Lag‐time effects on a naturally ventilated large thermometer screen

Abstract

AbstractSystematic natural ventilation effects on measured temperatures within a standard large wooden thermometer screen are investigated under summer conditions, using well‐calibrated platinum resistance thermometers. Under low ventilation (2 m wind speed u2 < 1.1 m s−1), the screen slightly underestimates daytime air temperature but overestimates air temperature nocturnally by 0.2°C. The screen's lag time L lengthens with decreasing wind speed, following an inverse power law relationship between L and u2. For u2 > 2 m s−1, L ∼ 2.5 min, increasing, when calm, to at least 15 min. Spectral response properties of the screen to air temperature fluctuations vary with wind speed because of the lag changes. Ventilation effects are particularly apparent at the higher (>25°C) temperatures, both through the lag effect and from solar heating. For sites where wind speed decreases with increasing daytime temperature, thermometer screen temperatures may consequently show larger uncertainties at the higher temperatures. Under strong direct beam solar radiation (>850 W m−2) the radiation effect is likely to be <0.4°C. Copyright © 2011 Royal Meteorological Society