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Ventilation effects on humidity measurements in thermometer screens

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Harrison, R. G. and Wood, C. R. (2012) Ventilation effects on humidity measurements in thermometer screens. Quarterly Journal of the Royal Meteorological Society, 138 (665). pp. 1114-1120. ISSN 1477-870X

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To link to this article DOI: 10.1002/qj.985

Abstract/Summary

Relative humidity (RH) measurements, as derived from wet-bulb and dry-bulb thermometers operated as a psychrometer within a thermometer screen, have limited accuracy because of natural ventilation variations. Standard RH calculations generally assume a fixed screen psychrometer coefficient, but this is too small during poor ventilation. By comparing a reference humidity probe—exposed within a screen containing a psychrometer—with wind-speed measurements under controlled conditions, a wind-speed correction for the screen psychrometer coefficient has been derived and applicable when 2-metre wind speeds fall below 3 ms–1. Applying this to hourly-averaged data reduced the mean moist RH bias of the psychrometer (over the reference probe) from 1.2% to 0.4%, and reduced the inter-quartile range of the RH differences from 2.0% to 0.8%. This correction is particularly amenable to automatic measurement systems.

Item Type:Article
Refereed:Yes
Divisions:Faculty of Science > School of Mathematical and Physical Sciences > Department of Meteorology
ID Code:24582
Uncontrolled Keywords:Psychrometer; hygrometer; wet-bulb; automatic weather station; supercooling.
Publisher:Royal Meteorological Society
Publisher Statement:This is a preprint of an article accepted (November 2011) for publication in Quarterly Journal of the Royal Meteorological Society Copyright ©2011,2012 Royal Meteorological Society

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