Hot air from the Sahara
Why does the climate cool down, even at night?
The country experienced the highest temperatures of summer on Wednesday, culminating at 36.5 degrees in Payerne. Even at altitude, the hot air of subtropical origin kept the thermometer over 25 degrees during the night.
The subtropical “broth” ensures high temperatures even at night: the evening sky above Zurich from the polyterrasse.
Photo: Urs power
- On Wednesday, the highest summer temperature was recorded in Payerne with 36.5 degrees.
- On the Banter and the Uetliberg, the thermometer did not drop below 25 degrees during the night.
- The heat wave continues until the weekend, but the risk of thunderstorm increases.
Switzerland knew Wednesday the highest temperatures this summer. According to Weatherthe record was set in Payerne (VD) with 36.5 degrees recorded shortly before 4 p.m. Geneva and Lausanne followed closely with 36.4 and 36.3 degrees respectively. The thermometer also climbed 36 degrees in Basel and Valais. On the set and in eastern Switzerland, the maximums remained a little more moderate, oscillating between 33 and 35 degrees.
The temperatures therefore remained slightly below the weather forecast. No new record was set in the plain for the month of August. Current records remain 39.3 degrees in Geneva (August 24, 2023) and 38.6 degrees in Basel (August 13, 2003).
According to Eugen Müller, meteorologist in Météosuisse, this phenomenon is explained by the southwest winds which transported Saharan dust to the Alpine space, between 2000 and 4000 meters above sea level. These particles have filtered solar radiation, preventing the air mass from revealing its full warming potential.
24 degrees at 1500 meters above sea level
To measure the importance of this potential, it is enough to observe another atmospheric floor. At average altitudes (Editor’s note: around 1500 meters)the arrival that came out of which had exceptionally rare characteristics in our latitudes.
View of the Zurich lake from the Uetliberg: at this altitude, the heat wave was felt in an intense particular way on Wednesday. The air has indeed not cooled during the night.
Doris Fanconi
Twice a day, Météosuisse launches from Payerne a weather balloon equipped with a radiosonde. This probe measures values such as temperature, humidity or direction and strength of the wind at different altitudes. “The ascent of the radiosonde on the night of Tuesday to Wednesday showed a temperature of 24.2 degrees at around 1500 meters above sea level,” explains Eugen Müller. It is the highest fourth value ever recorded in August. The record of all time – 25 degrees, measured on 1is August 1983 – was not narrowly reached.
Analyzes of experimental cards carried out by the meteorologist Markus Pfister suggest that the temperature could even have exceeded 25 degrees in certain regions at the altitude in question, in the far southwest of Switzerland as well as in Valais, where no report by radiosondeing was carried out.
Hot air from subtropical regions
Record or not, the air mass that invaded Switzerland on Wednesday was burning. No wonder when you know where it comes from. Thanks to retroprojections, meteorologists can trace the origin of the air at different altitudes, several days back.
These trajectories reveal the surprising air route that reached Switzerland on Wednesday at midnight. He first left the Western Mediterranean to cross Western Sahara and the Atlantic, then joined the Alpine space via Portugal and Spain.
The graph shows the origin of the air masses at different altitudes (from 2000 to 5,500 meters) Wednesday (midnight), in Switzerland.
wetter3.de/GFS
This mass of subtropical air has not only caused a marked heat in the plain, but also and above all in altitude weather stations. In the mountains, however, it is not so much the maximum temperatures of the day that were exceptional, but rather the nocturnal minimum. Thus, the Bantiger thermometer near Berne (1096 meters) did not drop below 25 degrees until the early hours of Wednesday. On the Zurich Tower of Uetliberg (1016 meters), the minimum night temperature was also 25 degrees.
This phenomenon is common in this type of meteorological situation. Unlike the plain, where air cools near the ground during the night, this effect does not operate in the mountains. Thus, even the summit of the Dôle, which culminates almost 1,700 meters near Geneva, had a tropical night with a minimum temperature of 21 degrees. As for the Jungfraujoch, at an altitude of 3580 meters, it maintained a temperature of 5 degrees, well above the freezing point.
The current heat wave will continue, but it gradually weakens. In French-speaking Switzerland, temperatures should remain over 30 degrees until around next week. In German -speaking Switzerland, they will descend below the heat wave on Saturday already. At the same time, the risk of thunderstorms intensifies day by day. From Saturday at the latest, it will be necessary to count with thunderstorms on a large part of the territory, including in the plain.
Translated from German by Olivia Beuhat
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