Ventilation effects on humidity measurements in thermometer screens

Abstract

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 is applicable when 2-metre wind speeds fall below 3 m s -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 interquartile range of the RH differences from 2.0% to 0.8%. This correction is particularly amenable to automatic measurement systems. © 2011 Royal Meteorological Society.