Accuracy of daily extreme air temperatures under natural variations in thermometer screen ventilation

Accurate air temperatures underpin environmental research. Most professional meteorological air temperature measurements still expose thermometers within traditional, naturally ventilated screens. Their representation of true air temperature depends on screen airflow, and therefore local winds. Accuracies of daily maximum (T_max) and minimum (T_min) air temperatures are assessed by comparison between a naturally ventilated large conventional screen and a co-located aspirated reference screen. In over 1200 days' data, the naturally ventilated T_min and T_max both showed small (median < 0.06°C) cold bias, but, in 1% of cases, warm T_max bias and cold T_min bias >|1°C|. The T_min cold bias is associated with calm clear nights, and the T_max warm bias events with calm winter days at low sun angles, allowing solar heating of the screen. The prevalence of poor natural ventilation, potentially affecting T_min and T_max, is estimated across European sites. Poor ventilation occurred at T_min for 12% of values, and at T_max for 4%. Climatological averaging will reduce these effects, but, without corroborating wind data, statistical changes in T_min or T_max, including identifying “Tropical Nights” (T_min > 20°C) or occurrences of winter extremes, may have limited value. Wider adoption of aspirated thermometer screens, with an initial overlap period, will largely eliminate these effects.