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.
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.”
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.
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.
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.
“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.
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.
“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.
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.”
“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.
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.
A new record was set in 2023, the study found, when Canadian fires produced 142 of the 169 pyrocumulonimbus (pyroCb) counted worldwide that year. They formed across five provinces and two territories between early May and late September 2023, with 81 tied to fires in British Columbia and Alberta.
What makes them dangerous is what happens underneath. The inventory attributes a specific set of hazards to the storms. Downdrafts that redirect a fire. Lightning capable of starting new ones. Hail, embers thrown far ahead of the front and updrafts strong enough to threaten firefighting aircraft. The study says a pyroCb forming over a fire poses an immediate threat to firefighters and civilians, impeding containment and contributing to increases in fire size and intensity. It also says the storms help spawn fire tornadoes, and that in 2020 the U.S. National Weather Service issued the first tornado warning ever written for one.
“These thunderstorms also, the pyroCbs, generate a lot of upward currents, they generate a lot of smoke and ash that travel to pretty great distances and affect the health of people downstream,” Kimbell said.
“PyroCbs have always happened. So there’s nothing really new about pyroCbs, but they’re not very common,” Kimbell said. “There’s nothing new about the process, about the phenomenon, just that we’re experiencing them right now.”
The inventory backs him up. It found significant year to year swings and no discernible trend across the 11 years it measured. Canada recorded none at all in 2020, then 50 the following year.
They are not unique to Canada either, Kimbell said, listing Asia, Russia, Australia “and of course lately in Europe.”
Canada is having a heavy wildfire season this year. More than 180 fires were burning in Ontario in mid-July, and the province is on pace for its worst year on record by early estimates. In B.C., the Pear Lake and Fiftynine Creek fires merged into a single 668-square-kilometre fire that destroyed properties.
Satellite analysis published by the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin showed a large wildfire southeast of Lytton producing several fire clouds on July 7. Cloud tops reached as low as minus 50 degrees C, cold enough to confirm ice had formed at the top. That ice is what separates a fire thunderstorm from an ordinary smoke cloud.
For anyone living near a fire, Kimbell said the cloud is not the thing to worry about.
“The big issue for people in the vicinity of the wildfire is the wildfire itself,” he said. “If you’re in the vicinity of a wildfire, then if told by the authorities to evacuate, you’re evacuating primarily because of the wildfire.”
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