges may be corrected for effects of exposure in the same way as other types of precipitation gauge. Heating devices can be used to allow for measurements during the cold season, particularly of solid precipitation. However, the performance of heated tipping-bucket gauges can be poor as a result of large errors due to both wind and evaporation of melting snow. Therefore, other types of gauges should be considered for use in winter precipitation measurement in regions where temperatures fall below 0°C for prolonged periods. However, the evaporation effect can be minimized by using instruments with controlled heating elements that maintain the temperature of the critical parts slightly above the melting point of water. 6.5.2.3

Calibration and maintenance

Calibration of the tipping bucket is usually accomplished by passing a known amount of water through the tipping mechanism at various rates and by adjusting the mechanism to the known volume. This procedure should be followed under laboratory conditions. The recommended calibration procedure for these gauges is available in Annex6.C.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

229

A proper field calibration, and field calibration check or field inspection should also be conducted on a regular basis as part of the routine maintenance and check, taking into account site and operational constraints. For catchment type rainfall intensity gauges, a recommended procedure by means of a portable device for reference flow rates is given in Annex6.E. Owing to the numerous error sources, the collection characteristics and calibration of tippingbucket raingauges are a complex interaction of many variables. Daily comparisons with the standard raingauge can provide useful correction factors, and is good practice. The correction factors may vary from station to station. Correction factors are generally greater than 1.0 (underreading) for low-intensity rain, and less than 1.0 (over-reading) for high-intensity rain. The relationship between the correction factor and intensity is not linear but forms a curve. Routine maintenance should include cleaning the accumulated dirt and debris from funnel and buckets, as well as ensuring that the gauge is level. It is highly recommended that the tipping mechanism be replaced with a newly calibrated unit on an annual basis. Timing intervals and dates of records must be checked. 6.5.3

Float gauge

In this type of instrument, the rain passes into a float chamber containing a light float. As the level of the water within the chamber rises, the vertical movement of the float is transmitted, by a suitable mechanism, to the movement of a pen on a chart or a digital transducer. By suitably adjusting the dimensions of the collector orifice, the float and the float chamber, any desired chart scale can be used. In order to provide a record over a useful period (24h are normally required) either the float chamber has to be very large (in which case a compressed scale on the chart or other recording medium is obtained), or a mechanism must be provided for emptying the float chamber automatically and quickly whenever it becomes full, so that the chart pen or other indicator returns to zero. Usually a siphoning arrangement is used. The actual siphoning process should begin precisely at the predetermined level with no tendency for the water to dribble over at either the beginning or the end of the siphoning period, which should not be longer than 15s. In some instruments, the float chamber assembly is mounted on knife edges so that the full chamber overbalances; the surge of the water assists the siphoning process, and, when the chamber is empty, it returns to its original position. Other rain recorders have a forced siphon which operates in less than 5s. One type of forced siphon has a small chamber that is separate from the main chamber and accommodates the rain that falls during siphoning. This chamber empties into the main chamber when siphoning ceases, thus ensuring a correct record of total rainfall. A heating device (preferably controlled by a thermostat) should be installed inside the gauge if there is a possibility that water might freeze in the float chamber during the winter. This will prevent damage to the float and float chamber and will enable rain to be recorded during that period. A small heating element or electric lamp is suitable where a mains supply of electricity is available, otherwise other sources of power may be employed. One convenient method uses a short heating strip wound around the collecting chamber and connected to a large-capacity battery. The amount of heat supplied should be kept to the minimum necessary in order to prevent freezing, because the heat may reduce the accuracy of the observations by stimulating vertical air movements above the gauge and increasing evaporation losses. A large undercatch by unshielded heated gauges, caused by the wind and the evaporation of melting snow, has been reported in some countries, as is the case for weighing gauges (see6.5.1.2). Apart from the fact that calibration is performed using a known volume of water, the maintenance procedures for this gauge are similar to those of the weighing-recording gauge (see6.5.1.3).

230

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

6.5.4 Other raingauges With the growth of measurement electronics technologies and smart instruments, other precipitation instruments have been developed in recent years. Their performance is approximately of the same quality as that of conventional tipping-bucket raingauges. However, these instruments provide rain intensity measurements with higher resolution compared to classic methods, starting from 0.01mm, and they are particularly suitable for areas that are difficult to access as they require less maintenance. These instruments are described in the present volume, Chapter14.

6.6

MEASUREMENT OF DEW, ICE ACCUMULATION AND FOG PRECIPITATION

6.6.1

Measurement of dew and leaf wetness

The deposition of dew is essentially a nocturnal phenomenon and, although relatively small in amount and locally variable, is of much interest in arid zones; in very arid regions, it may be of the same order of magnitude as the rainfall. The exposure of plant leaves to liquid moisture from dew, fog and precipitation also plays an important role in plant disease, insect activity, and the harvesting and curing of crops. In order to assess the hydrological contribution of dew, it is necessary to distinguish between dew formed: (a) As a result of the downward transport of atmospheric moisture condensed on cooled surfaces, known as dew-fall; (b) By water vapour evaporated from the soil and plants and condensed on cooled surfaces, known as distillation dew; (c) As water exuded by leaves, known as guttation. All three forms of dew may contribute simultaneously to the observed dew, although only the first provides additional water to the surface, and the latter usually results in a net loss. A further source of moisture results from fog or cloud droplets being collected by leaves and twigs and reaching the ground by dripping or by stem flow. The amount of dew deposited on a given surface in a stated period is usually expressed in units of kgm–2 or in millimetres depth of dew. Whenever possible, the amount should be measured to the nearest tenth of a millimetre. Leaf wetness may be described as light, moderate or heavy, but its most important measures are the time of onset or duration. A review of the instruments designed for measuring dew and the duration of leaf wetness, as well as a bibliography, is given in WMO (1992b). The following methods for the measurement of leaf wetness are considered. The amount of dew depends critically on the properties of the surface, such as its radiative properties, size and aspect (horizontal or vertical). It may be measured by exposing a plate or surface, which can be natural or artificial, with known or standardized properties, and assessing the amount of dew by weighing it, visually observing it, or making use of some other quantity such as electrical conductivity. The problem lies in the choice of the surface, because the results obtained instrumentally are not necessarily representative of the dew deposit on the surrounding objects. Empirical relationships between the instrumental measurements and the deposition of dew on a natural surface should, therefore, be established for each particular set of surface and exposure conditions; empirical relationships should also be established to distinguish between the processes of dew formation if that is important for the particular application.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

231

A number of instruments are in use for the direct measurement of the occurrence, amount and duration of leaf wetness and dew. Dew-duration recorders use either elements which themselves change in such a manner as to indicate or record the wetness period, or electrical sensors in which the electrical conductivity of the surface of natural or artificial leaves changes in the presence of water resulting from rain, snow, wet fog or dew. In dew balances, the amount of moisture deposited in the form of precipitation or dew is weighed and recorded. In most instruments providing a continuous trace, it is possible to distinguish between moisture deposits caused by fog, dew or rain by considering the type of trace. The only certain method of measuring net dew-fall by itself is through the use of a very sensitive lysimeter (see the present volume, Chapter10). In WMO (1992b) two particular electronic instruments for measuring leaf wetness are advocated for development as reference instruments, and various leaf-wetting simulation models are proposed. Some use an energy balance approach (the inverse of evaporation models), while others use correlations. Many of them require micrometeorological measurements. Unfortunately, there is no recognized standard method of measurement to verify them. 6.6.2

Measurement of ice accumulation

Ice can accumulate on surfaces as a result of several phenomena. Ice accumulation from freezing precipitation, often referred to as glaze, is the most dangerous type of icing condition. It may cause extensive damage to trees, shrubs and telephone and power lines, and create hazardous conditions on roads and runways. Hoar frost (commonly called frost) forms when air with a dewpoint temperature below freezing is brought to saturation by cooling. Hoar frost is a deposit of interlocking ice crystals formed by direct deposition on objects, usually of small diameter, such as tree branches, plant stems, leaf edges, wires, poles, and so forth. Rime is a white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water drops as they come into contact with an exposed object. 6.6.2.1

Measurement methods

At meteorological stations, the observation of ice accumulation is generally more qualitative than quantitative, primarily due to the lack of a suitable sensor. Ice accretion indicators, usually made of anodized aluminium, are used to observe and report the occurrence of freezing precipitation, frost or rime icing. Observations of ice accumulation can include both the measurement of the dimensions and the weight of the ice deposit as well as a visual description of its appearance. These observations are particularly important in mountainous areas where such accumulation on the windward side of a mountain may exceed the normal precipitation. A system consisting of rods and stakes with two pairs of parallel wires (one pair oriented north-south and the other east-west) can be used to accumulate ice. The wires may be suspended at any level, and the upper wire of each pair should be removable. At the time of observation, both upper wires are removed, placed in a special container, and taken indoors for melting and weighing of the deposit. The cross-section of the deposit is measured on the permanently fixed lower wires. Recording instruments are used in some countries for continuous registration of rime. A vertical or horizontal rod, ring or plate is used as the sensor, and the increase in the amount of rime with time is recorded on a chart. A simple device called an ice-scope is used to determine the appearance and presence of rime and hoar frost on a snow surface. The ice-scope consists of a round plywood disc, 30cm in diameter, which can be moved up or down and set at any height on a vertical rod fixed in the ground. Normally, the disc is set flush with the snow surface to collect the rime and hoar frost. Rime is also collected on a 20cm diameter ring fixed on the rod, 20cm from its upper end. A wire or thread 0.2 to 0.3mm in diameter, stretched between the ring and the top end of the rod, is used for the observation of rime deposits. If necessary, each sensor can be removed and weighed.

232

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

In the ISO 12494:2017 standard (ISO, 2017), which applies to ice accretion on all kinds of structures except electrical overhead line conductors, a standard ice-measuring device is described as follows: (a) A smooth cylinder with a diameter of 30mm placed with the axis vertical and rotating around the axis. The cylinder length should be a minimum of 0.5m, but, if heavy ice accretion is expected, the length should be 1m; (b) The cylinder is placed 10m above terrain; (c) Recordings of ice weight may be performed automatically. In Fikke et al. (2007), several types of ice detectors are identified, some of which are used for the start and end of icing periods while others are also able to quantify the ice accretion rate (usually expressed in kgm–2h–1). Many sensors are based on the measurement of the ice mass on a vertical tube used as a target for icing. An optical sensor (IR beam) detects the change of reflecting properties of a target tube when covered with ice. Another sensor, widely used for freezing rain, consists of a vibrating probe. Ice accreted on this probe changes the vibrating frequency, which allows both the detection of icing conditions and an estimate of the ice accretion rate. An internal probe heater is applied to melt the ice and keep the sensor within its operational limits. 6.6.2.2

Ice on pavements

Sensors have been developed and are in operation to detect and describe ice on roads and runways, and to support warning and maintenance programmes. VolumeIII, Chapter10 of the present Guide provides more specific information on this subject. With a combination of measurements, it is possible to detect dry and wet snow and various forms of ice. One sensor using two electrodes embedded in the road, flush with the surface, measures the electrical conductivity of the surface and readily distinguishes between dry and wet surfaces. A second measurement, of ionic polarizability, determines the ability of the surface, to hold an electrical charge; a small charge is passed between a pair of electrodes for a short time, and the same electrodes measure the residual charge, which is higher when there is an electrolyte with free ions, such as salty water. The polarizability and conductivity measurements together can distinguish between dry, moist and wet surfaces, frost, snow, white ice and some de-icing chemicals. However, because the polarizability of the non-crystalline black ice is indistinguishable from water under some conditions, the dangerous black ice state can still not be detected with the two sensors. In at least one system, this problem has been solved by adding a third specialized capacitive measurement which detects the unique structure of black ice. The above method is a passive technique. There is an active in situ technique that uses either a heating element, or both heating and cooling elements, to melt or freeze any ice or liquid present on the surface. Simultaneous measurements of temperature and of the heat energy involved in the thaw-freeze cycle are used to determine the presence of ice and to estimate the freezing point of the mixture on the surface. Most in situ systems include a thermometer to measure the road surface temperature. The quality of the measurement depends critically on the mounting (especially the materials) and exposure, and care must be taken to avoid radiation errors. There are two remote-sensing methods under development which lend themselves to carmounted systems. The first method is based on the reflection of IR and microwave radiation at several frequencies (about 3000nm and 3GHz, respectively). The microwave reflections can determine the thickness of the water layer (and hence the risk of aquaplaning), but not the ice condition. Two IR frequencies can be used to distinguish between dry, wet and icy conditions. It has also been demonstrated that the magnitude of reflected power at wavelengths around 2000nm depends on the thickness of the ice layer.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

233

The second method applies pattern recognition techniques to the reflection of laser light from the pavement, to distinguish between dry and wet surfaces, and black ice. 6.6.3

Measurement of fog precipitation

Fog consists of minute water droplets suspended in the atmosphere to form a cloud at the Earth’s surface. Fog droplets have diameters from about 1 to 40μm and fall velocities from less than 1 to approximately 5cms–1. In fact, the fall speed of fog droplets is so low that, even in light winds, the drops will travel almost horizontally. When fog is present, horizontal visibility is less than 1km; it is rarely observed when the temperature and dewpoint differ by more than 2°C. Meteorologists are generally more concerned with fog as an obstruction to vision than as a form of precipitation. However, from a hydrological standpoint, some forested high-elevation areas experience frequent episodes of fog as a result of the advection of clouds over the surface of the mountain, where the consideration of precipitation alone may seriously underestimate the water input to the watershed (Stadtmuller and Agudelo, 1990). More recently, the recognition of fog as a water supply source in upland areas (Schemenauer and Cereceda, 1994a) and as a wet deposition pathway (Schemenauer and Cereceda, 1991; Vong et al., 1991) has led to the requirement for standardizing methods and units of measurement. The following methods for the measurement of fog precipitation are considered. Although there have been a great number of measurements for the collection of fog by trees and various types of collectors over the last century, it is difficult to compare the collection rates quantitatively. The most widely used fog-measuring instrument consists of a vertical wire mesh cylinder centrally fixed on the top of a raingauge in such a way that it is fully exposed to the free flow of the air. The cylinder is 10cm in diameter and 22cm in height, and the mesh is 0.2cm by 0.2cm (Grunow, 1960). The droplets from the moisture-laden air are deposited on the mesh and drop down into the gauge collector where they are measured or registered in the same way as rainfall. Some problems with this instrument are its small size, the lack of representativeness with respect to vegetation, the storage of water in the small openings in the mesh, and the ability of precipitation to enter directly into the raingauge portion, which confounds the measurement of fog deposition. In addition, the calculation of fog precipitation by simply subtracting the amount of rain in a standard raingauge (Grunow, 1963) from that in the fog collector leads to erroneous results whenever wind is present. An inexpensive, 1 m2 standard fog collector and standard unit of measurement is proposed by Schemenauer and Cereceda (1994b) to quantify the importance of fog deposition to forested high-elevation areas and to measure the potential collection rates in denuded or desert mountain ranges. The collector consists of a flat panel made of a durable polypropylene mesh and mounted with its base 2m above the ground. The collector is coupled to a tippingbucket raingauge to determine deposition rates. When wind speed measurements are taken in conjunction with the fog collector, reasonable estimates of the proportions of fog and rain being deposited on the vertical mesh panel can be taken. The output of this collector results in litres of water. Since the surface area is 1m2, this gives a collection in lm–2.

