What is a fire thunderstorm? How wildfires create these ‘dragon clouds’ in France — and Canada

Beachgoers watch on a beach as clouds of smoke rise from a nearby wildfire, in Lacanau, France, on July 24, 2026. Photo by Maximilien Lamy/AFP via Getty Images

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A wildfire near Bordeaux burned hot enough to make its own thunderstorm this week, and the lightning it threw started new fires. It was the first pyrocumulonimbus cloud ever recorded in France, but Canada has seen hundreds of them in the last decade.

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Pyrocumulonimbus clouds, the fire-generated thunderstorms contributing to the blazes that forced tens of thousands of people from the region in southwest France this week, are rare in Europe. NASA has called them the “fire-breathing dragon of clouds.”

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The Gironde prefecture said Thursday that the fire has burned 42,000 hectares near Saumos, southwest of Bordeaux, and has held at that size since Tuesday. About 3,300 firefighters, 1,500 military personnel and 24 aircraft are assigned to it.

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Sophie Brocas, the prefect of Gironde, authorized residents of nine more municipalities to return home Thursday. That brought to 144,000 the number cleared to go back, and the A63 motorway reopened in both directions at 2 p.m., her office said. The prefecture said 231 firefighters have been injured and 240 homes destroyed, and that smoke has left the Nouvelle-Aquitaine region under an episode of degraded air quality. Smaller fires continue to burn in southern France, including one in Brignoles that has forced actor George Clooney and his family to evacuate their estate.

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The fire clouds form when a wildfire burns hot enough to punch a column of air into an unstable atmosphere, and they behave like thunderstorms because that is what they are.

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“So a pyrocumulonimbus cloud is a thunderstorm cloud that forms over an active, and usually a very active, wildfire,” said Peter Kimbell, a warning preparedness meteorologist with Environment and Climate Change Canada.

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The heat works the way the sun does on an ordinary summer afternoon, Kimbell said. Air rises, cools with height and condenses. What makes a fire cloud different is that the fire supplies its own water.

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“The moisture generated by the upward currents and also by the combustion of forest matter, this combustion actually releases the water vapour as well,” he said.

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The conditions have to line up. Hot and dry at the surface to produce the fire, then what Kimbell called “fairly good mid-level instability in the atmosphere, so temperatures cooling off with height fairly rapidly.”

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“So it’s a bit counterintuitive that you’d get thunderstorms above a hot, dry fire, but that’s what happens,” he said. Fire is associated with destruction, not with generating weather of its own.

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Canada records more of them than any other country. Researchers logged 350 pyrocumulonimbus events here between 2013 and 2023, according to the first worldwide inventory of the phenomenon, published last year in npj Climate and Atmospheric Science. The United States was second with 183, followed by northern Asia with 115 and Australia with 76. Outside those regions, researchers counted 19 events over the same period, primarily in Europe, Africa and South America. The inventory’s record ends in 2023, and no updated count has been published.