As a result of a new WMO regulation, KNMI lowered its thermometer screens around 1960 from 2.20 to 1.50 m above ground level. The present study analyses parallel air temperature measurements in screens at the two heights in De Bilt in the period 2017–2019. The results are compared with results of two one-year comparisons at KNMI stations De Bilt and Witteveen in the 1950s.
We found an increase in the diurnal temperature range on 1.50 m compared to 2.20 m, with a slight increase in daily maximum temperatures and a somewhat larger decrease in daily minimum temperatures. The effect on the mean daily temperatures is negligible. The effect of the lowering of the screen on annual minimum temperatures is a decrease of about 0.2 ◦C, while the effect on annual mean maximum temperature is an increase of about 0.1. ◦C. In general, the differences in 2.20 and 1.50 m air temperature show a seasonal cycle with the largest values in summer and the smallest in winter.
It is argued that there is a real effect on temperature of the lowering of thermometer screen in the Netherlands around 1960. The effect is small and depends on wind speed. Therefore, it is not feasible to correct the time series of all five principal stations in the Netherlands. The added value of the present study is that it gives an estimation of the order of magnitude of the temperature differences due to the lowering of the thermometer screens.
T. Brandsma. Effects of lowering thermometers screens in the Netherlands around 1960
KNMI number: TR-400, Year: 2022, Pages: 35