The extreme weather is being driven by atmospheric patterns that keep hot air trapped in place for days, with these factors worsened by global warming, experts say.

France's national temperature indicator -- an average of daytime and nighttime temperatures across 30 stations -- reached 29.8C on Tuesday, the hottest since measurements began in 1947.

With four more French departments being put under the highest heat alert category Wednesday, around 44 million people are affected, according to AFP calculations.

Added to the 31 departments currently on orange alert, more than 90 percent of the French population is exposed to extreme heat, with temperatures of 39C to 41C expected on Wednesday from Brittany to the Paris region, and in much of the southwest.

The heatwave caused the country's first major power outage of the latest bout of extreme weather, after a heat-related incident with a transformer left around 68,000 households on Wednesday without electricity in the northwestern Finistere department, the authorities said.

While teams worked through the night to fix the issue, which took place late Tuesday, power is not expected to be restored in full until the end of Wednesday at the earliest.

Up to 106,000 clients of the French power network were left without power by late Tuesday, as the scorching temperatures strains infrastructure built for the days before man-driven climate change made heatwaves longer, more frequent and more intense, according to scientists.

Sales of fans and air conditioners skyrocketed in a country where most buildings are not designed to cope with extreme heat.

On Monday, hypermarket operator Carrefour had sold 30,000 units by 6:30 pm -- "a thousand times more than on a normal day", CEO Alexandre Bompard said.

Sales on Amazon nearly doubled last week compared with the same period in 2025, while the electronics outlet Fnac Darty reported double-digit growth.

Thierry, an electrician in southwest France, said he was overwhelmed by requests for "emergency" air-conditioning installations.

"In theory, you have to submit a request to the owners' association general meeting" in residential complexes, "but people don't want to wait".

"It's difficult to live" alone and without air conditioning, said Martine Belloc, a 62-year-old retiree in Bordeaux, who on Tuesday went to La ManuCo, a coworking site that mobilised to welcome elderly people.

Forty people have drowned in France over recent days as they sought to cool down to escape record heat, as a heatwave swept across much of Europe.

Britain, Italy, Switzerland and Spain were also sweltering in extreme heat, with record temperatures in some areas disrupting schools and transport networks and forcing tourist sites - including the Eiffel Tower - to shut.

Europe is warming at more than twice the global average, according to the World Meteorological Organization, making such prolonger heat episodes increasingly likely.

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