ANNEX 6.A. STANDARD REFERENCE RAINGAUGE PIT

Reference raingauges are installed in a well-drained pit according to the design and specifications reported in the EN13798:2010 standard (CEN, 2010) to minimize environmental interference on measured rainfall intensities and protect against in-splash by a metal or plastic grating. The buried or sunken gauge (see Koschmider, 1934; Sieck et al., 2007) is expected to show a higher rainfall reading than a gauge above ground, with possible differences of 10% or more, when both instruments are working perfectly and accurately. Pits are preferably sited on ground level to avoid possible surface runoff (see general configuration in Figure6.A.1). The pit should be deep enough to accommodate the raingauge and to level the gauge's collector with the top of the grating (ground level) and centre it. The design of the pit takes into account dimensions of the raingauge and its method of installation. The base of the pit should have a recess (extra pit) to allow water to be drained. The square space of the grating is also adapted according to the raingauge collector's diameter in order to satisfy the standard requirements reported in CEN (2010). The sides of the pit are formed of bricks and concrete and are supported to prevent collapse. Supporting walls are built around the edges and a grating of approximately 1875 x 1875 x 120mm (L x W x H) is installed on the pit walls with the possibility to be lifted to give access to the raingauge for checks and maintenance operations. The grating distance is approximately 120–125mm. The grating is strong enough to walk on, to maintain its shape without distortion. To prevent in-splash from the top surface of the grating, the strips of the grating are at least 2mm thick and the distance between the edge of the central square and the ground is greater than 600mm (for further details see CEN, 2010). In Figure6.A.2, an example of a realization of four standard reference raingauge pits is provided, as reported in WMO (2009).

Figure6.A.1. A raingauge pit and its grating (ground-level configuration)

CHAPTER 6. MEASUREMENT OF PRECIPITATION

Figure6.A.2. Realization of the reference raingauge pits at Vigna di Valle, Italy (2007) during the WMO Field Intercomparison of Rainfall Intensity Gauges

235

ANNEX 6.B. PRECIPITATION INTERCOMPARISON SITES

The following text regarding precipitation intercomparison sites is based on statements made by CIMO at its eleventh session in 1994 and updated following its fifteenth session in 2010: The Commission recognized the benefits of national precipitation sites or centres where past, current and future instruments and methods of observation for precipitation can be assessed on an ongoing basis at evaluation stations. These stations should: (a) Operate the WMO recommended gauge configurations for rain (reference raingauge pit) and snow (DFIR). Installation and operation will follow specifications of the WMO precipitation intercomparisons. A DFIR installation is not required when only rain is observed; (b) Operate past, current and new types of operational precipitation gauges or other methods of observation according to standard operating procedures (SOPs) and evaluate the accuracy and performance against WMO recommended reference instruments; (c) Take auxiliary meteorological measurements that will allow the development of precipitation correction procedures and tests for their application; (d) Provide QC of data and archive all precipitation intercomparison data, including the related meteorological observations and the metadata, in a readily acceptable format, preferably digital; (e) Operate continuously for a minimum of 10 years; (f) Test all precipitation correction procedures available (especially those outlined in the final reports of the WMO intercomparisons) on the measurement of rain and solid precipitation; (g) Facilitate the conduct of research studies on precipitation measurements. It is not expected that the centres provide calibration or verification of instruments. They should make recommendations on national observation standards and should assess the impact of changes in observational methods on the homogeneity of precipitation time series in the region. The site would provide a reference standard for calibrating and validating radar or remote-sensing observations of precipitation.

ANNEX 6.C. STANDARDIZED PROCEDURE FOR LABORATORY CALIBRATION OF CATCHMENT TYPE RAINFALL INTENSITY GAUGES

1.

Principles

The calibration laboratory should be well prepared to perform calibrations of instruments to be used for operational practices. Apart from a well-designed reference system, the calibration procedures should be documented in full detail and set-up and staff should be well prepared before starting any calibration activity (see the ISO/IEC17025 standard (ISO/IEC, 2017) for details). The result of any calibration will be a calibration certificate presenting the results of the calibration (including corrections to be applied), allowing a compliance check with the relevant WMO recommendations. This certificate should also contain the measurement uncertainty for rainfall intensity. It should document the traceability of the rainfall intensity reference, the environmental conditions, such as temperature, and the applied time-averaging method. Rainfall intensity gauges should be calibrated using a calibration system that: (a) Has the capability of generating a constant water flow at various flow rates corresponding to the entire operational range of measurement (recommended range: from 0.2mmh–1 up to 2000mmh–1); (b) Is able to measure the flow by weighing the amount of water over a given period of time; (c) Is able to measure the output of the calibrated instrument at regular intervals or when a pulse occurs, which is typical for the majority of tipping-bucket raingauges. 2.

Requirements

(a) The calibration system should be designed to obtain uncertainties less than 1% for the generated rainfall intensity, and such performances should be reported and detailed; (b) In case of tipping-bucket raingauges, correct and suitable balancing of the buckets should be verified in order to guarantee a minimal variance of the tipping duration during the measurement process; (c) At least five reference intensities suitably spaced to cover the whole operating range of the instrument should be used; (d) The number of rainfall intensity reference setting points should be large enough to be able to determine a fitting curve by interpolation. The reference setting should be selected and well spaced so that the calibration curve can be established by interpolation in such a way that the uncertainty of the fitting curve is less than the required measurement uncertainty for the full range; (e) The calculation of flow rate is based on the measurements of mass and time; (f) The measurement of mass is better than 0.1%; (g) The duration of any test should be long enough to guarantee an uncertainty of less than 1% on the generated intensity; (h) The maximum time resolution for the measurement of rainfall intensities should be 1s;

238 (i)

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The following issues must be considered for any related laboratory activity in addressing possible error sources: (i)

The water quality/purity used for calibration should be well defined;

(ii) The reproducibility of the calibration conditions should be a priority; (iii) Suitable control and recording equipment should be used (such as PC-controlled); (iv) All acquisition systems must comply with electromagnetic compatibility to avoid parasitic pulses; (j)

The quantity, for which measurements of precipitation are generally reported, is height expressed in millimetres although weighing gauges measure mass. Since the density of rain depends on ambient temperature, the relationship between mass and the equivalent height of rainfall introduces an inaccuracy that must be taken into account during calibration and uncertainty calculation;

(k) The environmental conditions during each calibration must be noted and recorded: (i)

Date and hour (start/end);

(ii) Air temperature (°C); (iii) Water temperature (°C); (iv) Atmospheric pressure (hPa); (v) Ambient relative humidity (%); (vi) Any special condition that may be relevant to calibration (for example, vibrations); (vii) Evaporation losses must be estimated (mm); (l)

The number of tests performed for each instrument, their description in terms of time units and/or number of tips must be documented.

3.

Procedure from data interpretation

(a) The results should be presented in the form of a graph where the relative error is plotted against the reference intensity. The relative error is evaluated for each reference flow rate as:

e=

Im − Ir ⋅ 100% Ir

where Im is the intensity measured by the instrument and Ir the actual reference intensity provided to the instrument; (b) Ideally five tests, but a minimum of three, should be performed for each set of reference intensities, so that five error figures are associated with each instrument. The average error and the average values of Ir and Im are obtained by discarding the minimum and the maximum value of e obtained for each reference flow rate, then evaluating the arithmetic mean of the three remaining errors and reference intensity values. For each reference intensity, an error bar encompassing all the five error values used to obtain the average figures should be reported; (c) In addition, Ir versus Im can be plotted, where Im and Ir are average values, calculated as indicated above; all data are fitted with an interpolating curve, obtained as the best fit (linear, power law or second order polynomial are acceptable);

CHAPTER 6. MEASUREMENT OF PRECIPITATION

239

(d) In the graphs presenting the results, the ±5% limits must be drawn to allow an easy comparison of the results with the WMO recommendations; (e) In case water storage should occur for an intensity below the maximum declared intensity, the intensity at which water storage begins should be documented in the calibration certificate and intensities above this limit should not be considered; (f) In addition to measurements based on constant flow rates, the step response of each nontipping-bucket raingauge instrument should be determined. The step response should be measured by switching between two different constant flows, namely from 0mmh–1 to the reference intensity and back to 0mmh–1. The constant flow should be applied until the output signal of the instrument is stabilized, that is, when the further changes or fluctuation in the established rainfall intensity can be neglected with respect to the stated measurement uncertainty of the reference system. The sampling rate must be at least one per minute for those instruments that allow it. The time before stabilization is assumed as a measure of the delay of the instrument in measuring the reference rainfall intensity. Less than one minute delay is required for accurate rainfall intensity measurements. The response time should always be documented in the calibration certificate. 4.

Uncertainty calculation

The following sources of the measurement uncertainty should be considered and quantified: (a) Flow generator: Uncertainty on the flow steadiness deriving from possible variations in the constant flow generation mechanism, including pressure difference inside water content and in distribution pipes; (b) Flow measuring devices (both reference and device under calibration): Uncertainties due to the weighing apparatus, to time measurement and delays in acquisition and data processing and to the variation of experimental and ambient conditions such as temperature and relative humidity. These two sources of uncertainty are independent from each other; therefore a separate analysis can be performed, and results can be then combined into the uncertainty budget.

ANNEX 6.D. SUGGESTED CORRECTION PROCEDURES FOR PRECIPITATION MEASUREMENTS

The following text regarding the correction procedures for precipitation measurements is based on statements made by CIMO at its eleventh session in 1994. The correction methods are based on simplified physical concepts as presented in WMO (1987). They depend on the type of precipitation gauge applied. The effect of wind on a particular type of gauge has been assessed by using intercomparison measurements with the WMO reference gauges – the pit gauge for rain and the DFIR for snow, as is shown in WMO (1984) – and by the results of the WMO Solid Precipitation Measurement Intercomparison (WMO, 1998). The reduction of wind speed to the level of the gauge orifice should be made according to the following formula:

(

)(

uhp = log hz0−1 ⋅ log Hz0−1

)

−1

⋅ (1 − 0.024α ) u H

where uhp is the wind speed at the level of the gauge orifice; h is the height of the gauge orifice above ground; z0 is the roughness length (0.01m for winter and 0.03m for summer); H is the height of the wind speed-measuring instrument above ground; uH is the wind speed measured at the height H above ground; and α is the average vertical angle of obstacles around the gauge. The latter depends on the exposure of the gauge site and can be based either on the average value of direct measurements, on one of the eight main directions of the wind rose of the vertical angle of obstacles (in 360°) around the gauge, or on the classification of the exposure using metadata as stored in the archives of NMHSs. The classes are as follows: Class

Angle

Description

Exposed site

0–5

Only a few small obstacles such as bushes, a group of trees, a house

Mainly exposed site

6–12

Small groups of trees or bushes or one or two houses

Mainly protected site

13–19

Parks, forest edges, village centres, farms, groups of houses, yards

Protected site

20–26

Young forest, small forest clearing, park with big trees, city centres, closed deep valleys, strongly rugged terrain, leeward of big hills

Wetting losses occur with the moistening of the inner walls of the precipitation gauge. They depend on the shape and the material of the gauge, as well as on the type and frequency of precipitation. For example, for the Hellmann gauge they amount to an average of 0.3mm on a rainy and 0.15mm on a snowy day; the respective values for the Tretyakov gauge are 0.2mm and 0.1mm. Information on wetting losses for other types of gauges can be found in WMO (1982).

ANNEX 6.E. PROCEDURE FOR FIELD CALIBRATION OF CATCHMENT TYPE RAINFALL INTENSITY GAUGES

The field calibration is part of a routine field maintenance and check and should be performed on a regular basis. Its main purpose is to verify the operational status of precipitation gauges: to detect malfunctions, output anomalies and calibration drifts over time or between two laboratory calibrations. Field calibrations also provide valuable insight for data analysis and interpretation. The procedure is based on the same principles as laboratory calibration (given in Annex6.C), using the generation of constant intensity (stationary reference flow) within the gauge’s range of operational use. A field calibrator is typically composed of a cylindrical water tank of suitable capacity, a combination of air intakes and output nozzles for different rainfall intensities, and an electronic system to calculate the emptying time (see figure below). A suitable combination of air intakes and nozzles must be selected based on the precipitation gauge collector size and the intensity value chosen for the calibration. By opening the top tap and bottom nozzle, a constant flow is conveyed to the funnel of the gauge and, through the time of emptying and the conversion table (volume–time–intensity), it is possible to retrieve the reference intensity. Air intakes provide the pressure compensation, thus maintaining a constant push. From an operational viewpoint, the portable field calibrator permits rapid tests due to its very simple operation. The calibrator does not contain any sophisticated components and therefore provides a cost-effective solution for the metrological verification of precipitation intensity instruments. The repeatability of the field calibrator (and its accuracy) should be rigorously assessed in a laboratory before the operational use. The uncertainty should preferably be expressed as relative expanded uncertainty in relation to the statistical coverage interval (95% confidence level, k = 2) and should be lower than 2%. A statistical analysis of relative errors with respect to the field reference flow of the calibrator should be conducted for each field-calibrated precipitation gauge. At least 25–30 data points (normally 1min intensity values in mmh–1) should be recorded for each reference intensity (selected by the field calibrator). This makes it possible to assume a normal distribution of the data around the mean value and to better estimate the average and improve the accuracy of the results (central limit theorem). All tests must be performed in environmental conditions without precipitation or fog and with low wind flows (to avoid dynamic pressure perturbations to air intakes). The reference intensity should always be started at the beginning of a minute synchronized with the instrument clock or data-logger timer (official/station time-stamp).

Air intake Tap

Electrodes for emptying time

Air

Water

Nozzle Electronic timer Rainfall intensity (mm/h) = const

A simplified scheme of a portable field calibrator

242

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The minimum set of statistical parameters and metadata to be reported after each field calibration is listed below: (a) Date and time; (b) Reference intensity in mmh–1 (Iref ): constant intensity generated by the field calibrator; (c) Average (avgI) of intensity values (I1min) in mmh–1 of the precipitation gauge during the calibration, calculated as follows:

avgI =

1 N

N

j ) (6.E.1) ∑ ( I1min j =1

(d) Extremes (namely I+CL95% , I–CL95%) of an interval [avgI – δ(95%); avgI + δ(95%)] = [I+CL95%; I–CL95%] corresponding to the 95% confidence level. The amplitude δ(95%) is the half-width of the confidence interval calculated according to a normal or Student’s t probability distribution of samples (it includes a calculation of the standard deviation); (e) Relative error in percentage of the average intensity, calculated as follows:

 avgI − I ref  REavgI = 100 ⋅   (6.E.2) I ref  

(f) Relative errors in percentage of I+CL95% and I–CL95%, calculated as follows:

I −I  RE+CL95% = 100 ⋅  +CL95% ref  (6.E.3) I ref  

I − I ref RE−CL95% = 100 ⋅  −CL95% I ref 

  (6.E.4) 

The last three statistical parameters are used to calculate the gauge’s relative errors with regard to intensity with an uncertainty interval at the 95% confidence level for each reference intensity used during the calibration. The regular repetition of the field calibration and the comparison of results makes it possible to evaluate the stability of the calibration status and possible anomalies.

