French officials stated that a wildfire in southwest France became so intense that its smoke column developed into a thunderstorm, which subsequently generated ground-striking lightning and ignited further fires beyond the initial blaze.
This indicates the inferno was generating its own weather system rather than being solely driven by existing conditions. The thundercloud also produced violent, erratic winds capable of redirecting the flames.
This phenomenon, known as a pyrocumulonimbus or pyroCb, is uncommon in Europe but is more frequently documented in North America and Australia, where it occurs only over exceptionally severe fires. France’s national firefighters federation reported that it had never previously recorded one domestically.
The fire began near Saumos on July 22 and has since burned over 420 square kilometers (162 square miles) of forests and scrubland. It has damaged or destroyed more than 240 homes and necessitated the evacuation of 220,000 people in Gironde.
At approximately 6:20 p.m. on Friday, two days after the fire started, the departmental fire and rescue service confirmed the generation of a pyrocumulonimbus. The cloud weakened overnight as humidity increased before reforming several times.
An Electrified Black Thundercloud
Heat and smoke repeatedly surged upward into a vast black thundercloud that was electrified and internally illuminated.
Theodore M. Giannaros, a fire meteorologist at the National Observatory of Athens, commented, “Imagine a campfire so large and hot that the smoke rising off it turns into a storm-like cloud.” He added, “That is a pyroCb — a storm cloud born from the intense heat that a wildfire produces.”
More Than Just a Large Fire
Giannaros noted there is no specific temperature threshold for its formation; rather, the necessary condition is an atmosphere conducive to dry thunderstorms—specifically, very hot, very dry air near the surface overlying cooler, moister air. Hot air laden with smoke and moisture then rises rapidly from the flames. As it ascends and cools, water vapor condenses around ash particles. Above the freezing level, these droplets become ice crystals whose collisions generate electrical charges, similar to a conventional thunderstorm.
Lightning and Increased Destruction
Beneath this storm lies terrain already susceptible to burning. The cloud then amplifies the disaster that created it. Rising air draws powerful winds toward the fire, while downdrafts push gusts back to the surface. These shifts can alter the flames, divide them into multiple fronts, or propel them in a new direction.
“It’s a feedback loop, not a one-way effect,” Giannaros stated. “Once it forms, the cloud becomes its own weather system, sitting on top of the fire and making it harder to predict.”
These systems can also generate whirling columns of flame—typically a fire whirl, which Giannaros describes as a dust devil composed 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, resulting in the death of a fire inspector.
For firefighters, fresh outbreaks appearing beyond their crews and equipment can cause escape routes to close without warning. Direct attack may become impossible, forcing crews to retreat and focus on defending communities. Giannaros cautioned that the danger lies in the fire advancing faster than evacuations can be organized.
Until now, pyroCbs have been primarily considered an Australian and North American occurrence. Canada alone recorded a record 142 instances in 2023.
PyroCbs Have Been Rare in Europe
Portugal documented one during its severe 2017 fire season, and France has now joined this list. The continent is heating up faster than the global average—at roughly twice the rate since the 1980s, according to the EU’s Copernicus Climate Change Service and the World Meteorological Organization—with heatwaves becoming more frequent and severe and drought spreading across the south, turning forests into fuel.
Hotter and drier conditions increase the opportunity for extreme fires to form them, Giannaros said, though scientists still lack sufficient long-term data to establish a definitive trend. The European research project ROSETTA aims to address this knowledge gap.
He noted that the most common misconception is viewing the cloud as merely a byproduct. As long as it remains above the flames, it actively influences the fire's subsequent path.