Ventilation effects on humidity measurements in thermometer screens
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
To link to this item DOI: 10.1002/qj.985
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.
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