Natural ventilation effects on temperatures within Stevenson screens

Thermometer screen properties are poorly characterised at low wind speeds. Temperatures from a large thermometer screen have been compared with those from an automatically shaded open-air fine-wire resistance thermometer. For the majority of 5-minute average measurements obtained between July 2008 and 2009, the screen and fine-wire temperatures agreed closely, with a median difference <0.05 degrees C. At low wind speeds however, larger temperature differences occurred. When calm (wind speed at 2 metres, u(2), <= 0.1 m s(-1)), the difference between screen and open-air temperatures varied from -0.25 degrees C to +0.87 degrees C. At night with u(2) < 0.5 m s(-1), this difference was -0.14 degrees C to 0.39 degrees C, and, rarely, up to -0.68 degrees C to 1.38 degrees C. At the minimum in the daily temperature cycle, the semi-urban site at Reading had u(2) < 1 m s(-1) for 52% of the observations 1997-2008, u(2) < 0.5 m s(-1) for 34% and calm conditions for 20%. Consequently uncertainties in the minimum temperature measurements may arise from poor ventilation, which can propagate through calculations to daily average temperatures. In comparison with the daily minimum temperature, the 0900 UTC synoptic temperature measurement has a much lower abundance (5%) of calm conditions. Copyright (C) 2010 Royal Meteorological Society