24.06.2022, Hessen, Frankfurt/Main: Blitze entladen sich während eines Gewitters, das am Abend über die Frankfurter Innenstadt und seinen markanten Hochhäusern zieht. Foto: Arne Dedert/dpa +++ dpa-Bildfunk +++

June 27th is Dormouse Day – the day on which the weather for the rest of the summer can be read. At least that’s what old farmer’s rules say, such as “If the dormouse is wet, it rains non-stop” and “It rains on dormouse day, the rain doesn’t let go for seven weeks”.

Optimistic variants of the farmer’s rule predict rather good weather: “The weather on Dormouse Day may stay for another seven weeks”. But does this phrase really apply from a meteorological point of view?

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Until the 16th century, July 7th or 8th was known as dormouse day. The eleven-day shift dates back to the introduction of the Gregorian calendar in 1582. This corrected an inaccuracy of the previously valid Julian calendar.

From a meteorological point of view, however, the exact date of the dormouse makes no big difference: both the last week of June and the first week of July are relevant for the weather forecast. According to the German Weather Service (DWD), the weather situation stabilizes in most years during this period.

The decisive factor here is the course of the jet stream, a band of strong winds in the upper layer of the atmosphere. If it runs relatively far south over Europe, there will be a low in air pressure, which increases the likelihood of a cool and wet summer, according to the DWD. If, on the other hand, it runs relatively far north, the Azores High expands in the form of a wedge towards Central Europe. This promises rather warm, sunny days.

So the dormouse rule has a certain justification. However, you cannot rely on them one hundred percent. Statistically, the dormouse hit rate is around 55 to 70 percent in Germany. It is somewhat lower in the north and somewhat higher in the southern foothills of the Alps.

However, this hit rate only applies if the rule is interpreted for three or four weeks instead of seven. In addition, the weather can be predicted much better with computer models.

An explosive weather situation is currently prevailing in Germany, and in many regions people have to be prepared for the risk of severe weather. According to the DWD, there could be heavy rain, heavy gusts of wind and hail in large parts of the country by the weekend, in individual cases the grains can be as big as table tennis balls. “There is still steam in the boiler, because the weather conditions are not changing,” said a DWD meteorologist on Monday morning in Offenbach.

According to the information, the cause of the high risk of thunderstorms is a low-pressure area in the British Isles and a high-pressure area over Eastern Europe. They ensure that humid Mediterranean air flows into Germany from the south and south-west. “Exactly over the country there is an air mass limit, in the area of ​​which there are repeated showers and thunderstorms with an increased potential for severe weather,” explained the meteorologist.

There are huge temperature differences: while the DWD expects up to 36 degrees in the east on Monday, the values ​​​​in the west are just over 20 degrees. There is said to be light rain showers there.

In the eastern federal states, on the other hand, there were already official warnings of “extreme heat” in the morning. The DWD also expected the strongest thunderstorms there. On Tuesday, thunderstorms and storms are likely to brew, especially in the southern half, which will then move north on Wednesday night. “On Wednesday it will therefore be quite unstable with muggy temperatures almost throughout the country,” said the weather forecaster.

First of all, it has nothing to do with the nocturnal rodent. Instead, “dormouse” refers to a Christian legend.