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For the principal stations of KNMI in the 1901-1970 period there are two data sources available for calculating Tmean, Tn and Tx.

The first data source consists of the climatological measurements. These are available for the whole period and consist of three readings per day of a mercury thermometer and a minimum/maximum mercury thermometer at 8, 14 and 19 MLT2. Tn and Tx apply to the intervals between the observations times and are further denoted as Tn8, Tn14, Tn19, Tx8, Tx14 and Tx19. The temperature values at the remaining hours were deduced from thermograph readings. The thermograph readings were corrected towards the readings at 8, 14 and 19 MLT. Both the values of the regular thermometer and the minimum/maximum thermometer were used. As an illustration, Figure 1 shows for De Bilt the climatological means of the hourly temperatures and the several Tn and Tx values.

The second data source consists of the synoptical measurements. From 1946 onwards these measurements were done hourly at synoptic hours (1,2,…,24 UTC) mainly taken for weather forecasting purposes. First the instruments were read hourly in a Stevenson screen from a mercury thermometer but later resistance thermometers were used that could be read remotely. Till 1971, the synoptic measurements did not contain enough information to calculate 24 hour Tn Tx values for the 0-0 UTC interval.