Switzerland experiences first short heatwave of year

Switzerland experienced its first heatwave of the year over the Whitsun weekend, with temperatures in excess of 30 degrees. However, it wasn’t declared an official heatwave yet.

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The Federal Office of Meteorology and Climatology (MeteoSwiss) only refers to a heatwave when an average daily temperature of 25 degrees or more is reached for at least three consecutive days. If the criteria are only met for a maximum of two days, it is referred to as a short heatwave.

The decisive word is “average temperature”: this is the average over a full 24 hours, including the cool hours of the night and morning. Although temperatures exceeded 30 degrees during the day at many measuring stations, the official threshold was clearly missed.

At the Basel-Binningen measuring station, for example, temperatures reached 31 degrees. However, the average daily temperature was 22.9 degrees, below the 25-degree threshold. At the Sion station, the thermometer even climbed to 32.4 degrees, but here too the average daily temperature of 23 degrees Celsius remained well below the heatwave threshold.

Night-time cooling, a respite for the body

There is a medical reason why meteorologists base their heatwave warnings on 24-hour averages. For the human body and cardiovascular system, the main problem is not the short-lived heat peak in the afternoon. The night is decisive: if it remains heavy, humid and warm, the body is constantly working at full capacity. Blood vessels remain dilated, the pulse rate is high and sleep is not very restorative.

But as the nights cooled off again over the Whitsun weekend, the heart and circulatory system were able to take a much-needed restorative break.

Hot days earlier in the year

The term “hot day” is a different matter. For a day to be considered as such in the statistics, all that is required is for the mercury to exceed 30 degrees at its hottest point. Basel and Sion were therefore, by definition, hot days on Whitsun Sunday, although this was not enough to qualify as a heatwave.

The fact that it was so hot at Whitsun is “exceptional, but not a record”, wrote MeteoSwiss last week on its blog. In fact, the earliest hot day of the year goes back further in the calendar. At both the Basel-Binningen station (in 1945) and the Sion station (in 1958), the first hot day had already been recorded on May 10.

On average over the long term, however, the first hot day on the Swiss Plateau only occurs in June – usually on June 7 in Sion and June 9 in Basel-Binningen.

Translated from French by AI/jdp