2021 – A Slightly Cooler Than Average Year That Saw Extreme Rainfall in July in Certain Regions
Overall, the year was marked by very changeable and, in part, considerably duller weather than usual. The extreme rainfall on July 13 and 14 was a particularly striking weather event in 2021. The Climate Monitoring Station Aachen-Hörn of the RWTH Department of Geography has reported the following annual temperatures for 2021.
When it comes to air temperature, an annual average temperature of 10.7 degrees Celsius was reached in 2021, which is within the range of the average temperature (10.9 degrees Celsius) of the new Climate Normal lasting from 1991 to 2020.
Compared to the average of the relevant Climate Normal until 2020 – 9.7 degrees Celsius from 1961 to 1990 and the average from 1901 to 2000 for Aachen (Weather Station) of the German Weather Service DWD (9.6 degrees Celsius) – this value is still high, as it last fell below this in 2013 (9.9 degrees Celsius).
The highest temperature of the year was recorded on June 17, when it hit 33.0 degrees Celsius, while the lowest temperate was recorded on February 10, when it sunk to -10.6 degrees Celsius. In the Climate Normal of 1991 to 2020, such a maximum temperature was exceeded in 22 of the 30 years, while such a low value has not been recorded since 2008. A low like that of 2021 was recorded every fifth year on average.
Overall, despite the cool weather, three hot days (with a maximum temperature greater than or equal to 30 degrees Celsius) were still recorded, which is almost the same as the average for the period of 1961 to 1990 (3.5 days), but significantly lower than the value for 1991 to 2020 (7.6 days). Ice days (maximum temperature below 0 degrees Celsius) were recorded on five occasions, which is seven less than the average for the period 1961 to 1990 (but three more than the average for 1991 to 2020).
Even before the drastic weather change in the second half of June, two months were already colder than the 1961 to 1990 average (April and May), with August and November later joining this trend. However, as a whole, June was significantly warmer than the average of 1961 to 1990 (+4.4 Kelvin), with a monthly mean of 20.0 degrees Celsius, while the winter months all saw much higher lows than the typical lows in this period (e.g. January: -1.9 degrees Celsius = +15.5 Kelvin).
One extreme weather event to note was the heavy rainfall on July 13 and 14, 2021. The heavy rainfall of 55 millimeters and 98 millimeters was recorded on both days respectively, i.e. a total of 153 millimeters of precipitation were measured at the DWD's Aachen-Orsbach Weather Station. Due to the special structure of the rainfall, there were very large spatial and temporal differences, not only locally, but also over a large area, especially on the northern side of the Eifel, which usually sees little precipitation. A preliminary assessment of the RADOLAN radar precipitation data of the DWD carried out at RWTH’s Department of Geography showed the following average precipitation totals: 100 millimeters in the upper Ahr valley, 101 millimeters in the upper Indes valley, and 117 millimeters in the upper Erft valley on July 13 and 14.
The precipitation total of 789 millimeters expected for 2021 (up to and including December 31) at the Climate Measuring Station Aachen-Hörn is – despite the extreme rainfall in July – still somewhat below the average of the years 1961 to 1990 at Aachen (Weather Station) of the DWD (828 millimeters) or well below the average of the Climate Normal of 1991 to 2020 at the Climate Measuring Station Aachen-Hörn. In North Rhine-Westphalia, it is assumed that the water deficit in the deeper soil layers will continue, especially taking into account the fact that much of the heavy rainfall could not seep away but only run off.