For the third time this summer, France is crossed by violent storms after a period of heat wave.
After having suffocated in front of a thermometer which rose to more than 35°C, France is now preparing to face the torrential rains of thunderstorms. Six departments on the shores of the Mediterranean Sea (Hérault, Aude, Lozère, Tarn, Aveyron, Gard) have been placed on orange alert by The Weather Channel*. The rest of France is called for caution, especially towns bordering rivers that could overflow.
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After the driest July on record since 1959, “violent thunderstorms and heavy accumulations of rain with a risk of flooding» are in turn sweeping across the territory from Tuesday to Wednesday, August 17. And this, for the third time this summer. Heat waves always seem to be followed by lightning and thunder. A melody that sounds like an eternal refrain. Is there a real causal link between heat waves and thunderstorms?
Heat, the only ingredient?
«The correlation link is not so obvious“, analyzes Régis Crépet, meteorologist for The Weather Channel*. To better understand it, it is necessary to take an interest in the making of a storm. This last “is the meeting between two masses of air, one very hot and another cold“. After burning for days in the sun, the ground lets out a mass of hot air which rises to meet molecules whose temperature is close to zero degrees.
Large cumulonimbus clouds then form, the functioning of which resembles powerful vacuum cleaners that feed on the different air masses. Inside the black clouds, the meeting between hot and cold, which have opposite electric charges, is a source of lightning. The warmer the air rising from the ground, the more the encounter with the cold atmosphere creates major storms.
For example, the storms which swept over France last June followed a long wave of heat waves, particularly early for the season. Another example, if we go back to July 26, 2019, lightning struck the territory after several days with mercury above 35°C. This date even marks the most important lightning activity of the summer.
A year later, on August 12, 2020, while France had been struggling to breathe for several days, violent storms put an end to the heatwave episode.
But is heat the only trigger for a storm? “One ingredient is not enough, continues the meteorologist. The heat is one of them, that’s for sure, but that’s not all.
Importance of humidity
A thunderstorm cannot form only with warm air masses, it also needs moisture. Stéphane Schmitt is a researcher at the Météorages centre, the official body for recording storm events in France. “There are only very very rarely thunderstorms in the Sahara because there is no exchange of air masses and the air is particularly dry there.“, he gives as an example.
How then can we explain that the month of June 2022 was the most struck down month since 1959 when there was no rainfall in the spring? “The early and intense heat wave permanently anchored a mass of hot air on the territory», explains Météorages, in a press release. Added to this was “humidity carried by the Atlantic“. So, according to the organization, “all the ingredients have been brought together to generate severe stormy waves».
Certainly more violent phenomena
Faced with hotter and drier summers, should we expect more thunderstorms? “It is still difficult to establish a direct correlation, nuance Stéphane Schmitt, who does not observe a general increase over twenty years. But we see more large-scale phenomena. On June 4, Météo France issued the most extensive storm warning in its history, extending it to 65 departments, or 3/4 of the country.
«For each additional degree, extreme precipitation intensifies by 7%“, warn the IPCC scientists in their 2021 report. If the link between a scorching summer and stormy episodes is not systematic, it seems that the latter, fueled by global warming, are more violent, possibly even destroy roads and homes. For Régis Crépet, the recent storms are indeed “shorter but more intense“. In addition to the heat and humidity of the soil, “it’s really the atmospheric condition that creates storms.»
*The Weather Channel is a property of the Figaro group