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A pyrocumulonimbus cloud seen near the village of Saumos in France
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A pyrocumulonimbus cloud, a looming thunderstorm generated by an intensely hot wildfire, has been observed with radar in France for the first time. These deadly “fire clouds” of rising hot air, smoke and moisture, which accelerate blazes with strong winds and soaring embers, are expected to become more common in Europe as wildfires there get bigger.
“You have this tilted column that is sucking air and then propagating embers in front, and it also creates lightning. That’s another way of lighting up fuels in front of the fire, so these cells are unstoppable,” says Jean-Baptiste Filippi at the University of Corsica, France. “You cannot stop a thunderstorm, and neither can you stop a firestorm, a strong pyrocumulonimbus.”
Alongside huge blazes in Spain, France has been suffering a wildfire in the Gironde region, which President Emmanuel Macron has called the country’s most severe crisis since the second world war. The fire has burned more than 420 square kilometres since last week, coming within 15 kilometres of the city of Bordeaux and forcing around 220,000 people to evacuate. It generated a pyrocumulonimbus cloud on 24 July, according to regional officials. Embers and lightning ignited new fires around the original inferno.
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“We faced another firestorm, the same incredible, unprecedented, unknown phenomenon as the one we saw on Friday, when we realised we were entering a new dimension,” Gironde prefect Sophie Brocas told journalists.
It is a miracle this pyrocumulonimbus-fuelled fire hasn’t claimed any casualties, Filippi says.
Sometimes called the “fire-breathing dragon of clouds”, pyrocumulonimbus are typically generated by massive wildfires in the forests of North America, Siberia and Australia. They’ve spurred on infamous fires like the Black Saturday bushfires that killed 173 people in Australia in 2009 and the conflagration that razed the Canadian oil city of Fort McMurray in 2016. Canada’s record-breaking wildfires in 2023 generated 142 of these storms.
In Europe, however, there have been only a handful of confirmed pyrocumulonimbus in Spain and Portugal, including one during Portugal’s devastating Pedrógão Grande fire in 2017, which destroyed hundreds of homes and killed 66 people. But that appears to be changing.
“You have to have two things happening at the same time: an extreme heat source and an unstable atmosphere that interacts with the fire,” says Francesca Di Giuseppe at the European Centre for Medium-Range Weather Forecasts. “If… the heat from the fire increases, like it’s projected to increase from our studies, then yes, you should see more of these events.”
This firestorm begins to take shape when temperatures as high as 800°C send a plume of smoke into the sky. As the hot air rises and cools, water vapour condenses onto the ash to form pyrocumulus clouds, like the fluffy white clouds you might see on a summer day, only brown or grey.
If the plume is hot enough and the atmosphere is unstable enough, the cloud can keep rising to reach the upper troposphere at 10,000 metres, the cruising altitude of a passenger plane. It sometimes reaches the stratosphere, spreading as much smoke as a moderate volcano. It becomes a pyrocumulonimbus when the static electricity from colliding ice particles sparks thunder and lightning.
Although rain from the cloud often evaporates in the hot air, this kind of thunderstorm nonetheless creates its own weather system. The cloud’s updraft pulls in air, fanning the flames with winds reaching 160 kilometres per hour and accelerating the fire’s spread by up to five times, according to Filippi.
Meanwhile, strong downdrafts create erratic gusts that carry embers off to start new fires. Dozens of lightning strikes can ignite blazes as far as 100 kilometres ahead of the main fire. It can also create fire tornadoes of twisting flames. As the winds, embers and lightning strengthen and expand the wildfire, it releases more heat and smoke, a feedback loop that can amplify the rising cloud for many hours.
Both wind shear higher in the atmosphere and erratic gusts at the ground can take the fire in unpredictable directions, making it nearly impossible to contain its acceleration or plan targeted evacuations.
“The fire front is constantly shifting. We cannot fight it directly. It’s a David-versus-Goliath scenario,” Eric Brocardi of the National Firefighters Federation of France told local media about the Gironde fire on 2 August.
Wildfire plumes have expanded in height since 2003 in the western US, creating more pyrocumulonimbus clouds, an increase that’s projected to continue.
Pyrocumulonimbus will have more chances to occur in Europe as well, as the fastest-warming continent experiences more extreme heat , such as the record-breaking heatwave in June, and wildfires. Increasing “climate whiplash” could also contribute, such as when a wet spring last year boosted vegetation growth on the Iberian peninsula, only for record summer heat to dry it out . This led to Europe’s worst-ever wildfires, including a pyrocumulonimbus fire in Spain.
“We need to adapt, so get prepared,” Filippi says. “Burn-proof and reduce the amount of fuel around your house.”
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