REFERENCES AND FURTHER READING

Chan, Y.W., C.L. Yu and K.H. Tam, 2015: Inter-comparison of raingauges on rainfall amount and intensity measurements in a tropical environment. Journal of Geodesy and Geomatics Engineering, 1:12–25. Colli, M., L.G. Lanza and P.W. Chan, 2013a: Co-located tipping-bucket and optical drop counter RI measurements and a simulated correction algorithm. Atmospheric Research, 119:3–12. Colli, M., L.G. Lanza and P. La Barbera, 2013b: Performance of a weighing rain gauge under laboratory simulated time-varying reference rainfall rates. Atmospheric Research, 131:3–12. European Committee for Standardization (CEN), 2010: Hydrometry – Specification for a Reference Raingauge Pit, EN13798:2010. Fikke, S., G. Ronsten, A. Heimo, S. Kunz, M. Ostrozlik, P.E. Persson, J. Sabata, B. Wareing, B. Wichura, J.Chum, T. Laakso, K. Säntti and L. Makkonen, 2007: COST-727: Atmospheric Icing on Structures; Measurements and data collection on icing: State of the Art, MeteoSwiss, No.75. Goodison, B.E., J.R. Metcalfe, R.A. Wilson and K. Jones, 1988: The Canadian automatic snow depth sensor: A performance update. Proceedings of the Fifty-sixth Annual Western Snow Conference, Atmospheric Environment Service, Canada, pp.178–181. Goodison, B.E., B. Sevruk and S. Klemm, 1989: WMO solid precipitation measurement intercomparison: Objectives, methodology and analysis. In: International Association of Hydrological Sciences, 1989: Atmospheric deposition. Proceedings, Baltimore Symposium (May, 1989), IAHS Publication No.179, Wallingford. Grunow, J., 1960: The productiveness of fog precipitation in relation to the cloud droplet spectrum. In: American Geophysical Union, 1960, Physics of precipitation. Geophysical Monograph No.5, Proceedings of the Cloud Physics Conference (3–5June 1959, Woods Hole, Massachusetts), Publication No.746, pp.110–117. ———, 1963: Weltweite Messungen des Nebelniederschlags nach der Hohenpeissenberger Methode. In: International Union of Geodesy and Geophysics, General Assembly (Berkeley, California, 19–31August 1963), International Association of Scientific Hydrology Publication No.65, pp.324–342. Habib, E., W.F. Krajewski and A. Kruger, 2001: Sampling errors of tipping-bucket rain gauge measurements. Journal of Hydrologic Engineering, 6:159–166. International Organization for Standardization, 2017: Atmospheric Icing of Structures, ISO12494:2017. Geneva. International Organization for Standardization/International Electrotechnical Commission, 2017: General Requirements for the Competence of Testing and Calibration Laboratories, ISO/IEC17025:2017. Geneva. Koschmieder, H., 1934: Methods and results of definite rain measurements; III. Danzig Report (1). Monthly Weather Review, 62:5–7. Legates, D.R. and C.J. Willmott, 1990: Mean seasonal and spatial variability in gauge-corrected, global precipitation. Int. J. Climatology, 10:111–127. Nespor, V. and B. Sevruk, 1999: Estimation of wind-induced error of rainfall gauge measurements using a numerical simulation. Journal of Atmospheric and Oceanic Technology, 16(4):450–464. Niemczynowicz, J., 1986: The dynamic calibration of tipping-bucket raingauges. Nordic Hydrology, 17:203–214. Rinehart, R.E., 1983: Out-of-level instruments: Errors in hydrometeor spectra and precipitation measurements. Journal of Climate and Applied Meteorology, 22:1404–1410. Schemenauer, R.S. and P. Cereceda, 1991: Fog water collection in arid coastal locations. Ambio, 20(7):303–308. ———, 1994a: Fog collection’s role in water planning for developing countries. Natural Resources Forum, 18(2):91–100. ———, 1994b: A proposed standard fog collector for use in high-elevation regions. Journal of Applied Meteorology, 33(11):1313–1322. Sevruk, B., 1974a: Correction for the wetting loss of a Hellman precipitation gauge. Hydrological Sciences Bulletin, 19(4):549–559. ———, 1974b: Evaporation losses from containers of Hellman precipitation gauges. Hydrological Sciences Bulletin, 19(2):231–236. ———, 1984: Comments on “Out-of-level instruments: Errors in hydrometeor spectra and precipitation measurements”. Journal of Climate and Applied Meteorology, 23:988–989.

244

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Sevruk, B. and V. Nespor, 1994: The effect of dimensions and shape of precipitation gauges on the windinduced error. In: Global Precipitation and Climate Change (M. Desbois and F. Desalmand, eds.). NATO ASI Series, Springer Verlag, Berlin, I26:​231–246. Sevruk, B. and L. Zahlavova, 1994: Classification system of precipitation gauge site exposure: Evaluation and application. International Journal of Climatology, 14(6):681–689. Sieck, L.C., S.J. Burges and M. Steiner, 2007: Challenges in obtaining reliable measurements of point rainfall. Water Resources Research, 43:1–23. Slovak Hydrometeorological Institute and Swiss Federal Institute of Technology, 1993: Precipitation measurement and quality control. Proceedings of the International Symposium on Precipitation and Evaporation (B. Sevruk and M. Lapin, eds.) (Bratislava, 20–24September1993), VolumeI, Bratislava and Zurich. Smith, J.L., H.G. Halverson and R.A. Jones, 1972: Central Sierra Profiling Snowgauge: A Guide to Fabrication and Operation. USAEC Report TID-25986, National Technical Information Service, U.S. Department of Commerce, Washington DC. Stadtmuller, T. and N. Agudelo, 1990: Amount and variability of cloud moisture input in a tropical cloud forest. In: Proceedings of the Lausanne Symposia (August/November), IAHS Publication No.193, Wallingford. Stagnaro, M., M. Colli, L.G. Lanza and P.W. Chan, 2016: Performance of post-processing algorithms for rainfall intensity using measurements from tipping-bucket rain gauges. Atmospheric Measurement Techniques, 9(12):5699-5706. Vong, R.J., J.T. Sigmon and S.F. Mueller, 1991: Cloud water deposition to Appalachian forests. Environmental Science and Technology, 25(6):1014–1021. World Meteorological Organization, 1972: Evaporation losses from storage gauges (B. Sevruk). In: Distribution of Precipitation in Mountainous Areas, Geilo Symposium (Norway, 31July– 5August1972), VolumeII – technical papers (WMO-No.326). Geneva. ———, 1982: Methods of Correction for Systematic Error in Point Precipitation Measurement for Operational Use (B.Sevruk). Operational Hydrology Report No.21 (WMO-No.589). Geneva. ———, 1984: International Comparison of National Precipitation Gauges with a Reference Pit Gauge (B.Sevruk and W.R. Hamon). Instruments and Observing Methods Report No.17 (WMO/TD‑No.38). Geneva. ———, 1985: International Organizing Committee for the WMO Solid Precipitation Measurement Intercomparison. Final report of the first session (distributed to participants only). Geneva. ———, 1986: Papers Presented at the Workshop on the Correction of Precipitation Measurements (B.Sevruk, ed.) (Zurich, Switzerland, 1–3 April1985). Instruments and Observing Methods Report No.25 (WMO/TD-No.104). Geneva. ———, 1987: Instruments Development Inquiry (E. Prokhorov). Instruments and Observing Methods Report No.24 (WMO/TD-No.231). Geneva. ———, 1989a: Catalogue of National Standard Precipitation Gauges (B. Sevruk and S. Klemm). Instruments and Observing Methods Report No.39 (WMO/TD-No.313). Geneva. ———, 1989b: International Workshop on Precipitation Measurements (B. Sevruk, ed.) (St Moritz, Switzerland, 3–7December 1989). Instruments and Observing Methods Report No.48 (WMO/TDNo.328). Geneva. ———, 1992a: Snow Cover Measurements and Areal Assessment of Precipitation and Soil Moisture (B.Sevruk, ed.). Operational Hydrology Report No.35 (WMO-No.749). Geneva. ———, 1992b: Report on the Measurement of Leaf Wetness (R.R. Getz). Agricultural Meteorology Report No.38 (WMO/TD-No.478). Geneva. ———, 1998: WMO Solid Precipitation Measurement Intercomparison: Final Report (B.E. Goodison, P.Y.T. Louie and D. Yang). Instruments and Observing Methods Report No.67 (WMO/TD-No.872). Geneva. ———, 2006: WMO Laboratory Intercomparison of Rainfall Intensity Gauges (L.G.Lanza, M. Leroy, C.Alexandropoulos, L. Stagi and W. Wauben). Instruments and Observing Methods Report No.84 (WMO/TD-No.1304). Geneva. ———, 2008: Guide to Hydrological Practices (WMO-No.168), VolumeI. Geneva. ———, 2009: WMO Field Intercomparison of Rainfall Intensity Gauges (E. Vuerich, C. Monesi, L.G. Lanza, L.Stagi, E. Lanzinger). Instruments and Observing Methods Report No.99 (WMO/ TD‑No.1504). Geneva.

CHAPTER 6. MEASUREMENT OF PRECIPITATION

245

Yang, D., J.R. Metcalfe, B.E. Goodison and E. Mekis, 1993: True Snowfall: An evaluation of the Double Fence Intercomparison Reference Gauge. Proceedings of the Fiftieth Eastern Snow Conference, Quebec City, 8–10June 1993, Quebec, Canada, pp.105–111. Yang, D., B.E. Goodison, J.R. Metcalfe, V.S.Golubev, E. Elomaa, T. Gunther, R. Bates, T.Pangburn, C.L.Hanson, D. Emerson, V.Copaciu and J.Milkovic, 1995: Accuracy of Tretyakov precipitation gauge: results of WMO intercomparison. Hydrological Processes, 9:877–895.

CHAPTER 7. MEASUREMENT OF RADIATION

7.1

GENERAL

The various fluxes of radiation to and from the Earth’s surface are among the most important variables in the heat economy of the Earth as a whole and at any individual place at the Earth’s surface or in the atmosphere. Radiation measurements are used for the following purposes: (a) To study the transformation of energy within the Earth–atmosphere system and its variation in time and space; (b) To analyse the properties and distribution of the atmosphere with regard to its constituents, such as aerosols, water vapour, ozone, and so on; (c) To study the distribution and variations of incoming, outgoing and net radiation; (d) To satisfy the needs of biological, medical, agricultural, architectural and industrial activities with respect to radiation; (e) To verify satellite radiation measurements and algorithms. Such applications require a widely distributed regular series of records of solar and terrestrial surface radiation components and the derivation of representative measures of the net radiation. In addition to the publication of serial values for individual observing stations, an essential objective must be the production of comprehensive radiation climatologies, whereby the daily and seasonal variations of the various radiation constituents of the general thermal budget may be more precisely evaluated and their relationships with other meteorological elements better understood. A very useful account of the operation and design of networks of radiation stations is contained in WMO(1986). VolumeV of the present Guide describes the scientific principles of the measurements and gives advice on QA, which is most important for radiation measurements. The Baseline Surface Radiation Network(BSRN) Operations Manual (WMO,2005a) gives an overview of the latest state of radiation measurements. Following normal practice in this field, errors and uncertainties are expressed in this chapter as a 66% confidence interval of the difference from the true quantity, which is similar to a standard deviation of the population of values. Where needed, specific uncertainty confidence intervals are indicated, and uncertainties are estimated using the ISO method (ISO/IEC, 2008/JCGM,2008). For example, 95% uncertainty implies that the stated uncertainty is for a confidence interval of95%. 7.1.1

Definitions

Annex 7.A contains the nomenclature of radiometric and photometric quantities. It is based on definitions recommended by the International Radiation Commission of the International Association of Meteorology and Atmospheric Sciences and by the International Commission on Illumination (CIE). Annex7.B gives the meteorological radiation quantities, symbols and definitions. Radiation quantities.  These may be classified into two groups according to their origin, namely solar and terrestrial radiation. In the context of this chapter, “radiation” can imply a process or apply to multiple quantities. For example, “solar radiation” could mean solar energy, solar exposure or solar irradiance (see Annex7.B). Solar energy.  This is the electromagnetic energy emitted by the sun. The solar radiation incident on the top of the terrestrial atmosphere is called extraterrestrial solar radiation; 97%of

CHAPTER 7. MEASUREMENT OF RADIATION

247

which is confined to the spectral range290 to 3000nm is called solar (or sometimes short-wave) radiation. Part of the extra-terrestrial solar radiation penetrates through the atmosphere to the Earth’s surface, while part of it is scattered and/or absorbed by the gas molecules, aerosol particles, cloud droplets and cloud crystals in the atmosphere. Terrestrial radiation.  This is the long-wave electromagnetic energy emitted by the Earth’s surface and by the gases, aerosols and clouds of the atmosphere; it is also partly absorbed within the atmosphere. For a temperature of 300K, 99.99% of the power of the terrestrial radiation has a wavelength longer than 3000nm and about 99% longer than 5000nm. For lower temperatures, the spectrum is shifted to longer wavelengths.

Since the spectral distributions of solar and terrestrial radiation overlap very little, they can very often be treated separately in measurements and computations. In meteorology, the sum of both types is called total radiation.

Light.  This is the radiation visible to the human eye. The spectral range of visible radiation is defined by the spectral luminous efficiency for the standard observer. The lower limit is taken to be between 360 and 400nm, and the upper limit between760 and 830nm (CIE,1987). The radiation of wavelengths shorter than about 400nm is termed UV, and longer than about 800nm, IR radiation. The UV range is sometimes divided into three subranges (IEC,1987):

UV-A: 315–400 nm UV-B: 280–315 nm UV-C: 100–280 nm

7.1.2

Units and scales

7.1.2.1

Units

The SI is to be preferred for meteorological radiation variables. A general list of the units is given in Annexes7.A and7.B. 7.1.2.2

Standardization

The responsibility for the calibration of radiometric instruments rests with the World, Regional and National Radiation Centres, the specifications for which are given in Annex7.C. Furthermore, the World Radiation Centre(WRC) at Davos is responsible for maintaining the basic reference, the World Standard Group(WSG) of instruments, which is used to establish the WRR. During international comparisons, organized every five years, the standards of the regional centres are compared with the WSG, and their calibration factors are adjusted to the WRR. They, in turn, are used to transmit the WRR periodically to the national centres, which calibrate their network instruments using their own standards. Definition of the World Radiometric Reference In the past, several radiation references or scales have been used in meteorology, namely the Ångström scale of 1905, the Smithsonian scale of 1913, and the international pyrheliometric scale of 1956 (IPS1956). The developments in absolute radiometry in recent years have very much reduced the uncertainty of radiation measurements. With the results of many comparisons of 15individual absolute pyrheliometers of 10different types, a WRR has been defined. The old scales can be transferred into the WRR using the following factors:

WRR = 1.026 Angstrom scale 1905 WRR = 0.977 Smithsonian scale 1913 WRR = 1.022 IPS 1956

248

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The WRR is accepted as representing the physical units of total irradiance within0.3% (99%uncertainty of the measured value). Realization of the World Radiometric Reference: World Standard Group To guarantee the long-term stability of the new reference, a group of at least four absolute pyrheliometers of different design is used as the WSG. At the time of incorporation into this group, the instruments are given a reduction factor to correct their readings to the WRR. To qualify for membership of this group, a radiometer must fulfil the following specifications: (a) Stability must be better than 0.2% of the measured value over timescales of decades; (b) The 95% uncertainty of the series of measurements with the instrument must lie within the limits of the uncertainty of the WRR; (c) The instrument has to have a different design from the other WSG instruments. To meet the stability criteria, the instruments of the WSG are the subjects of an intercomparison at least once a year, and, for this reason, WSG is kept at the WRC Davos. Computation of world radiometric reference values To calibrate radiometric instruments, the reading of a WSG instrument, or one that is directly traceable to the WSG, should be used. During international pyrheliometer comparisons(IPCs), the WRR value is calculated from the mean of at least three participating instruments of the WSG. To yield WRR values, the readings of the WSG instruments are always corrected with the individual reduction factor, which is determined at the time of their incorporation into the WSG. The calculation of the mean value of the WSG, serving as the reference, may be jeopardized by the failure of one or more radiometers. To address this issue CIMO resolved1 that at each IPC an ad hoc group should be established comprising the Rapporteur on Meteorological Radiation Instruments (or designate) and at least five members, including the chair. This group assesses the stability of the WSG instruments, and selects instruments to be used in the calculation of the WRR. The director of the comparison must participate in the group’s meetings as an expert. The group should discuss the preliminary results of the comparison, based on criteria defined by the WRC, evaluate the reference and recommend the updating of the calibration factors. 7.1.3

Meteorological requirements

7.1.3.1

Data to be reported

Irradiance and radiant exposure are the quantities most commonly recorded and archived, with averages and totals of over 1h. There are also many requirements for data over shorter periods, down to 1min or even tens of seconds (for some energy applications). Daily totals of radiant exposure are frequently used, but these are expressed as a mean daily irradiance. Measurements of atmospheric extinction must be made with very short response times to reduce the uncertainties arising from variations in the air mass. For radiation measurements, it is particularly important to record and make available information about the circumstances of the observations. This includes the type and traceability of the instrument, its calibration history, and its location in space and time, spatial exposure and maintenance record.

1

Recommended by CIMO at its eleventh session (1994).

