Meteorologists like to occupy themselves with the future. Maybe that’s why they are ahead of the game. Because whereas for others the summer only ends on 21 September, for meteorologists today autumn has already started. But now first a retrospect: what meteorological summer have we had?
The first thing that stands out, of course, is the long heat wave in August. It lasted thirteen days in De Bilt, where KNMI’s main measuring station is located.
For a heat wave, at least five summer days (maximum temperature above 25 degrees) are needed in a row, of which three days are tropical (maximum temperature above 30 degrees). The heat wave in August counted no less than nine tropical days. Eight of these were in a row: a record. It was also striking that the nights were exceptionally warm.
Also in 2018 there was a heat wave of thirteen days, but it only had four tropical days. For the previous heat wave of similar intensity, we have to go back to 2006. From 15 July there was then a heat wave of sixteen days, with eight tropical days.
In total there were 29 summer days and 11 tropical days this summer. Normally there are on average 21 and four.
Although the number of summer days was higher than last year, the temperature was slightly lower. It was 18.3 degrees Celsius on average this summer, compared to 18.4 in 2019. Both years score far above the long-term average, which is calculated over the years 1981 to 2010. That is 17 degrees over the summer months.
July was almost one degree cooler than average. “All the more remarkable that this year has ended up in the top ten warmest years since 1901. That has to do with the record heat in June and extremely warm August,” says CCeit weatherman Gerrit Hiemstra. In June it was almost 2 degrees and in August almost 3 degrees warmer than average.
The highest temperature was measured on 8 August. The measuring station in Arcen in Limburg then indicated a temperature of 37 degrees. Last year, a record temperature of 40.7 degrees was measured in Gilze-Rijen.
The drought this summer led to a call from water companies to use less water during the heat wave. Natuurmonumenten warned provinces and water boards that the drought of the past three years has caused irreparable damage to flora and fauna in several nature areas. The spring of 2020 was already the driest ever recorded.
Exactly how dry was it this summer? In De Bilt it was only a little bit drier than average. There was 218 millimetres of rain in the three summer months, where it is normally 220. July in particular was drier than normal. Then 51 millimetres fell, where 81 millimetres is normal. June and August were slightly wetter than average.
Regionally there were big differences. In the east and southeast there was very little rain, in Arcen for example only 89 millimetres. In places where it did rain, the showers were often very heavy. In several places this led to flooding. Hiemstra: “These kinds of extreme showers become much more normal.”
At the same time, in the above-average wet months, there was also above-average sunshine. In June there were 228 sunshine hours (normal 194) and in August 220 (normal 188).
A total of 657 hours of sunshine in the summer. A lot less than in 2018 and 2019, when the sun was shining 752 and 733 hours, but still far above what is usual: 588 hours.
But how will this summer go down in history? In its report, KNMI refers to a ‘very warm’ summer. From a historical perspective, this seems more than justified: since the measurements began in 1901, only five summers have been warmer.
Hiemstra: “In the past, warm and cool summers alternated, now it is very warm for the third summer in a row. I think this summer is a model for the summers we will have in the future”