Ventilation effects on humidity measurements in thermometer screens

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. Copyright © 2011 Royal Meteorological Society