CHAPTER 7. MEASUREMENT OF RADIATION

7.1.3.2

249

Uncertainty

There are no formally agreed statements of required uncertainty for most radiation quantities, but uncertainty is discussed in the sections of this chapter dealing with the various types of measurements, and best practice uncertainties are stated for the Global Climate Observing System’s (GCOS) Baseline Surface Radiation Network (seeWMO,2005a). It may be said generally that good quality measurements are difficult to achieve in practice, and for routine operations they can be achieved only with modern equipment and redundant measurements. Some systems still in use fall short of best practice as the lesser performance has been acceptable for many applications. However, data of the highest quality are increasingly in demand. Statements of uncertainty for net radiation and radiant exposure are given in the present volume, Chapter1, Annex1.A. The required 95% uncertainty for radiant exposure for a day, stated by WMO for international exchange, is 0.4MJm–2 for ≤8MJm–2 and 5% for >8MJm–2. 7.1.3.3

Sampling and recording

The uncertainty requirements can best be satisfied by making observations at a sampling period less than the 1/etime constant of the instrument, even when the data to be finally recorded are integrated totals for periods of up to 1h, or more. The data points may be integrated totals or an average flux calculated from individual samples. Digital data systems are greatly to be preferred. Chart recorders and other types of integrators are much less convenient, and the resultant quantities are difficult to maintain at adequate levels of uncertainty. 7.1.3.4

Times of observation

In a worldwide network of radiation measurements, it is important that the data be homogeneous not only for calibration, but also for the times of observation. Therefore, all radiation measurements should be referred to what is known in some countries as local apparent time, and in others as true solar time. However, standard or universal time(UT) is attractive for automatic systems because it is easier to use, but is acceptable only if a reduction of the data to true solar time does not introduce a significant loss of information (that is to say, if the sampling and storage rates are high enough, as indicated in 7.1.3.3 above). See Annex7.D for useful formulae for the conversion from standard to solar time. 7.1.4

Measurement methods

Meteorological radiation instruments are classified using various criteria, namely the type of variable to be measured, the field of view, the spectral response, the main use, and the like. The most important types of classifications are listed in Table7.1. The quality of the instruments is characterized by items (a) to (h) below. The instruments and their operation are described in 7.2 to 7.4 below. WMO(1986) provides a detailed account of instruments and the principles according to which they operate. Absolute radiometers are self-calibrating, meaning that the irradiance falling on the sensor is replaced by electrical power, which can be accurately measured. The substitution, however, cannot be perfect; the deviation from the ideal case determines the uncertainty of the radiation measurement. Most radiation sensors, however, are not absolute and must be calibrated against an absolute instrument. The uncertainty of the measured value, therefore, depends on the following factors, all of which should be known for a well-characterized instrument: (a) Resolution, namely, the smallest change in the radiation quantity which can be detected by the instrument; (b) Drifts of sensitivity (the ratio of electrical output signal to the irradiance applied) over time;

250

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Table 7.1. Meteorological radiation instruments Viewing angle (sr) (see Figure 7.1)

Instrument classification

Parameter to be measured

Main use

Absolute pyrheliometer

Direct solar radiation

Primary standard

Pyrheliometer

Direct solar radiation

(a) Secondary standard for calibrations (b) Network

5 x 10 –3 to 2.5 x 10 –2

Spectral pyrheliometer

Direct solar radiation in broad spectral bands (e.g. with OG530, RG630, etc. filters)

Network

5 x 10 –3 to 2.5 x 10 –2

Sunphotometer

Direct solar radiation in narrow spectral bands (e.g.at 500±2.5nm, 368 ± 2.5nm)

(a) Standard (b) Network

Pyranometer

(a) Global (solar) radiation (b) Diffuse sky (solar) radiation (c) Reflected solar radiation

(a) Working standard (b) Network

Spectral pyranometer

Global (solar) radiation in broadband spectral ranges (e.g. with OG530, RG630, etc. filters)

Network

Net pyranometer

Net global (solar) radiation

(a) Working standard (b) Network

Pyrgeometer

(a) Upward long-wave radiation (downward-looking) (b) Downward long-wave radiation (upward-looking)

Network

Pyrradiometer

Total radiation

Working standard

Net pyrradiometer

Net total radiation

Network

5 x 10 –3 (approx. 2.5˚ half angle)

1 x 10 –3 to 1 x 10 –2 (approx. 2.3˚ full angle)

CHAPTER 7. MEASUREMENT OF RADIATION

251

(c) Changes in sensitivity owing to changes of environmental variables, such as temperature, humidity, pressure and wind; (d) Non-linearity of response, namely, changes in sensitivity associated with variations in irradiance; (e) Deviation of the spectral response from that postulated, namely the blackness of the receiving surface, the effect of the aperture window, and so on; (f) Deviation of the directional response from that postulated, namely cosine response and azimuth response; (g) Time constant of the instrument or the measuring system; (h) Uncertainties in the auxiliary equipment. Instruments should be selected according to their end-use and the required uncertainty of the derived quantity. Certain instruments perform better for particular climates, irradiances and solar positions.

7.2

MEASUREMENT OF DIRECT SOLAR RADIATION

Direct solar radiation is measured using pyrheliometers, the receiving surfaces of which are arranged to be normal to the solar direction. Using apertures, only the radiation from the sun and a narrow annulus of the sky is measured, the latter radiation component is sometimes referred to as circumsolar radiation or aureole radiation. In modern instruments, this extends out to a half-angle of about 2.5° on some models, and to about 5° from the sun’s centre (corresponding, respectively, to 6·10 –3 and 2.4·10 –2sr). The pyrheliometer mount must allow for the rapid and smooth adjustment of the azimuth and elevation angles. A sighting device is usually included in which a small spot of light or solar image falls upon a mark in the centre of the target when the receiving surface is exactly normal to the direct solar beam. For continuous recording, it is advisable to use automatic sun-following equipment (sun tracker). For all new designs of direct solar radiation instruments, it is recommended that the opening half‑angle be 2.5°(6·10 –3sr) and the slope angle1°. For the definition of these angles refer to Figure7.1.

R

Front aperture

d r

Receiving surface

Figure 7.1. View-limiting geometry: The opening half-angle is arctan R/d; the slope angle is arctan (R–r)/d.

252

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

During the comparison of instruments with different view-limiting geometries, the aureole radiation influences the readings more significantly for larger slope and aperture angles. The difference can be as great as 2% between the two apertures mentioned above for an air massof1.0. In order to enable climatological comparison of direct solar radiation data during different seasons, it may be necessary to reduce all data to a mean Sun–Earth distance: EN = E / R 2 (7.1)

where EN is the solar radiation, normalized to the mean Sun–Earth distance, which is defined to be one astronomical unit(AU) (see Annex7.D); E is the measured direct solar radiation, and Ris the Sun–Earth distance inAUs. 7.2.1

Direct solar radiation

Some of the characteristics of operational pyrheliometers (other than primary standards) are given in Table7.2 (adapted from ISO,1990a), with indicative estimates of the uncertainties of measurements made with them if they are used with appropriate expertise andQC. Cheaper pyrheliometers are available (see ISO,1990a), but without effort to characterize their response, the resulting uncertainties reduce the quality of the data, and, given that a sun tracker is required, in most cases the incremental cost for a good pyrheliometer is minor. The estimated uncertainties are based on the following assumptions: (a) Instruments are well-maintained, calibrated, correctly aligned and clean; (b) 1min and 1h figures are for clear-sky irradiances at solar noon; (c) Daily exposure values are for clear days at mid-latitudes. 7.2.1.1

Primary standard pyrheliometers

An absolute pyrheliometer can define the scale of total irradiance without resorting to reference sources or radiators. The limits of uncertainty of the definition must be known; the quality of this knowledge determines the reliability of an absolute pyrheliometer. Only specialized laboratories should operate and maintain primary standards. Details of their construction and operation are given in WMO (1986). However, for the sake of completeness, a brief account is given here. All absolute pyrheliometers of modern design use cavities as receivers and electrically calibrated, differential heat-flux meters as sensors. At present, this combination has proved to yield the lowest uncertainty possible for the radiation levels encountered in solar radiation measurements (namely, up to1.5kWm–2). Normally, the electrical calibration is performed by replacing the radiative power by electrical power, which is dissipated in a heater winding as close as possible to where the absorption of solar radiation takes place. The uncertainties of such an instrument’s measurements are determined by a close examination of the physical properties of the instrument and by performing laboratory measurements and/ or model calculations to determine the deviations from ideal behaviour, that is, how perfectly the electrical substitution can be achieved. This procedure is called characterization of the instrument. The following specification should be met by an absolute pyrheliometer (an individual instrument, not a type) to be designated and used as a primary standard: (a) At least one instrument out of a series of manufactured radiometers has to be fully characterized. The95% uncertainty of this characterization should be less than 2Wm–2 under the clear-sky conditions suitable for calibration (seeISO, 1990a). The95% uncertainty (for all components of the uncertainty) for a series of measurements should not exceed 4Wm–2 for any measured value;

253

CHAPTER 7. MEASUREMENT OF RADIATION

Table 7.2. Characteristics of operational pyrheliometers Characteristic

High qualitya

Good qualityb

< 15 s

< 30 s

2 W m–2

4 W m–2

0.51

1

0.1

0.5

1

2

Non-linearity (percentage deviation from the responsivity at 500Wm–2 due to the change of irradiance within 100Wm–2 to 1100Wm–2)

0.2

0.5

Spectral sensitivity (percentage deviation of the product of spectral absorptance and spectral transmittance from the corresponding mean within the range 300 to 3000nm)

0.5

1.0

Tilt response (percentage deviation from the responsivity at 0° tilt (horizontal) due to change in tilt from 0° to 90° at 1000Wm–2)

0.2

0.5

0.9

1.8

kJ m–2

0.56

1

%

0.7

1.5

kJ m–2

21

54

%

0.5

1.0

kJ m–2

200

400

Response time (95% response) Zero offset (response to 5 K h–1 change in ambient temperature) Resolution (smallest detectable change in W m–2) Stability (percentage of full scale, change/year) Temperature response (percentage maximum error due to change of ambient temperature within an interval of 50K)

Achievable uncertainty, 95% confidence level (see above) 1 min totals

1 h totals

Daily totals

%

Notes: a Near state of the art; suitable for use as a working standard; maintainable only at stations with special facilities and staff. b Acceptable for network operations.

(b) Each individual instrument of the series must be compared with the one which has been characterized, and no individual instrument should deviate from this instrument by more than the characterization uncertainty as determined in(a) above; (c) A detailed description of the results of such comparisons and of the characterization of the instrument should be made available upon request; (d) Traceability to the WRR by comparison with the WSG or some carefully established reference with traceability to the WSG is needed in order to prove that the design is within the state of the art. The latter is fulfilled if the 95% uncertainty for a series of measurements traceable to the WRR is less than1Wm–2. 7.2.1.2

Secondary standard pyrheliometers

An absolute pyrheliometer which does not meet the specification for a primary standard or which is not fully characterized can be used as a secondary standard if it is calibrated by comparison with the WSG with a 95% uncertainty for a series of measurements less than1Wm–2. Other types of instruments with measurement uncertainties similar or approaching those for primary standards may be used as secondary standards.

254

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

The Ångström compensation pyrheliometer has been and still is, used as a convenient secondary standard instrument for the calibration of pyranometers and other pyrheliometers. It was designed by K.Ångström as an absolute instrument, and the Ångström scale of 1905 was based on it; now it is used as a secondary standard and must be calibrated against a standard instrument. The sensor consists of two platinized manganin strips, each of which is about 18mm long, 2mm wide and about 0.02mm thick. They are blackened with a coating of candle soot or with an optical matt black paint. A thermo-junction of copper-constantan is attached to the back of each strip so that the temperature difference between the strips can be indicated by a sensitive galvanometer or an electrical micro-voltmeter. The dimensions of the strip and front diaphragm yield opening half‑angles and slope angles as listed in Table7.3. Table 7.3. View-limiting geometry of Ångström pyrheliometers Angle Opening half-angle Slope angle

Vertical

Horizontal

5° – 8°

~2°

0.7° – 1.0°

1.2° – 1.6°

The measurement set consists of three or more cycles, during which the left- or right-hand strip is alternately shaded from or exposed to the direct solar beam. The shaded strip is heated by an electric current, which is adjusted in such a way that the thermal electromagnetic force of the thermocouple and, hence, the temperature difference between the two strips approximate zero. Before and after a measuring sequence, the zero is checked either by shading or by exposing both strips simultaneously. Depending on which of these methods is used and on the operating instructions of the manufacturer, the irradiance calculation differs slightly. The method adopted for the IPCs uses the following formula:

E = K ⋅ iL ⋅ iR (7.2)

where Eis the irradiance in W m–2; Kis the calibration constant determined by comparison with a primary standard (Wm–2A–2); and iLiRis the current in amperes measured with the left- or right‑hand strip exposed to the direct solar beam, respectively. Before and after each series of measurements, the zero of the system is adjusted electrically by using either of the foregoing methods, the zeros being called “cold” (shaded) or “hot” (exposed), as appropriate. Normally, the first reading, say iR, is excluded and only the following iL–iR pairs are used to calculate the irradiance. When comparing such a pyrheliometer with other instruments, the irradiance derived from the currents corresponds to the geometric mean of the solar irradiances at the times of the readings of iL andiR. The auxiliary instrumentation consists of a power supply, a current-regulating device, a nullmeter and a current monitor. The sensitivity of the nullmeter should be about 0.05·10 –6 A per scale division for a lowinput impedance (10kΩ). Under these conditions, a temperature difference of about 0.05K between the junction of the copper‑constantan thermocouple causes a deflection of one scale division, which indicates that one of the strips is receiving an excess heat supply amounting to about0.3%. The uncertainty of the derived direct solar irradiance is highly dependent on the qualities of the current-measuring device, whether a moving-coil milliammeter or a digital multimeter which measures the voltage across a standard resistor and on the operator’s skill. The fractional error in the output value of irradiance is twice as large as the fractional error in the reading of the electric current. The heating current is directed to either strip by means of a switch and is normally controlled by separate rheostats in each circuit. The switch can also cut the current off so that the zero can be determined. The resolution of the rheostats should be sufficient to allow the nullmeter to be adjusted to within one half of a scale division.

CHAPTER 7. MEASUREMENT OF RADIATION

7.2.1.3

255

Field and network pyrheliometers

These pyrheliometers generally make use of a thermopile as the detector. They have similar view‑limiting geometry as standard pyrheliometers. Older models tend to have larger fields of view and slope angles. These design features were primarily designed to reduce the need for accurate sun tracking. However, the larger the slope (and opening) angle, the larger the amount of aureole radiation sensed by the detector; this amount may reach several per cent for high optical depths and large limiting angles. With new designs of sun trackers, including computerassisted trackers in both passive and active (sun-seeking) configurations, the need for larger slope angles is unnecessary. However, a slope angle of1° is still required to ensure that the energy from the direct solar beam is distributed evenly on the detector; and allows for minor sun tracker pointing errors of the order of0.1°. The intended use of the pyrheliometer may dictate the selection of a particular type of instrument. Some manually oriented models are used mainly for spot measurements, while others, installed on a sun tracker, are designed specifically for the long-term monitoring of direct irradiance. Before deploying an instrument, the user must consider the significant differences found among operational pyrheliometers as follows: (a) The field of view of the instrument; (b) Whether the instrument measures both the long-wave and short-wave portion of the spectrum (namely, whether the aperture is open or covered with a glass or quartz window); (c) The temperature compensation or correction methods; (d) The magnitude and variation of the zero irradiance signal; (e) If the instrument can be installed on an automated tracking system for long-term monitoring; (f) If, for the calibration of other operational pyrheliometers, differences (a) to (c) above are the same, and if the pyrheliometer is of the quality required to calibrate other network instruments. 7.2.1.4

Calibration of pyrheliometers

All pyrheliometers, other than absolute pyrheliometers, must be calibrated by comparison using the sun as the source with a pyrheliometer that has traceability to the WSG and a likely uncertainty of calibration equal to or better than the pyrheliometer being calibrated. As all solar radiation data must be referred to the WRR, absolute pyrheliometers also use a factor determined by comparison with the WSG and not their individually determined one. After such a comparison (for example, during the periodically organized IPCs) such a pyrheliometer can be used as a standard to calibrate, again by comparison with the sun as a source, secondary standards and field pyrheliometers. Secondary standards can also be used to calibrate field instruments, but with increased uncertainty. The quality of sun-source calibrations may depend on the aureole influence if instruments with different view-limiting geometries are compared. Also, the quality of the results will depend on the variability of the solar irradiance, if the time constants and zero irradiance signals of the pyrheliometers are significantly different. Lastly, environmental conditions, such as temperature, pressure and net long-wave irradiance, can influence the results. If a very high quality of calibration is required, only data taken during very clear and stable days should be used. The procedures for the calibration of field pyrheliometers are given in an ISO standard (ISO,1990b).

