What are fire clouds or pyrocumulonimbus? How they formed over France's wildfires — Explained
National Firefighters Federation of France (FNSPF) said the giant wildfire in the country's southwest had generated a pyrocumulonimbus — a phenomenon they described as never before observed in France.
AS firefighters battle one of France's worst wildfire seasons in recent years, an unusual atmospheric phenomenon has drawn the attention of scientists: a pyrocumulonimbus (pyroCb), also known as a fire cloud or cumulonimbus flammagenitus.
National Firefighters Federation of France (FNSPF) said the giant wildfire in the country's southwest had generated a pyrocumulonimbus — a phenomenon they described as never before observed in France. The appearance of these towering clouds underscores not only the intensity of the fires but also the dangerous feedback loop they can create, worsening wildfire conditions.
What is a pyrocumulonimbus or fire cloud?
A pyrocumulonimbus is essentially a thunderstorm created by a wildfire rather than by the sun heating the Earth's surface.
According to NASA, these clouds form when an intense fire releases enormous amounts of heat, smoke and moisture into the atmosphere. The superheated air rises rapidly like a giant chimney, carrying ash and water vapour high into the sky.
As the rising air cools, the moisture condenses into clouds. If the updraft is powerful enough, the cloud grows into a towering thunderstorm known as a pyrocumulonimbus.
Scientists also refer to it as cumulonimbus flammagenitus, reflecting its fire-generated origin.
How do fire clouds form?
The process unfolds in several stages:
-A massive wildfire heats the surrounding air to extreme temperatures.
-Hot air mixed with smoke and moisture rises rapidly.
-As it climbs higher, the air cools and water vapour condenses, forming a pyrocumulus cloud.
-If the plume continues rising into colder layers of the atmosphere, ice crystals develop inside the cloud.
-These ice particles generate electrical charges, transforming the cloud into a fully developed pyrocumulonimbus thunderstorm.
Unlike ordinary thunderstorms powered by solar heating, the energy driving these clouds comes almost entirely from the wildfire itself.
Why are pyrocumulonimbus clouds dangerous?
Fire clouds are not merely a visual spectacle—they can significantly worsen wildfire behaviour.
1. They create their own weather
Pyrocumulonimbus clouds generate powerful and unpredictable winds that can rapidly change a wildfire's direction and intensity.
These winds may spread flames into previously unaffected areas, making firefighting operations much more difficult.
2. They produce lightning
Like conventional thunderstorms, pyroCb clouds can produce lightning.
Lightning strikes may ignite entirely new wildfires several kilometres away from the original blaze, multiplying the emergency.
3. They can generate severe weather
Scientists say these clouds may also produce:
-Strong wind gusts
-Heavy turbulence
-Hail
-Intense downdrafts
-In rare cases, tornado-like vortices
These conditions create additional hazards for emergency responders and nearby communities.
Why are scientists paying close attention?
NASA describes pyrocumulonimbus clouds as giant atmospheric chimneys because they can inject enormous amounts of smoke into the lower stratosphere.
Unlike smoke trapped in the lower atmosphere—which is typically washed out within days—stratospheric smoke can remain suspended for months or even years.
Scientists say the largest pyroCb events can loft smoke into the atmosphere in quantities comparable to those released by a moderate volcanic eruption.
Because the smoke reaches such high altitudes, it can spread across continents and even around the globe, potentially influencing climate and air quality far from the wildfire itself.
Have they been observed before?
Although rare, pyrocumulonimbus clouds have been documented during some of the world's most destructive wildfire seasons.
One notable example occurred during NASA and NOAA's FIREX-AQ mission in August 2019, when NASA's DC-8 flying laboratory flew directly through a pyrocumulonimbus forming over a wildfire in eastern Washington state.
Scientists used the mission to study the chemistry of wildfire smoke and better understand its effects on air quality, weather and climate.
NASA notes that while fire-generated clouds themselves are relatively common during intense wildfires, only a small number of fully developed pyrocumulonimbus events have been extensively documented because they require exceptionally intense fires and favourable atmospheric conditions.
Why did one form over France?
The wildfire in southwest France has burned tens of thousands of hectares amid prolonged heatwaves, dry vegetation and strong winds.
These extreme conditions supplied the enormous heat needed to drive smoke and moisture high into the atmosphere, allowing a pyrocumulonimbus to develop.
Meteorologists have also warned that temperatures across Spain and parts of France could approach 40°C, with low humidity and dry winds creating ideal conditions for further fire growth and potentially more fire-cloud formation.
Does climate change play a role?
Scientists caution that climate change does not directly create pyrocumulonimbus clouds, but it increases the likelihood of the conditions that produce them.
More frequent and intense heatwaves, prolonged droughts and larger, hotter wildfires mean the atmosphere is increasingly capable of generating these extreme fire-induced thunderstorms.
