Weather. "Up to 35°C": Why are heat waves becoming more frequent in September?

This Friday is a peak heatwave day in France, with temperatures expected to climb to 35°C in some places, a phenomenon that is becoming increasingly common in September due to climate change.

Météo-France predicts a "summer surge" that will affect almost the entire country , with temperatures reaching 32°C in Bordeaux, 29°C in Paris, 28°C in Toulouse, and 27°C in Lille. "35°C could be reached in the plains of the Southwest, which remains very rare after September 15," Météo-France predicts. Gironde, Lot-et-Garonne, Landes, and Gers are among the departments where it is expected to be the hottest.

In the northern half, "30°C could be reached locally in Hauts-de-France, Normandy, the Paris Basin, or Burgundy," the forecasters add. The only exception: in the parts of the Northwest most exposed to maritime influence, Brittany and Cotentin, a mild day is expected, with 19°C in Brest, for example. Already on Thursday, temperatures reached or exceeded 33°C at three weather stations in Occitanie and Gironde.

Météo-France spoke of a "remarkable, but not unprecedented, heat spike," as evidenced by the 32°C in Paris on September 18, 1961. At the time, the heat caused a completely different fear than that of global warming today: road accidents linked to a spike in travel to the countryside. The weekend resulted in more than 30 deaths, reported the daily newspaper Le Monde .

Apart from another September 18 with 35°C in Bordeaux, in 1987, Météo-France notes precedents all dating from the ^21st century, in 2003, 2020 and 2023. The historically hottest September in France was recorded in 2023, with a temperature 3.6°C higher than the average for the years 1990 to 2010.

The country experienced an "unprecedented heatwave" from September 3 to 11, during the return to school and the opening of the Rugby World Cup. It was the first time that an orange alert for the risk of a heatwave had been triggered on a September day, on the 7th in this case. The mercury then climbed to 35.1° in Paris. This time, there is no alert from Météo-France, unlike the intense heatwaves of June and August 2025 .

The warming of summers is disrupting the calendar of the grape harvest month of September in France in particular. For the past forty years, the harvest has been taking place earlier and earlier , as the grapes ripen more quickly.

"Many regions are telling us they will have finished harvesting before the end of September. That's particularly early. Champagne was almost finished by September 5th, which is very exceptional," explained Bernard Farges, president of the National Interprofessional Committee for Wines with Designation of Origin.

The month is also becoming increasingly desirable for tourists, even if the school calendar or the habits of the working world allow few of them to take advantage of it. In the Europ Assistance 2025 Annual Barometer, among more than 1,000 people surveyed online in February-March by Ipsos, 37% of respondents in France designated September as an ideal month to go away, more than August (35%), June and July (32% each).

Scientific consensus links human activity, particularly greenhouse gas emissions, to global warming . The latter is making heat waves longer and more intense, as well as earlier and later. Temperatures are rising even faster in France than the global average.