How 'fire-generated thunderstorms' make wildfires more deadly

What the report says

BBC World News reports that wildfires around Bordeaux, in France’s Gironde region, have forced tens of thousands of evacuations while producing an unusual weather phenomenon known as pyrocumulonimbus, or fire-generated thunderstorm clouds. French firefighting authorities cited by the BBC said the phenomenon has contributed to the fires near Bordeaux and had not previously been observed in France.

According to the BBC, these clouds form when intense heat from a large wildfire drives air upward, carrying smoke, ash and water vapour high into the atmosphere. As the rising air cools, moisture condenses into cloud; if the fire is strong enough and the atmosphere is unstable, it can develop into a towering thunderstorm-type cloud. The resulting turbulent winds can scatter embers in multiple directions, while lightning can ignite additional fires, making the blaze harder to predict and contain.

The BBC said Gironde officials described conditions overnight on Monday as broadly stable, but warned that firefighting could be complicated by another heatwave forecast for France, with temperatures potentially reaching 40C this week. A senior response official warned the Gironde fire could continue burning for weeks and may take months to fully extinguish.

The report places the French fires in the context of a broader European summer marked by extreme heat and limited rainfall, leaving landscapes more vulnerable to ignition and spread. The BBC also notes earlier pyrocumulonimbus-linked firestorms in Australia’s 2019-20 Black Summer and in the western United States in 2021, while scientists expect climate change to raise the likelihood of conditions that can support such extreme fire behaviour.