In 20 years' time, will France resemble Oklahoma, the American state known as the perfect playground for storms? The question may seem provocative, but it's a very serious one, posed by two researchers who, on Wednesday, April 3, published the first climatological study of "derechos" in France. They've shown that these gigantic storm systems – currently rare in France – are likely to intensify under the impact of climate change. Atmospheric instability has already risen in recent years.
Much studied in the US, these phenomena remain relatively unknown in France. They made a splash in the media in 2022, when a very violent derecho swept across Corsica, on the night of August 17. Winds of up to 225 kilometers per hour were combined with intense rain, hail and 100,000 bolts of lightning. The toll came to five deaths and extensive damage. In all, 12 people lost their lives along the monster storm's ill-fated path, from the Balearic Islands to Italy and Austria.
"This alarmed us because we didn't know much about this phenomenon. The event was unprecedented in its scale and intensity, and also the fact that it occurred over the Mediterranean," said climatologist Davide Faranda, director of research at CNRS (French National Center for Scientific Research) and one of the two authors of the study, published in the journal Weather and Climate Dynamics. At the time, the French weather agency Météo-France was roundly criticized for not having placed the beautiful island on red alert.
By combining satellite data and observations, the two scientists have arrived at a classification for derechos in France. They counted 38 phenomena between 2000 and 2022, only during the warm season (May to September), an average of 1.7 per year. This number is similar to that in Germany, but much lower than in the US, where between 10 and 15 are recorded each year.
To qualify, thunderstorm systems must extend over 100 kilometers, cover a distance of more than 400 kilometers, "and have a lifetime of several hours, during which they produce very strong wind gusts, in excess of 90 km/h," explained Lucas Fery, a PhD student in climate physics at the French Atomic Energy and Alternative Energies Commission (CEA), and first author of the study.
The winds generated by this phenomenon move in a straight line, hence their name, which means "straight" in Spanish – a term first used in 1888. These regional systems are so named in contrast to tornadoes, which are circular winds extending over a few hundred meters; tropical cyclones, which rotate around an eye over a few hundred kilometers; and storms, which are large depressions extending over some 1,000 kilometers.
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