Bonn The heavy rains last month ensured that it was the rainiest Bonn July in 173 years. July was also unusually cool; the weather pattern altogether was not the norm.
There was a very unusual weather pattern this year in July: no days over 30°C (86°F) and no nights where it didn’t fall to under 20°C (68°F), extreme thunderstorms with enormous amounts of rain and less sunshine - this is the summary from the former chief statistician of the city of Bonn, Klaus Kosack. He draws on data from the Bonn-Endenich weather station of the University of Bonn and his weather database.
■ Temperatures: With an average temperature of 19.5 degrees Celsius, July of 2021 landed in 36th place. The warmest July to date was in 2006 with 23.9 degrees Celsius on average; the coolest July in Bonn was in 1907 with 15.0 degrees Celsius. On 14 days, the temperature climbed above 25 degrees Celsius but it never reached 30°. The highest temperature measured was 29.0 degrees Celsius on the 25th. The last time there was a July without a day reaching 30°C was in 2011, when we had the lowest monthly temperature of 11.8 degrees Celsius on July 21. July was 0.8 degrees Celsius cooler than June.
■ Sun: Without a stable high pressure system over Bonn, the sun didn’t have much of a chance to prevail: With 171 hours of sunshine, July landed in 38th place behind 1997 (176 hours) and ahead of 1973 (167 hours). Compared to the long-term average, the sun already shone 108 hours longer this year. On four days there were more than 13 hours of sun; on three days there were no hours of sunshine whatsoever.
■ Precipitation: July 2021 was extremely wet: A total of 222 liters per square meter fell on 14 days, more than three times the amount of a normal July. This set a new monthly record since January 1848. The most rain fell on the 14th with 130.4 liters per square meter. Low pressure system "Bernd" was responsible for this. There has never been so much rain in one day in Bonn. The last time a record rainfall was measured was on August 21, 1996 (90.4 liters per square meter). This exceeded the previous July record of 2014 by ten liters per square meter. 84 percent of July's precipitation fell on days when more than ten liters per square meter fell. The rainfall total of last year has already almost been reached.
This also had an impact on the Rhine water level. On the 17th, the level climbed to a record high of 7.71 meters, a new July record for Bonn since 1901.
■ Compared with 2020: July 2020 was 0.3 degrees Celsius warmer than the current July. Last year, the minimum temperature was slightly lower at 10.6 degrees Celsius, but the maximum was 8.5 degrees higher at 37.5 degrees Celsius. The hottest July day so far was on July 25, 2019, with 41.9 degrees Celsius. In 2020, there were 37 liters per square meter of rain and the sun shone 54 hours longer.
Orig. text: Klaus Kosack