256

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

From recent experience at IPCs, a period of five years between traceable calibrations to the WSG should suffice for primary and secondary standards. Field pyrheliometers should be calibrated every one to two years; the more prolonged the use and the more rigorous the conditions, the more often they should be calibrated. 7.2.2

Exposure

For continuous recording and reduced uncertainties, an accurate sun tracker that is not influenced by environmental conditions is essential. Sun tracking to within 0.2° is required, and the instruments should be inspected at least once a day, and more frequently if weather conditions so demand (with protection against adverse conditions). The principal exposure requirement for monitoring direct solar radiation is freedom from obstructions to the solar beam at all times and seasons of the year. Furthermore, the site should be chosen so that the incidence of fog, smoke and airborne pollution is as typical as possible of the surrounding area. For continuous observations, typically a window is used to protect the sensor and optical elements against rain, snow, and so forth. Care must be taken to ensure that such a window is kept clean and that condensation does not appear on the inside.

7.3

MEASUREMENT OF GLOBAL AND DIFFUSE SKY RADIATION

The solar radiation received from a solid angle of 2πsr on a horizontal surface is referred to as global radiation. This includes radiation received directly from the solid angle of the sun’s disc, as well as diffuse sky radiation that has been scattered in traversing the atmosphere. The instrument needed for measuring solar radiation from a solid angle of 2πsr into a plane surface and a spectral range from 300 to 3000nm is the pyranometer. The pyranometer is sometimes used to measure solar radiation on surfaces inclined in the horizontal and in the inverted position to measure reflected global radiation. When measuring the diffuse sky component of solar radiation, the direct solar component is screened from the pyranometer by a shading device (see7.3.3.3). Pyranometers normally use thermo-electric, photoelectric, pyro-electric or bimetallic elements as sensors. Since pyranometers are exposed continually in all weather conditions they must be robust in design and resist the corrosive effects of humid air (especially near the sea). The receiver should be hermetically sealed inside its casing, or the casing must be easy to take off so that any condensed moisture can be removed. Where the receiver is not permanently sealed, a desiccator is usually fitted in the base of the instrument. The properties of pyranometers which are of concern when evaluating the uncertainty and quality of radiation measurement are: sensitivity, stability, response time, cosine response, azimuth response, linearity, temperature response, thermal offset, zero irradiance signal and spectral response. Further advice on the use of pyranometers is given in ISO(1990c) and WMO(2005a). Table 7.4 (adapted from ISO, 1990a) describes the characteristics of pyranometers of various levels of performance, with the uncertainties that may be achieved with appropriate facilities, well-trained staff and good QC under the sky conditions outlined in7.2.1.

257

CHAPTER 7. MEASUREMENT OF RADIATION

Table 7.4. Characteristics of operational pyranometers Characteristic

High qualitya

Good qualityb

Moderate qualityc

< 15 s

< 30 s

< 60 s

7 W m–2

15 W m–2

30 W m–2

2 W m–2

4 W m–2

8 W m–2

1 W m–2

5 W m–2

10 W m–2

Stability (change per year, percentage of full scale)

0.8

1.5

3.0

Directional response for beam radiation (the range of errors caused by assuming that the normal incidence responsivity is valid for all directions when measuring, from any direction, a beam radiation whose normal incidence irradiance is 1000Wm–2)

10 W m–2

20 W m–2

30 W m–2

Temperature response (percentage maximum error due to any change of ambient temperature within an interval of 50K)

2

4

8

0.5

1

3

Spectral sensitivity (percentage deviation of the product of spectral absorptance and spectral transmittance from the corresponding mean within the range 300 to 3000nm)

2

5

10

Tilt response (percentage deviation from the responsivity at 0˚ tilt (horizontal) due to change in tilt from 0˚ to 90˚ at 1000Wm–2)

0.5

2

5

3% 2%

8% 5%

20% 10%

Response time (95% response) Zero offset: (a) Response to 200 W m–2 net thermal radiation (ventilated) (b) Response to 5 K h–1 change in ambient temperature Resolution (smallest detectable change)

Non-linearity (percentage deviation from the responsivity at 500Wm–2 due to any change of irradiance within the range 100 to 1000Wm–2)

Achievable uncertainty (95% confidence level): Hourly totals Daily totals

Notes: a Near state of the art; suitable for use as a working standard; maintainable only at stations with special facilities and staff. b Acceptable for network operations. c Suitable for low-cost networks where moderate to low performance is acceptable.

7.3.1

Calibration of pyranometers

The calibration of a pyranometer consists of the determination of one or more calibration factors and the dependence of these on environmental conditions, such as: (a) Angular distribution of irradiance; (b) Calibration methods; (c) Directional response of the instrument; (d) Inclination of instrument; (e) Irradiance level; (f) Net long-wave irradiance for thermal offset correction;

258

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

(g) Spectral distribution of irradiance; (h) Temperature; (i)

Temporal variation.

The users of pyranometers must recognize that the uncertainty of observations will increase when the sensor exposure conditions deviate from the conditions in which the pyranometer was calibrated. Normally, it is necessary to specify the test environmental conditions, which can be quite different for different applications. The method and conditions must also be given in some detail in the calibration certificate. There are a variety of methods for calibrating pyranometers using the sun or laboratory sources. These include the following: (a) By comparison with a standard pyrheliometer for the direct solar irradiance and a calibrated shaded pyranometer for the diffuse sky irradiance; (b) By comparison with a standard pyrheliometer using the sun as a source, with a removable shading disc for the pyranometer; (c) With a standard pyrheliometer using the sun as a source and two pyranometers to be calibrated alternately measuring global and diffuse irradiance; (d) By comparison with a standard pyranometer using the sun as a source, under other natural conditions of exposure (for example, a uniform cloudy sky and direct solar irradiance not statistically different from zero); (e) In the laboratory, on an optical bench with an artificial source, either normal incidence or at some specified azimuth and elevation, by comparison with a similar pyranometer previously calibrated outdoors; (f) In the laboratory, with the aid of an integrating chamber simulating diffuse sky radiation, by comparison with a similar type of pyranometer previously calibrated outdoors. These are not the only methods; (a), (b) and (c) and (d) are commonly used. However, it is essential that, except for (b), either the zero irradiance signals for all instruments are known or pairs of identical model pyranometers in identical configurations are used. Ignoring these offsets and differences can bias the results significantly. Method (c) is considered to give very good results without the need for a calibrated pyranometer. It is difficult to determine a specific number of measurements on which to base the calculation of the pyranometer calibration factor. However, the standard error of the mean can be calculated and should be less than the desired limit when sufficient readings have been taken under the desired conditions. The principal variations (apart from fluctuations due to atmospheric conditions and observing limitations) in the derived calibration factor are due to the following: (a) Departures from the cosine law response, particularly at solar elevations of less than 10° (for this reason it is better to restrict calibration work to occasions when the solar elevation exceeds30°); (b) The ambient temperature; (c) Imperfect levelling of the receiver surface;

CHAPTER 7. MEASUREMENT OF RADIATION

259

(d) Non-linearity of instrument response; (e) The net long-wave irradiance between the detector and the sky. The pyranometer should be calibrated only in the position of use. When using the sun as the source, the apparent solar elevation should be measured or computed (to the nearest0.01°) for this period from solar time (see Annex7.D). The mean instrument or ambient temperature should also be noted. 7.3.1.1

By reference to a standard pyrheliometer and a shaded reference pyranometer

In this method, described in ISO (1993), the pyranometer’s response to global irradiance is calibrated against the sum of separate measurements of the direct and diffuse components. Periods with clear skies and steady radiation (as judged from the record) should be selected. The vertical component of the direct solar irradiance is determined from the pyrheliometer output, and the diffuse sky irradiance is measured with a second pyranometer that is continuously shaded from the sun. The direct component is eliminated from the diffuse sky pyranometer by shading the whole outer dome of the instrument with a disc of sufficient size mounted on a slender rod and held some distance away. The diameter of the disc and its distance from the receiver surface should be chosen in such a way that the screened angle approximately equals the aperture angles of the pyrheliometer. Rather than using the radius of the pyranometer sensor, the radius of the outer dome should be used to calculate the slope angle of the shading disc and pyranometer combination. This shading arrangement occludes a close approximation of both the direct solar beam and the circumsolar sky irradiance as sensed by the pyrheliometer. On a clear day, the diffuse sky irradiance is less than 15% of the global irradiance; hence, the calibration factor of the reference pyranometer does not need to be known very accurately. However, care must be taken to ensure that the zero irradiance signals from both pyranometers are accounted for, given that for some pyranometers under clear sky conditions the zero irradiance signal can be as high as 15% of the diffuse sky irradiance. The calibration factor is then calculated according to:

E ⋅ sin h + Vs ks = V ⋅ k (7.3)

or:

k = (E sin h + Vs ks ) / V (7.4)

where Eis the direct solar irradiance measured with the pyrheliometer (Wm–2), Vis the global irradiance output of the pyranometer to be calibrated(µV); Vs is the diffuse sky irradiance output of the shaded reference pyranometer(µV), his the apparent solar elevation at the time of reading; kis the calibration factor of the pyranometer to be calibrated (Wm–2µV–1); and ksis the calibration factor of the shaded reference pyranometer (Wm–2µV–1), and all the signal measurements are taken simultaneously. The direct, diffuse and global components will change during the comparison, and care must be taken with the appropriate sampling and averaging to ensure that representative values are used. 7.3.1.2

By reference to a standard pyrheliometer

This method, described in ISO (1993), is similar to the method of the preceding paragraph, except that the diffuse sky irradiance signal is measured by the same pyranometer. The direct component is eliminated temporarily from the pyranometer by shading the whole outer dome of the instrument as described in7.3.1.1. The period required for occulting depends on the steadiness of the radiation flux and the response time of the pyranometer, including the

260

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

time interval needed to bring the temperature and long-wave emission of the glass dome to equilibrium; 10times the thermopile 1/e time constant of the pyranometer should generally be sufficient. The difference between the representative shaded and unshaded outputs from the pyranometer is due to the vertical component of direct solar irradianceE measured by the pyrheliometer. Thus:

E ⋅ sin h = (Vun − Vs ) ⋅ k (7.5)

or:

k = (E ⋅ sin h) / (Vun − Vs ) (7.6)

where E is the representative direct solar irradiance at normal incidence measured by the pyrheliometer (Wm–2); Vunis the representative output signal of the pyranometer (µV) when in unshaded (or global) irradiance mode; Vs is the representative output signal of the pyranometer (µV) when in shaded (or diffuse sky) irradiance mode; his the apparent solar elevation, and kis the calibration factor (Wm–2µV–1), which is the inverse of the sensitivity (µVW–1m2). Both the direct and diffuse components will change during the comparison, and care must be taken with the appropriate sampling and averaging to ensure that representative values of the shaded and unshaded outputs are used for the calculation. To reduce uncertainties associated with representative signals, a continuous series of shade and un-shade cycles should be performed and time-interpolated values used to reduce temporal changes in global and diffuse sky irradiance. Since the same pyranometer is being used in differential mode, and the difference in zero irradiance signals for global and diffuse sky irradiance is negligible, there is no need to account for zero irradiances in equation7.6. 7.3.1.3

Alternate calibration using a pyrheliometer

This method uses the same instrumental set-up as the method described in7.3.1.1, but only requires the pyrheliometer to provide calibrated irradiance data (E), and the two pyranometers are assumed to be un-calibrated (Forgan,1996). The method calibrates both pyranometers by solving a pair of simultaneous equations analogous to equation7.3. Irradiance signal data are initially collected with the pyrheliometer and one pyranometer (pyranometerA) measures global irradiance signals(VgA) and the other pyranometer (pyranometerB) measures diffuse irradiance signals (VdB) over a range of solar zenith angles in clear sky conditions. After sufficient data have been collected in the initial configuration, the pyranometers are exchanged so that pyranometerA, which initially measured the global irradiance signal, now measures the diffuse irradiance signal(VdA), and vice versa with regard to pyranometerB. The assumption is made that for each pyranometer the diffuse(kd) and global(kg) calibration coefficients are equal, and the calibration coefficient for pyranometerA is given by:

= kA k= gA kdA (7.7)

with an identical assumption for pyranometer B coefficients. Then for a timet0 in the initial period a modified version of equation7.3is:

E(t0 )sin(h(t0 )) = kAVgA (t0 ) − kBVdB(t0 ) (7.8)

For timet1 in the alternate period when the pyranometers are exchanged:

E(t1)sin(h(t1)) = kBVgB(t1) − kAVdA (t1) (7.9)

As the only unknowns in equations 7.8 and 7.9 are kA and kB, these can be solved for any pair of times (t0, t1). Pairs covering a range of solar elevations provide an indication of the directional response. The resultant calibration information for both pyranometers is representative of the global calibration coefficients and produces almost identical information to method7.3.1.1, but without the need for a calibrated pyranometer. As with method 7.3.1.1, to produce coefficients with minimum uncertainty this alternate method requires that the irradiance signals from the pyranometers be adjusted to remove any

CHAPTER 7. MEASUREMENT OF RADIATION

261

estimated zero irradiance offset. To reduce uncertainties due to changing directional response it is recommended to use a pair of pyranometers of the same model and observation pairs when sinh(t0)~sinh(t1). The method is ideally suited to automatic field monitoring situations where three solar irradiance components (direct, diffuse and global) are monitored continuously. Experience suggests that the data collection necessary for the application of this method may be conducted during as little as one day with the exchange of instruments taking place around solar noon. However, at a field site, the extended periods and days either side of the instrument change may be used for data selection, provided that the pyrheliometer has a valid calibration. 7.3.1.4

By comparison with a reference pyranometer

As described in ISO (1992), this method entails the simultaneous operation of two pyranometers mounted horizontally, side by side, outdoors for a sufficiently long period to acquire representative results. If the instruments are of the same model and monitoring configuration, only one or two days of comparison should be sufficient. The more pronounced the difference between the types of pyranometer configurations, the longer the period of comparison required. A long period, however, could be replaced by several shorter periods covering typical conditions (clear, cloudy, overcast, rainfall, snowfall, and so on). The derivation of the instrument factor is straightforward, but, in the case of different pyranometer models, the resultant uncertainty is more likely to be a reflection of the difference in model, rather than the stability of the instrument being calibrated. Data selection should be carried out when irradiances are relatively high and varying slowly. Each mean value of the ratioR of the response of the test instrument to that of the reference instrument may be used to calculate k=R·kr, where kris the calibration factor of the reference, and k is the calibration factor being derived. During a sampling period, provided that the time between measurements is less than the 1/etime constant of the pyranometers, data collection can occur during times of fluctuating irradiance. The mean temperature of the instruments or the ambient temperature should be recorded during all outdoor calibration work to allow for any temperature effects. 7.3.1.5

By comparison in the laboratory

There are two methods which involve laboratory-maintained artificial light sources providing either direct or diffuse irradiance. In both cases, the test pyranometer and a reference standard pyranometer are exposed under the same conditions. In one method, the pyranometers are exposed to a stabilized tungsten-filament lamp installed at the end of an optical bench. A practical source for this type of work is a0.5 to 1.0kW halogen lamp mounted in a water-cooled housing with forced ventilation and with its emission limited to the solar spectrum by a quartz window. This kind of lamp can be used if the standard and the instrument to be calibrated have the same spectral response. For general calibrations, a high‑pressure xenon lamp with filters to give an approximate solar spectrum should be used. When calibrating pyranometers in this way, reflection effects should be excluded from the instruments by using black screens. The usual procedure is to install the reference instrument and measure the radiant flux. The reference is then removed and the measurement repeated using the test instrument. The reference is then replaced and another determination is made. Repeated alternation with the reference should produce a set of measurement data of good precision (about 0.5%). In the other method, the calibration procedure uses an integrating light system, such as a sphere or hemisphere illuminated by tungsten lamps, with the inner surface coated with highly reflective diffuse-white paint. This offers the advantage of simultaneous exposure of the reference pyranometer and the instrument to be calibrated. Since the sphere or hemisphere simulates a sky with an approximately uniform radiance, the angle errors of the instrument at 45° dominate.

