Impact of two different sized Stevenson screens on air temperature measurements

In this study we evaluated the impact of the size of two naturally ventilated wooden Stevenson screens on air temperature measurements in the first-order meteorological station of Calamocha (northeastern Iberian Peninsula, Spain). The 1-year field experiment consisted of comparing air temperatures measured at the two most commonly sized Stevenson screens used by the Spanish Meteorological State Agency (AEMET) since last century; the medium-sized Stevenson screen employed at the second-order weather stations, versus the large-sized Stevenson screen mainly used at the first-order meteorological stations. The main objective was to report the air temperature difference between these two differently sized Stevenson screens, and to study the impact on the observed differences of some weather elements (i.e. relative humidity, wind speed, total cloud cover, atmospheric pressure and global solar radiation). The results show that the medium-sized Stevenson screen tended to overheat daily maximum air temperatures (0.54 degrees C on yearly average) and also air temperatures recorded at 1300 UTC. The differences on daily minimum air temperatures were negligible (-0.11 degrees C on yearly average). This overheating bias (not statistically significant) occurred under anticyclonic situations that lead to clear skies, high solar radiation, weak winds and low relative humidity. The bias appeared throughout the whole year but in particular during the warm season from May through October. Air temperature observations from the nearby station Daroca confirmed an overheating bias introduced by a change from a large-sized Stevenson screen to a medium-sized one in Calamocha.