RESOLVING THE LEDGER
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PARIS (AP) — A wildfire in southwest France grew so powerful that its smoke column became a thunderstorm, generating lightning that struck the ground and ignited new fires beyond the original blaze, French officials said.
Put another way, that means the inferno was no longer simply being propelled by the weather. It was making its own. The thundercloud also produced violent, erratic winds capable of driving the flames in new directions.
The phenomenon, called a pyrocumulonimbus, or pyroCb, is rare in Europe and more commonly documented in North America and Australia, where it occurs only over some of the most extreme fires. France’s national firefighters federation said it had never before recorded one in the country.
The blaze erupted near Saumos on July 22 and has since burned more than 420 square kilometers (162 square miles) of forests and scrubland, damaged or destroyed more than 240 homes and forced 220,000 people to evacuate in Gironde.
At about 6:20 p.m. Friday, two days after the fire began, the departmental fire and rescue service said it had generated a pyrocumulonimbus. The cloud weakened overnight as humidity rose, then formed again several times.
An electrified black thundercloud
Repeatedly, heat and smoke have surged upward into a vast black thundercloud, electrified and lit from within.
“Imagine a campfire so large and hot that the smoke rising off it turns into a storm-like cloud,” said Theodore M. Giannaros, a fire meteorologist at the National Observatory of Athens.
“That is a pyroCb — a storm cloud born from the intense heat that a wildfire produces.”
It takes more than a big fire.
There is no set temperature at which one forms, Giannaros said; what is needed is the kind of atmosphere that breeds dry thunderstorms — very hot, very dry air near the ground, with cooler and moister air above it.
Hot air loaded with smoke and moisture then rises rapidly from the flames. As it climbs and cools, water vapor condenses around particles of ash.
Above the freezing level, droplets become ice crystals whose collisions separate electrical charges, as in an ordinary thunderstorm.
Lightning and more destruction follow
Beneath this storm lies a landscape already primed to burn. The cloud then amplifies the disaster that created it.
Rising air pulls powerful winds toward the fire, while downdrafts punch gusts back to the surface.
Those shifts can turn the flames, divide them into several fronts or send them racing in a new direction.
“It’s a feedback loop, not a one-way effect,” Giannaros said. “Once it forms, the cloud becomes its own weather system, sitting on top of the fire and making it harder to predict.”
The systems can also spin up whirling columns of flame — usually a fire whirl, which Giannaros describes as a dust devil made of fire and heat instead of dust.
True tornadoes are rarer, though one struck Redding, California, during the 2018 Carr Fire with winds equivalent to an EF-3, killing a fire inspector.
For firefighters, escape routes can close without warning as fresh outbreaks appear beyond their crews and equipment.
Direct attack may become impossible, forcing crews to retreat and defend communities.
The danger, Giannaros said, is fire moving faster than evacuations can be organized.
Until now, pyroCbs have been essentially an Australian and North American phenomenon. Canada alone produced a record 142 of them in 2023.
In Europe, pyroCbs have been rare
Portugal recorded one during its deadly 2017 fire season. France has now joined the list.
The continent is heating faster than any other on Earth — at roughly twice the global average since the 1980s, according to the EU’s Copernicus Climate Change Service and the World Meteorological Organization — with heat waves growing more frequent and severe and drought spreading across the south, curing forests into fuel.
Hotter and drier conditions give extreme fires more opportunity to form them, Giannaros said, but scientists still lack enough long-term data to establish a trend.
The European research project ROSETTA aims to close that gap.
The most common misunderstanding, he said, is that the cloud is merely a side effect. As long as it hangs above the flames, it is helping decide where the fire goes next.
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