262

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

As the cosine error at these angles is normally low, the repeatability of integrating-sphere measurements is generally within 0.5%. As for the source used to illuminate the sphere, the same considerations apply as for the first method. 7.3.1.6

Routine checks on calibration factors

There are several methods for checking the constancy of pyranometer calibration, depending upon the equipment available at a particular station. Every opportunity to check the performance of pyranometers in the field must be seized. At field stations where carefully preserved standards (either pyrheliometers or pyranometers) are available, the basic calibration procedures described above may be employed. Where standards are not available, other techniques can be used. If there is a simultaneous record of direct solar radiation, the two records can be examined for consistency by the method used for direct standardization, as explained in7.3.1.2. This simple check should be applied frequently. If there are simultaneous records of global and diffuse sky radiation, the two records should be frequently examined for consistency. In periods of total cloud, the global and diffuse sky radiation should be identical, and these periods can be used when a shading disc is used for monitoring diffuse sky radiation. When using shading bands it is recommended that the band be removed so that the diffuse sky pyranometer is measuring global radiation and its data can be compared to simultaneous data from the global pyranometer. The record may be verified with the aid of a travelling working standard sent from the central station of the network or from a nearby station. Lastly, if calibrations are not performed at the site, the pyranometer can be exchanged for a similar one sent from the calibration facility. Either of the last two methods should be used at least once a year. Pyranometers used for measuring reflected solar radiation should be moved into an upright position and checked using the methods described above. 7.3.2

Performance of pyranometers

Considerable care and attention to details are required to attain the desirable standard of uncertainty. A number of properties of pyranometers and measurement systems should be evaluated so that the uncertainty of the resultant data can be estimated. For example, it has been demonstrated that, for a continuous record of global radiation without ancillary measurements of diffuse sky and direct radiation, an uncertainty better than5% in daily totals represents the result of good and careful work. Similarly, when a protocol similar to that proposed by WMO(2005a) is used, uncertainties for daily total can be of the order of2%. 7.3.2.1

Sensor levelling

For accurate global radiation measurements with a pyranometer it is essential that the spirit level indicate when the plane of the thermopile is horizontal. This can be tested in the laboratory on an optical levelling table using a collimated lamp beam at about a 20° elevation. The levelling screws of the instrument are adjusted until the response is as constant as possible during rotation of the sensor in the azimuth. The spirit-level is then readjusted, if necessary, to indicate the horizontal plane. This is called radiometric levelling and should be the same as physical levelling of the thermopile. However, this may not be true if the quality of the thermopile surface is not uniform. 7.3.2.2

Change of sensitivity due to ambient temperature variation

Thermopile instruments exhibit changes in sensitivity with variations in instrument temperature. Some instruments are equipped with integrated temperature compensation circuits in an effort to maintain a constant response over a large range of temperatures. The temperature coefficient

CHAPTER 7. MEASUREMENT OF RADIATION

263

of sensitivity may be measured in a temperature-controlled chamber. The temperature in the chamber is varied over a suitable range in 10°C steps and held steady at each step until the response of the pyranometers has stabilized. The data are then fitted with a smooth curve. If the maximum percentage difference due to temperature response over the operational ambient range is 2% or more, a correction should be applied on the basis of the fit of the data. If no temperature chamber is available, the standardization method with pyrheliometers (see7.3.1.1, 7.3.1.2 or7.3.1.3) can be used at different ambient temperatures. Attention should be paid to the fact that not only the temperature, but also, for example, the cosine response (namely, the effect of solar elevation) and non-linearity (namely, variations of solar irradiance) can change the sensitivity. 7.3.2.3

Variation of response with orientation

The calibration factor of a pyranometer may very well be different when the instrument is used in an orientation other than that in which it was calibrated. Inclination testing of pyranometers can be conducted in the laboratory or with the standardization method described in7.3.1.1 or7.3.1.2. It is recommended that the pyranometer be calibrated in the orientation in which it will be used. A correction for tilting is not recommended unless the instrument’s response has been characterized for a variety of conditions. 7.3.2.4

Variation of response with angle of incidence

The dependence of the directional response of the sensor upon solar elevation and azimuth is usually known as the Lambert cosine response and the azimuth response, respectively. Ideally, the solar irradiance response of the receiver should be proportional to the cosine of the zenith angle of the solar beam, and constant for all azimuth angles. For pyranometers, it is recommended that the cosine error (or percentage difference from ideal cosine response) be specified for at least two solar elevation angles, preferably 30° and 10°. A better way of prescribing the directional response is given in Table7.4, which specifies the permissible error for all angles. Only lamp sources should be used to determine the variation of response with the angle of incidence because the spectral distribution of the sun changes with the angle of elevation. Using the sun as a source, an apparent variation of response with solar elevation angle could be observed which, in fact, is a variation due to non-homogeneous spectral response. 7.3.2.5

Uncertainties in hourly and daily totals

As most pyranometers in a network are used to determine hourly or daily exposures (or exposures expressed as mean irradiances), it is evident that the uncertainties in these values are important. Table 7.4 lists the expected maximum deviation from the true value, excluding calibration errors. The types of pyranometers in the third column of Table7.4 (namely, those of moderate quality) are not suitable for hourly or daily totals, although they may be suitable for monthly and yearlytotals. 7.3.3

Installation and maintenance of pyranometers

The site selected to expose a pyranometer should be free from any obstruction above the plane of the sensing element and, at the same time, should be readily accessible. If it is impracticable to obtain such an exposure, the site must be as free as possible of obstructions that may shadow it at any time in the year. The pyranometer should not be close to light-coloured walls or other objects likely to reflect solar energy onto it; nor should it be exposed to artificial radiation sources.

264

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

In most places, a flat roof provides a good location for mounting the radiometer stand. If such a site cannot be obtained, a stand placed some distance from buildings or other obstructions should be used. If practicable, the site should be chosen so that no obstruction, in particular within the azimuth range of sunrise and sunset over the year, should have an elevation exceeding 5°. Other obstructions should not reduce the total solar angle by more than 0.5sr. At stations where this is not possible, complete details of the horizon and the solid angle subtended should be included in the description of the station. A site survey should be carried out before the initial installation of a pyranometer whenever its location is changed or if a significant change occurs with regard to any surrounding obstructions. An excellent method of doing this is to use a survey camera that provides azimuthal and elevation grid lines on the negative. A series of exposures should be made to identify the angular elevation above the plane of the receiving surface of the pyranometer and the angular range in azimuth of all obstructions throughout the full 360° around the pyranometer. If a survey camera is not available, the angular outline of obscuring objects may be mapped out by means of a theodolite or a compass and clinometer combination. The description of the station should include the altitude of the pyranometer above sea level (that is, the altitude of the station plus the height of pyranometer above the ground), together with its geographical longitude and latitude. It is also most useful to have a site plan, drawn to scale, showing the position of the recorder, the pyranometer, and all connecting cables. The accessibility of instrumentation for frequent inspection is probably the most important single consideration when choosing a site. It is most desirable that pyranometers and recorders be inspected at least daily, and preferably more often. The foregoing remarks apply equally to the exposure of pyranometers on ships, towers and buoys. The exposure of pyranometers on these platforms is a very difficult and sometimes hazardous undertaking. Seldom can an instrument be mounted where it is not affected by at least one significant obstruction (for example, a tower). Because of platform motion, pyranometers are subject to wave motion and vibration. Precautions should be taken, therefore, to ensure that the plane of the sensor is kept horizontal and that severe vibration is minimized. This usually requires the pyranometer to be mounted on suitably designed gimbals. 7.3.3.1

Correction for obstructions to a free horizon

If the direct solar beam is obstructed (which is readily detected on cloudless days), the record should be corrected wherever possible to reduce uncertainty. Only when there are separate records of global and diffuse sky radiation can the diffuse sky component of the record be corrected for obstructions. The procedure requires first that the diffuse sky record be corrected, and the global record subsequently adjusted. The fraction of the sky itself which is obscured should not be computed, but rather the fraction of the irradiance coming from that part of the sky which is obscured. Since the diffuse sky radiation from elevations below 5° contributes less than 1% to the diffuse sky radiation, it can normally be neglected. Attention should be concentrated on objects subtending angles of 10° or more, as well as those which might intercept the solar beam at any time. In addition, it must be borne in mind that light‑coloured objects can reflect solar radiation onto the receiver. Strictly speaking, when determining corrections for the loss of diffuse sky radiation due to obstacles, the variance in sky radiance over the hemisphere should be taken into account. However, the only practical procedure is to assume that the radiance is isotropic, that is, the same from all parts of the sky. In order to determine the relative reduction in diffuse sky irradiance for obscuring objects of finite size, the following expression may be used:

∆ Esky = π −1 ∫ ∫ sin θ cos θ dθ dϕ (7.10) φΘ

where θ is the angle of elevation; ϕ is the azimuth angle, Θ is the extent in elevation of the object; and φ is the extent in azimuth of the object.

CHAPTER 7. MEASUREMENT OF RADIATION

265

The expression is valid only for obstructions with a black surface facing the pyranometer. For other objects, the correction has to be multiplied by a reduction factor depending on the reflectivity of the object. Snow glare from a low sun may even lead to an opposite sign for the correction. 7.3.3.2

Installation of pyranometers for measuring global radiation

A pyranometer should be securely attached to whatever mounting stand is available, using the holes provided in the tripod legs or in the baseplate. Precautions should always be taken to avoid subjecting the instrument to mechanical shocks or vibration during installation. This operation is best effected as follows. First, the pyranometer should be oriented so that the emerging leads or the connector are located poleward of the receiving surface. This minimizes heating of the electrical connections by the sun. Instruments with Moll-Gorcynski thermopiles should be oriented so that the line of thermo-junctions (the long side of the rectangular thermopile) points east-west. This constraint sometimes conflicts with the first, depending on the type of instrument, and should have priority since the connector could be shaded, if necessary. When towers are nearby, the instrument should be situated on the side of the tower towards the Equator, and as far away from the tower as practical. Radiation reflected from the ground or the base should not be allowed to irradiate the instrument body from underneath. A cylindrical shading device can be used, but care should be taken to ensure that natural ventilation still occurs and is sufficient to maintain the instrument body at ambient temperature. The pyranometer should then be secured lightly with screws or bolts and levelled with the aid of the levelling screws and spirit-level provided. After this, the retaining screws should be tightened, taking care that the setting is not disturbed so that, when properly exposed, the receiving surface is horizontal, as indicated by the spirit-level. The stand or platform should be sufficiently rigid so that the instrument is protected from severe shocks and the horizontal position of the receiver surface is not changed, especially during periods of high winds and strong solar energy. The cable connecting the pyranometer to its recorder should have twin conductors and be waterproof. The cable should be firmly secured to the mounting stand to minimize rupture or intermittent disconnection in windy weather. Wherever possible, the cable should be properly buried and protected underground if the recorder is located at a distance. The use of shielded cable is recommended; the pyranometer, cable and recorder being connected by a very low resistance conductor to a common ground. As with other types of thermo-electric devices, care must be exercised to obtain a permanent copper-to-copper junction between all connections prior to soldering. All exposed junctions must be weatherproof and protected from physical damage. After identification of the circuit polarity, the other extremity of the cable may be connected to the data‑collection system in accordance with the relevant instructions. 7.3.3.3

Installation of pyranometers for measuring diffuse sky radiation

For measuring or recording separate diffuse sky radiation, the direct solar radiation must be screened from the sensor by a shading device. Where continuous records are required, the pyranometer is usually shaded either by a small metal disc held in the sun’s beam by a sun tracker, or by a shadow band mounted on a polar axis. The first method entails the rotation of a slender arm synchronized with the sun’s apparent motion. If tracking is based on sun-synchronous motors or solar almanacs, frequent inspection is essential to ensure proper operation and adjustment, since spurious records are otherwise difficult to detect. Sun trackers with sun-seeking systems minimize the likelihood of such problems. The second method involves frequent personal attention at the site and significant corrections to the record on account of the appreciable screening of diffuse sky radiation by the shading arrangement. Assumptions about the sky radiance distribution and band dimensions are

266

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

required to correct for the band and increase the uncertainty of the derived diffuse sky radiation compared to that using a sun-seeking disc system. Annex7.E provides details on the construction of a shading ring and the necessary corrections to be applied. A significant error source for diffuse sky radiation data is the zero irradiance signal. In clear sky conditions the zero irradiance signal is the equivalent of 5 to 10Wm–2 depending on the pyranometer model, and could approach 15% of the diffuse sky irradiance. The Baseline Surface Radiation Network(BSRN) Operations Manual (WMO,2005a) provides methods to minimize the influence of the zero irradiance signal. The installation of a diffuse sky pyranometer is similar to that of a pyranometer which measures global radiation. However, there is the complication of an equatorial mount or shadow-band stand. The distance to a neighbouring pyranometer should be sufficient to guarantee that the shading ring or disc never shadows it. This may be more important at high latitudes where the sun angle can be very low. Since the diffuse sky radiation from a cloudless sky may be less than one tenth of the global radiation, careful attention should be given to the sensitivity of the recording system. 7.3.3.4

Installation of pyranometers for measuring reflected radiation

The height above the surface should be 1 to 2m. In summertime, the ground should be covered by grass that is kept short. For regions with snow in winter, a mechanism should be available to adjust the height of the pyranometer in order to maintain a constant separation between the snow and the instrument. Although the mounting device is within the field of view of the instrument, it should be designed to cause less than 2% error in the measurement. Access to the pyranometer for levelling should be possible without disturbing the surface beneath, especially if it is snow. 7.3.3.5

Maintenance of pyranometers

Pyranometers in continuous operation should be inspected at least once a day and perhaps more frequently, for example when meteorological observations are being made. During these inspections, the glass dome of the instrument should be wiped clean and dry (care should be taken not to disturb routine measurements during the daytime). If frozen snow, glazed frost, hoar frost or rime is present, an attempt should be made to remove the deposit very gently (at least temporarily), with the sparing use of a de-icing fluid, before wiping the glass clean. A daily check should also ensure that the instrument is level, that there is no condensation inside the dome, and that the sensing surfaces are still black. In some networks, the exposed dome of the pyranometer is ventilated continuously by a blower to avoid or minimize deposits in cold weather, and to minimize the temperature difference between the dome and the case. The temperature difference between the ventilating air and the ambient air should not be more than about 1K. If local pollution or dust forms a deposit on the dome, it should be wiped very gently, preferably after blowing off most of the loose material or after wetting it a little, in order to prevent the surface from being scratched. Such abrasive action can appreciably alter the original transmission properties of the material. Desiccators should be kept charged with active material (usually a colour-indicating silica gel). 7.3.3.6

Installation and maintenance of pyranometers on special platforms

Very special care should be taken when installing equipment on such diverse platforms as ships, buoys, towers and aircraft. Radiation sensors mounted on ships should be provided with gimbals because of the substantial motion of the platform.

CHAPTER 7. MEASUREMENT OF RADIATION

267

If a tower is employed exclusively for radiation equipment, it may be capped by a rigid platform on which the sensors can be mounted. Obstructions to the horizon should be kept to the side of the platform farthest from the Equator, and booms for holding albedometers should extend towards the Equator. Radiation sensors should be mounted as high as is practicable above the water surface on ships, buoys and towers, in order to keep the effects of water spray to a minimum. Radiation measurements have been taken successfully from aircraft for a number of years. Care must be exercised, however, in selecting the correct pyranometer and proper exposure. Particular attention must be paid during installation, especially for systems that are difficult to access, to ensure the reliability of the observations. It may be desirable, therefore, to provide a certain amount of redundancy by installing duplicate measuring systems at certain critical sites.

7.4

MEASUREMENT OF TOTAL AND LONG-WAVE RADIATION

The measurement of total radiation includes both short wavelengths of solar origin (300 to 3000nm) and longer wavelengths of terrestrial and atmospheric origin (3000 to 100000nm). The instruments used for this purpose are pyrradiometers. They may be used for measuring either upward or downward radiation flux components, and a pair of them may be used to measure the differences between the two, which is the net radiation. Single-sensor pyrradiometers, with an active surface on both sides, are also used for measuring net radiation. Pyrradiometer sensors must have a constant sensitivity across the whole wavelength range from 300 to 100000nm. The measurement of long-wave radiation can be accomplished either directly using pyrgeometers, or indirectly by subtracting the measured global radiation from the total radiation measured. Most pyrgeometers eliminate the short wavelengths by means of filters which have approximately constant transparency to long wavelengths while being almost opaque to the shorter wavelengths (300 to 3000nm). Some pyrgeometers – either without filters or filters that do not eliminate radiation below 3000nm – can be used only during the night. The long-wave flux L¯ measured by a pyrgeometer or a pyrradiometer has two components, the black-body flux from the surface temperature of the sensing element and the net radiative flux measured by the receiver:

L− = L* + σ Ts4 (7.11)

σ is the Stefan-Boltzmann constant (5.6704·10 –8Wm–2K–1); Tsis the underlying surface temperature(K); L¯is the irradiance measured either by a reference pyrgeometer or calculated from the temperature of the black-body cavity capping the upper receiver (Wm–2); L*is the net radiative flux at the receiver (Wm–2). Measuring the short-wave component measured by a pyrradiometer follows the description in7.3. 7.4.1

Instruments for the measurement of long-wave radiation

Over the last decade, significant advances have been made in the measurement of terrestrial radiation by pyrgeometers particularly with the advent of the silicon domed pyrgeometer, and as a result pyrgeometers provide the highest accuracy measurements of terrestrial radiation. Nevertheless, the measurement of terrestrial radiation is still more difficult and less understood than the measurement of solar irradiance, Table7.5 provides an analysis of the sources of errors.

268

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Table 7.5. Sources of error in pyrradiometric measurements Elements influencing the measurements Screening properties

Nature of influence on pyrradiometers With domes Spectral characteristics of transmission

Without domes None

Effects on the precision of measurements

Methods for determining these characteristics

(a) Spectral variations in calibration coefficient

(a) Determine spectrally the extinction in the screen

(b) The effect of reduced incident radiation on the detector due to short-wave diffusion in the domes (depends on thickness)

(b) Measure the effect of diffuse sky radiation or measure the effect with a varying angle of incidence

(c) Ageing and other variations in the sensors

(c) Spectral analysis: compare with a new dome; determine the extinction of the dome

Convection effects

Changes due to non-radiative energy exchanges: sensordome environment (thermal resistance)

Changes due to nonradiative energy exchanges: sensorair (variation in areal exchange coefficient)

Uncontrolled changes due to wind gusts are critical in computing the radiative flux divergence in the lowest layer of the atmosphere

Study the dynamic behaviour of the instrument as a function of temperature and speed in a wind tunnel

Effects of hydrometeors (rain, snow, fog, dew, frost) and dust

Variation of the spectral transmission plus the nonradiative heat exchange by conduction and change

Variation of the spectral character of the sensor and of the dissipation of heat by evaporation

Changes due to variations in the spectral characteristics of the sensor and to non‑radiative energy transfers

Study the influence of forced ventilation on the effects

Properties of the sensor surface (emissivity)

Depends on the spectral absorption of the blackening substance on the sensor

Changes in calibration coefficient (a) As a function of spectral response (b) As a function of intensity and azimuth of incident radiation (c) As a function of temperature effects

(a) Spectrophotometric analysis of the calibration of the absorbing surfaces (b) Measure the sensor’s sensitivity variability with the angle of incidence

Temperature effects

Non-linearity of the sensor as a function of temperature

A temperature coefficient is required

Study the influence of forced ventilation on these effects

Asymmetry effects

(a) Differences between the thermal capacities and resistance of the upward- and downward-facing sensors (b) Differences in ventilation of the upward- and downward-facing sensors (c) Control and regulation of sensor levelling

(a) Influence on the time constant of the instrument (b) Error in the determination of the calibration factors for the two sensors

(a) Control the thermal capacity of the two sensor surfaces (b) Control the time constant over a narrow temperature range

CHAPTER 7. MEASUREMENT OF RADIATION

269

Pyrgeometers have developed in two forms. In the first form, the thermopile receiving surface is covered with a hemispheric dome inside which an interference filter is deposited. In the second form, the thermopile is covered with a flat plate on which the interference filter is deposited. In both cases, the surface on which the interference filter is deposited is made of silicon. The first style of instrument provides a full hemispheric field of view, while for the second a 150° field of view is typical and the hemispheric flux is modelled using the manufacturer’s procedures. The argument used for the latter method is that the deposition of filters on the inside of a hemisphere has greater imprecision than the modelling of the flux below 30° elevations. Both types of instruments are operated on the principle that the measured output signal is the difference between the irradiance emitted from the source and the black-body radiative temperature of the instrument. In general, pyrgeometer derived terrestrial radiation can be approximated by a modification to equation7.11:

(

)

L− = L* + k2σ Ts4 + k3σ Td4 − Ts4 (7.12)

where k2 takes into account the emission properties of the thermopile and uncertainties of the temperature measurement of the cold surface of the thermopile; k3is the instrument dome sensitivity to IR irradiance (µV/(Wm–2)); and Td is the dome temperature(K). The net radiative flux measured by the receiver, L*, is defined as:

(

)

L* = U C 1 + k1σ Ts3 (7.13)

where C is the sensitivity of the receiver (μV/(W m–2)), and k1 is a residual temperature coefficient of the receiver. While state-of-the-art pyrgeometers have a temperature correction circuitry implemented in their receiver to bring k1 very close to zero (as described in7.3.2.2), it is still recommended to determine k1 by a laboratory characterization as described in7.4.3. Several recent comparisons have been made using instruments of similar manufacture in a variety of measurement configurations. These studies have indicated that, following careful calibration, fluxes measured at night agree to within ±1Wm–2, but in periods of high solar energy the difference between unshaded instruments can be significant. The reason for the differences is that the silicon dome and the associated interference filter may transmit solar radiation and is not a perfect reflector of solar energy. Thus, a solar contribution may reach the sensor, and solar heating of the dome occurs. By shading the instrument similarly to that used for diffuse solar measurements, ventilating it as recommended by ISO(1990a), and measuring the temperature of the dome and the instrument case, this discrepancy can be reduced to ±2Wm–2. Based upon these and other comparisons, the following recommendations should be followed for the measurement of long-wave radiation: (a) When using pyrgeometers that have a built-in battery circuit to emulate the black-body condition of the instrument, extreme care must be taken to ensure that the battery is well maintained. Even a small change in the battery voltage will significantly increase the measurement error. If at all possible, the battery should be removed from the instrument, and the case and dome temperatures of the instrument should be measured according to the manufacturer’s instructions; (b) Where possible, both the case and dome temperatures of the instrument should be measured and used in the determination of irradiance; (c) The instrument should be ventilated; (d) For best results, the instrument should be shaded from direct solar irradiance by a small sun‑tracking disc as used for diffuse sky radiation measurement. These instruments should be calibrated at National or Regional Calibration Centres by using reference pyrgeometers traceable to the World Infrared Standard Group(WISG) of Pyrgeometers of the WRC Davos that is governed under the framework described in AnnexF.

270 7.4.2

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Instruments for the measurement of total radiation

One problem with instruments for measuring total radiation is that there are no absorbers which have a completely constant sensitivity over the extended range of wavelengths concerned. Similarly, it is difficult to find suitable filters that have constant transmission between 300 and 100000nm. Therefore, the recommended practice for measuring total radiation is to perform simultaneous separate measurements of short- and long-wave radiation using a pyranometer and a pyrgeometer, respectively. The use of thermally sensitive sensors requires a good knowledge of the heat budget of the sensor. Otherwise, it is necessary to reduce sensor convective heat losses to near zero by protecting the sensor from the direct influence of the wind. The technical difficulties linked with such heat losses are largely responsible for the fact that net radiative fluxes are determined less precisely than global radiation fluxes. In fact, different laboratories have developed their own pyrradiometers on technical bases which they consider to be the most effective for reducing the convective heat transfer in the sensor. During the last few decades, pyrradiometers have been built which, although not perfect, embody good measurement principles. Thus, there is a great variety of pyrradiometers employing different methods for eliminating, or allowing for, wind effects, as follows: (a) No protection, in which case empirical formulae are used to correct for wind effects; (b) Determination of wind effects by the use of electrical heating; (c) Stabilization of wind effects through artificial ventilation; (d) Elimination of wind effects by protecting the sensor from the wind. The long-wave component of a pyrradiometer is described in equation7.11. Table 7.5 provides an analysis of the sources of error arising in pyrradiometric measurements and proposes methods for determining these errors. It is difficult to determine the uncertainty likely to be obtained in practice. In situ comparisons at different sites between different designs of pyrradiometer yield results manifesting differences of up to 5% to 10% under the best conditions. In order to improve such results, an exhaustive laboratory study should precede the in situ comparison in order to determine the different effects separately. Deriving total radiation by independently measuring the short-wave and long-wave components achieves the highest accuracies and is recommended over the pyrradiometer measurements. Short‑wave radiation can be measured using the methods outlined in7.2 and7.3, while long‑wave radiation can be measured with pyrgeometers. Table7.6 lists the characteristics of pyrradiometers of various levels of performance, and the uncertainties to be expected in the measurements obtained from them. 7.4.3

Calibration of pyrgeometers

Pyrradiometers and net pyrradiometers can be calibrated for short-wave radiation using the same methods as those used for pyranometers (see7.3.1) using the sun and sky as the source. In the case of one-sensor net pyrradiometers, the downward-looking side must be covered by a cavity of known and steady temperature. Long-wave radiation calibration of reference radiometers is best done in the laboratory with black‑body cavities, but night-time comparison to reference instruments is preferred for network

271

CHAPTER 7. MEASUREMENT OF RADIATION

Table 7.6. Characteristics of operational pyrradiometers Characteristic

High qualitya

Good qualityb

Moderate qualityc

1

5

10

Stability (annual change; % of full scale)

2%

5%

10%

Cosine response error at 10° elevation

3%

7%

15%

Azimuth error at 10° elevation (additional to cosine error) (deviation from mean)

3%

5%

10%

Temperature dependence (–20 °C to 40 °C) (deviation from mean)

1%

2%

5%

0.5%

2%

5%

2%

5%

10%

Resolution (W m–2)

Non-linearity (deviation from mean) Variation in spectral sensitivity integrated over 200nm intervals from 300 to 75000nm

Notes: a Near state of the art; maintainable only at stations with special facilities and specialist staff. b Acceptable for network operations. c Suitable for low-cost networks where moderate to low performance.

measurements. In the case of calibration of the sensor the downward fluxL¯ is measured separately by using a pyrgeometer or provided by a black-body cavity. In which case, signal V from the net radiative flux received by the instrument (via equation7.11) amounts to: V = L∗ ⋅ K or K = V L∗ (7.14)

where V is the output of the instrument (µV); and K is sensitivity (µV/(Wm–2)). The instrument sensitivities should be checked periodically in situ by careful selection of well‑described environmental conditions with slowly varying fluxes. Pyrgeometers should also be checked periodically to ensure that the transmission of short-wave radiation has not changed. The symmetry of net pyrradiometers requires regular checking. This is done by inverting the instrument, or the pair of instruments, in situ and noting any difference in output. Differences of greater than 2% of the likely full scale between the two directions demand instrument recalibration because either the ventilation rates or absorption factors have become significantly different for the two sensors. Such tests should also be carried out during calibration or installation. 7.4.4

Installation of pyrradiometers and pyrgeometers

Pyrradiometers and pyrgeometers are generally installed at a site which is free from obstructions, or at least has no obstruction with an angular size greater than 5° in any direction, and which has a low sun angle at all times during the year. A daily check of the instruments should ensure that: (a) The instrument is level; (b) Each sensor and its protection devices are kept clean and free from dew, frost, snow and rain; (c) The domes do not retain water (any internal condensation should be dried up); (d) The black receiver surfaces have emissivities very close to 1.

272

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Since it is not generally possible to directly measure the reflected solar radiation and the upward long-wave radiation exactly at the surface level, it is necessary to place the pyrradiometers, or pyranometers and pyrgeometers at a suitable distance from the ground to measure these upward components. Such measurements integrate the radiation emitted by the surface beneath the sensor. For those instruments which have an angle of view of 2πsr and are installed 2m above the surface, 90% of all the radiation measured is emitted by a circular surface underneath having a diameter of 12m (this figure is 95% for a diameter of 17.5m and 99% for one of 39.8m), assuming that the sensor uses a cosine detector. This characteristic of integrating the input over a relatively large circular surface is advantageous when the terrain has large local variations in emittance, provided that the net pyrradiometer can be installed far enough from the surface to achieve a field of view which is representative of the local terrain. The output of a sensor located too close to the surface will show large effects caused by its own shadow, in addition to the observation of an unrepresentative portion of the terrain. On the other hand, the readings from a net pyrradiometer located too far from the surface can be rendered unrepresentative of the fluxes near that surface because of the existence of undetected radiative flux divergences. Usually, a height of 2m above short homogeneous vegetation is adopted, while in the case of tall vegetation, such as a forest, the height should be sufficient to eliminate local surface heterogeneities adequately. 7.4.5

Recording and data reduction

In general, the text in 7.1.3 applies to pyrradiometers and pyrgeometers. Furthermore, the following effects can specifically influence the readings of these radiometers, and they should be recorded: (a) The effect of hydrometeors on non-protected and non-ventilated instruments (rain, snow, dew, frost); (b) The effect of wind and air temperature; (c) The drift of zero of the data system. This is much more important for pyrradiometers, which can yield negative values, than for pyranometers, where the zero irradiance signal is itself a property of the net irradiance at the sensor surface. Special attention should be paid to the position of instruments if the derived long-wave radiation requires subtraction of the solar irradiance component measured by a pyranometer; the pyrradiometer and pyranometer should be positioned within 5m of each other and in such a way that they are essentially influenced in the same way by their environment.

7.5

MEASUREMENT OF SPECIAL RADIATION QUANTITIES

7.5.1

Measurement of daylight

Illuminance is the incident flux of radiant energy that emanates from a source with wavelengths between 380 and 780nm and is weighted by the response of the human eye to energy in this wavelength region. The CIE has defined the response of the human eye to photons with a peak responsivity at 555nm. Figure7.2 and Table7.7 provide the relative response of the human eye normalized to this frequency. Luminous efficacy is defined as the relationship between radiant emittance(Wm–2) and luminous emittance(lm). It is a function of the relative luminous sensitivityV(λ) of the human eye and a normalizing factor Km(683) describing the number of lumens emitted per watt of EMR from a monochromatic source of 555.19nm (the freezing point of platinum), as follows:

780

Φv = Km

∫ Φ ( λ ) V ( λ ) d λ (7.15)

380

273

CHAPTER 7. MEASUREMENT OF RADIATION

Relative response

1.0 0.8 0.6 0.4 0.2 0.0

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 7.2. Relative luminous sensitivity V(λ) of the human eye for photopic vision where Φv is the luminous flux (lmm–2 or lx); Φ(λ) is the spectral radiant flux (Wm–2nm–1); V(λ)is the sensitivity of the human eye, and Kmis the normalizing constant relating luminous to radiation quantities. Thus, 99% of the visible radiation lies between 400 and730nm. Quantities and units for luminous variables are given in Annex7.A. 7.5.1.1

Instruments

Illuminance meters comprise a photovoltaic detector, one or more filters to yield sensitivity according to the V(λ) curve, and often a temperature control circuit to maintain signal stability. The CIE has developed a detailed guide to the measurement of daylight (CIE,1994) which describes expected practices in the installation of equipment, instrument characterization, data‑acquisition procedures and initialQC. The measurement of global illuminance parallels the measurement of global irradiance. However, the standard illuminance meter must be temperature controlled or corrected from at least –10°C to 40°C. Furthermore, it must be ventilated to prevent condensation and/or frost from coating the outer surface of the sensing element. Illuminance meters should normally be able to measure fluxes over the range 1 to 20000lx. Within this range, uncertainties should remain within the limits of Table7.8. These values are based upon CIE recommendations (CIE,1987), but only for uncertainties associated with high-quality illuminance meters specifically intended for external daylight measurements. Diffuse sky illuminance can be measured following the same principles used for the measurement of diffuse sky irradiance. Direct illuminance measurements should be taken with instruments having a field of view whose open half-angle is no greater than 2.85° and whose slope angle is less than1.76°. 7.5.1.2

Calibration

Calibrations should be traceable to a Standard Illuminant A following the procedures outlined in CIE(1987). Such equipment is normally available only at national standards laboratories. The calibration and tests of specification should be performed yearly. These should also include tests to determine ageing, zero setting drift, mechanical stability and climatic stability. It is also recommended that a field standard be used to check calibrations at each measurement site between laboratory calibrations.

274

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

Table 7.7. Photopic spectral luminous efficiency values (unity at wavelength of maximum efficacy) Wavelength (nm)

Photopic V(λ)

Wavelength (nm)

Photopic V(λ)

380

0.00004

590

0.757

390

0.00012

600

0.631

400

0.0004

610

0.503

410

0.0012

620

0.381

420

0.0040

630

0.265

430

0.0116

640

0.175

440

0.023

650

0.107

450

0.038

660

0.061

460

0.060

670

0.032

470

0.091

680

0.017

480

0.139

690

0.0082

490

0.208

700

0.0041

500

0.323

710

0.0021

510

0.503

720

0.00105

520

0.710

730

0.00052

530

0.862

740

0.00025

540

0.954

750

0.00012

550

0.995

760

0.00006

560

0.995

770

0.00003

570

0.952

780

0.000015

580

0.870

Table 7.8. Specification of illuminance meters Specification

Uncertainty percentage

V(λ) match

2.5

UV response

0.2

IR response

0.2

Cosine response

1.5

Fatigue at 10 klx

0.1

Temperature coefficient

0.1 K–1

Linearity

0.2

Settling time

0.1 s

CHAPTER 7. MEASUREMENT OF RADIATION

7.5.1.3

275

Recording and data reduction

The CIE has recommended that the following climatological variables be recorded: (a) Global and diffuse sky daylight illuminance on horizontal and vertical surfaces; (b) Illuminance of the direct solar beam; (c) Sky luminance for 0.08 sr intervals (about 10°·10°) all over the hemisphere; (d) Photopic albedo of characteristic surfaces such as grass, earth and snow. Hourly or daily integrated values are usually needed. The hourly values should be referenced to true solar time. For the presentation of sky luminance data, stereographic maps depicting isolines of equal luminance are most useful.

7.6

MEASUREMENT OF ULTRAVIOLET RADIATION

Measurements of solar UV radiation are in demand because of its effects on the environment and human health, and because of the enhancement of radiation at the Earth’s surface as a result of ozone depletion (Kerr and McElroy, 1993) and changes in other parameters like clouds and aerosols. The UV spectrum is conventionally divided into three parts, as follows: (a) UV-A is the band with wavelengths of 315 to 400nm, namely, just outside the visible spectrum. It is usually2 less biologically active, and its intensity at the Earth’s surface does not vary significantly with atmospheric ozone content; (b) UV-B is defined as radiation in the 280 to 315nm band. It is biologically active and its intensity at the Earth’s surface depends on the atmospheric ozone column, depending on wavelength. A frequently used expression of its biological activity is its erythemal effect, which is the extent to which it causes the reddening of human skin; (c) UV-C, in wavelengths of 100 to 280nm, is completely absorbed in the atmosphere and does not occur naturally at the Earth’s surface. UV-B is the band on which most interest is centred for measurements of UV radiation. An alternative, but now non-standard, definition of the boundary between UV-A and UV-B is 320nm rather than315nm. Measuring UV radiation is difficult because of the small amount of energy reaching the Earth’s surface, the variability due to changes in stratospheric ozone levels, and the rapid increase in the magnitude of the flux with increasing wavelength. Figure7.3 illustrates changes in the spectral irradiance between 290 and 325nm at the top of the atmosphere and at the surface in Wm–2nm–1. Global UV irradiance is strongly affected by atmospheric phenomena such as clouds, and to a lesser extent by atmospheric aerosols. The influence of surrounding surfaces is also significant because of multiple scattering. This is especially the case in snow-covered areas. Difficulties in the standardization of UV radiation measurement stem from the variety of uses to which the measurements are put (WMO, 2003,2011). Unlike most meteorological measurements, standards based upon global needs have not yet been reached. In many countries, measurements of UV radiation are not taken by Meteorological Services, but by health or environmental protection authorities. This leads to further difficulties in the standardization of

2

The phytoplankton photosynthesis action spectrum, for example, has an important component in the UV-A.

276

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1.0E+1

Irradiance (W m–2 nm–1)

1.0E+0 1.0E-1 1.0E-2 1.0E-3 1.0E-4

Extra-terrestrial irradiance

1.0E-5

Surface irradiance (250 milliatmosphere centimetre ozone) Surface irradiance (300 milliatmosphere centimetre ozone)

1.0E-6

Surface irradiance (350 milliatmosphere centimetre ozone)

1.0E-7 280.00

290.00

300.00

310.00

320.00

330.00

Wavelength (nm)

Figure 7.3. Model results illustrating the effect of increasing ozone levels on the transmission of UV-B radiation through the atmosphere instruments and methods of observation. Standards are necessary for compatible observations, QA and QC of measurements, and data archiving, as well as for connecting measurements with the user communities (WMO,2003). Guidelines and standard procedures have been developed on how to characterize and calibrate UV broadband instruments, spectroradiometers and filter radiometers used to measure solar UV irradiance (seeWMO, 1996, 1999a, 1999b, 2001, 2008, 2010a). Although not available commercially yet, guides and standard procedures have also been developed for array spectroradiometers (WMO,2010b). Application of the recommended procedures for data QA performed at sites operating instruments for solar UV radiation measurements will ensure a valuable UV radiation database. This is needed to derive a climatology of solar UV irradiance in space and time for studies of the Earth’s climate. Recommendations for measuring sites and instrument specifications are also provided in these documents. Requirements for UV‑B measurements were put forward in the WMO Global Atmosphere Watch(GAW) Programme (WMO, 1993, 2001, 2003, 2010a, 2010b, 2011, 2014). For UV-B global spectral irradiance, requirements depend on the objective. Specifications for less demanding objectives are reproduced in Table7.9 (WMO,2001). The following instrument descriptions are provided for general information and for assistance in selecting appropriate instrumentation. 7.6.1

Instruments

Three general types of instruments are available commercially for the measurement of UV radiation. The first class of instruments use broadband filters. These instruments integrate over either the UV-B or UV-A spectrum or the entire broadband UV region responsible for affecting human health. The second class of instruments use one or more interference filters to integrate over discrete portions of the UV-A and/or UV-B spectrum. The third class of instruments are spectroradiometers that measure across a pre-defined portion of the spectrum sequentially, or simultaneously, using a fixed passband.

CHAPTER 7. MEASUREMENT OF RADIATION

277

Table 7.9. GAW Programme requirements for UV-B global spectral irradiance measurements Characteristic

Requirements

Cosine error

(a) < ±10% for incidence angles < 60° (b) < ±10% for integrated isotropic radiance

a

Minimum spectral range

290–325 nmb

Bandwidth (full width half maximum (FWHM))

< 1 nm

Wavelength precision

< ±0.05 nm

Wavelength accuracy

< ±0.1 nm

Slit function

< 10 –3 of maximum at 2.5 FWHM away from centre

Sampling wavelength interval < FWHM Maximum irradiance

> 1 W m–2 nm–1 at 325 nm and, if applicable, 2 W m–2 nm–1 at 400 nm (noon maximum)

Detection threshold

< 5 · 10 –5 W m–2 nm–1 (for signal-to-noise ratio (SNR) = 1 at 1nm FWHM)

Stray light

< 5 · 10 –4 W m–2 nm–1 when the instrument is exposed to the sun at minimum solar zenith angle

Instrument temperature

Monitored and sufficiently stable to maintain overall instrument stability

Scanning duration

< 10 min per spectrum, e.g. for ease of comparison with models

Overall calibration uncertaintyc

< ±10% (unless limited by detection threshold)

Scan date and time

Recorded with each spectrum such that timing is known to within 10s at each wavelength

Ancillary measurements required

Direct normal spectral irradiance or diffuse spectral irradiance Total ozone column, e.g. derived from measurements of direct normal spectral irradiance Erythemally weighted irradiance, measured with a broadband radiometer Atmospheric pressure Cloud amount Illuminance, measured with a luxmeter Direct irradiance at normal incidence measured with a pyrheliometer Visibility

Data frequency

At least one scan per hour and additionally a scan at local solar noon

Notes: a Smaller cosine errors would be desirable, but are unrealistic for the majority of the instruments that are currently in use. b The overall calibration uncertainty is expressed at 95% confidence level and includes all uncertainties associated with the irradiance calibration (for example, uncertainty of the standard lamps, transfer uncertainties, alignment errors during calibration, and drift of the instrument between calibrations). For more details, see Bernhard and Seckmeyer (1999), Cordero et al. (2008), and Cordero et al. (2013). c An extension to longer wavelengths is desirable for the establishment of a UV climatology with respect to biological applications, see WMO (2001, 2010b).

7.6.1.1

Broadband sensors

Most, but not all, broadband sensors are designed to measure a UV spectrum that is weighted by the erythemal function proposed by McKinlay and Diffey(1987) and reproduced in Figure7.4. Another action spectrum found in some instruments is that of Parrishetal. (1982). Two methods (and their variations) are used to accomplish this hardware weighting.

278

GUIDE TO INSTRUMENTS AND METHODS OF OBSERVATION - VOLUME I

1.00E+0

Erythemal action spectra

1.00E-1

McKinlay and Diffey (1987) Parrish et al. (1982) normalized to 1 at 250 nm

1.00E-2

1.00E-3

1.00E-4

1.00E-5 250.00

300.00

350.00

400.00

Wavelength (nm)

Figure 7.4. Erythemal curves Source: Parrish et al. (1982) and McKinlay and Diffey (1987)

One of the means of obtaining erythemal weighting is to first filter out nearly all visible wavelength light using UV-transmitting, black-glass blocking filters. The remaining radiation then strikes a UV-sensitive phosphor. In turn, the green light emitted by the phosphor is filtered again by using coloured glass to remove any non-green visible light before impinging on a gallium arsenic or a gallium arsenic phosphorus photodiode. The quality of the instrument is dependent on such items as the quality of the outside protective quartz dome, the cosine response of the instrument, the temperature stability, and the ability of the manufacturer to match the erythemal curve with a combination of glass and diode characteristics. Instrument temperature stability is crucial, both with respect to the electronics and the response of the phosphor to incident UV radiation. Phosphor efficiency decreases by approximately 0.5%K–1 and its wavelength response curve is shifted by approximately 1nm longer every 10K. This latter effect is particularly important because of the steepness of the radiation curve at these wavelengths. More recently, instruments have been developed to measure erythemally weighted UV irradiance using thin film metal interference filter technology and specially developed silicon photodiodes. These overcome many problems associated with phosphor technology, but must contend with very low photodiode signal levels and filter stability. Other broadband instruments use one or the other measurement technology to measure the complete spectra by using either a combination of glass filters or interference filters. The bandpass is as narrow as 20nm FWHM to as wide as 80nm FWHM for instruments measuring a combination of UV-A and UV-B radiation. Some manufacturers of these instruments provide simple algorithms to approximate erythemal dosage from the unweighted measurements. The basic maintenance of these instruments consists of ensuring that the domes are cleaned, the instrument is levelled, the desiccant (if provided) is active, and the heating/cooling system is working correctly, if so equipped. QC and QA as well as detailed maintenance should be done by well-experienced staff.

CHAPTER 7. MEASUREMENT OF RADIATION

7.6.1.2

279

Narrowband sensors

The definition of narrowband for this classification of instrument is vague. The widest bandwidth for instruments in this category is 10nm FWHM. The narrowest bandwidth at present for commercial instruments is of the order of 2nm FWHM (WMO, 2010a). These sensors use one or more interference filters to obtain information about a portion of the UV spectra. The simplest instruments consist of a single filter, usually at a wavelength that can be measured by a good-quality, UV enhanced photodiode, although more than one filter is desirable. Specifications required for this type of instrument (WMO,2010a) are given in Table7.10. Wavelengths near 305nm are typical for such instruments. The out-of-band rejection of such filters should be equal to, or greater than, 10 –6 throughout the sensitive region of the detector. Higher quality instruments of this type either use Peltier cooling to maintain a constant temperature near 20°C or heaters to increase the instrument filter and diode temperatures to above normal ambient temperatures, usually 40°C. However, the latter alternative markedly reduces the life of interference filters. A modification of this type of instrument uses a photomultiplier tube instead of the photodiode. This allows the accurate measurement of energy from shorter wavelengths and lower intensities at all measured wavelengths. Manufacturers of instruments that use more than a single filter often provide a means of reconstructing the complete UV spectrum and determining biologically effective doses for a variety of action spectra, the total column ozone amount and cloud attenuation, through modelled relationships developed around the measured wavelengths (WMO,2010a). Single wavelength instruments are used similarly to supplement the temporal and spatial resolution of more sophisticated spectrometer networks or for long-term accurate monitoring of specific bands to detect trends in the radiation environment. The construction of the instruments must be such that the radiation passes through the filter close to normal incidence so that wavelength shifting to shorter wavelengths is avoided.

Table 7.10. Requirements for UV-B global narrowband irradiance measurements Characteristic

Requirements

Stray light including sensitivity to visible and IR radiation

< 1% contribution to the signal of wavelengths outside 2.5 FWHM for a solar zenith angle less than 70°

Stability over time on timescales up to a year

Signal change: Currently in use: better than 5% Desired: 2%

Minimum number of channels

At least one channel with centre wavelength < 310nm and at least one with centre wavelength > 330nm

Maximum irradiance

Signal of the instruments must not saturate at radiation levels encountered on the Earth’s surface

Detection threshold

SNR = 3 for irradiance at solar zenith angle of 80° and total ozone column of 300Dobson units

Instrument temperature

Monitored and sufficiently stable to maintain overall instrument stability

Response time

Guide To Instruments and Methods of Observation: Volume I - Measurement of Meteorological Variables - PDFCOFFEE.COM (2025)

References

Top Articles
Latest Posts
Recommended Articles
Article information

Author: Merrill Bechtelar CPA

Last Updated:

Views: 6482

Rating: 5 / 5 (50 voted)

Reviews: 89% of readers found this page helpful

Author information

Name: Merrill Bechtelar CPA

Birthday: 1996-05-19

Address: Apt. 114 873 White Lodge, Libbyfurt, CA 93006

Phone: +5983010455207

Job: Legacy Representative

Hobby: Blacksmithing, Urban exploration, Sudoku, Slacklining, Creative writing, Community, Letterboxing

Introduction: My name is Merrill Bechtelar CPA, I am a clean, agreeable, glorious, magnificent, witty, enchanting, comfortable person who loves writing and wants to share my knowledge and understanding with you.