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- Firefighters in southwestern France reported a rare, unprecedented fire-weather phenomenon above the large Gironde wildfire.
- These clouds form when intense wildfire heat drives superheated air, smoke, ash and moisture upward, eventually condensing into pyrocumulus and sometimes pyrocumulonimbus.
- Pyrocumulonimbus clouds can reach 10 to 15 kilometres high, sometimes enter the stratosphere, and generate lightning, hail and powerful winds.
- The clouds can spread new fires by lightning or burning material, while climate change increases the conditions that make such events more likely.
The images appear more like a volcanic eruption than a typical forest fire. A huge dark cloud rises above the flames, reaching high into the sky. However, it is not simply a dense column of smoke.
The phenomenon is known as “fire clouds”, or pyrocumulonimbus (pyroCb), and it is a thunderstorm-like cloud created by the fire itself. Its appearance can dramatically change the behaviour of a wildfire, making it more dangerous and unpredictable.
Scientists have renewed their focus on the phenomenon as major fires in countries including France and Spain develop amid extreme heat, prolonged drought and strong winds.
In southwestern France, firefighters described a rare and unprecedented fire-weather phenomenon above the large Gironde wildfire.
How are fire clouds formed?
A large wildfire releases enormous amounts of heat. Superheated air, together with smoke, ash and moisture released by burning vegetation, rises rapidly, creating a powerful upward current.
As this column rises, temperatures and atmospheric pressure decrease, causing water vapour to condense and initially form a pyrocumulus cloud.
According to scientific explanations, this type of cloud forms when the heat from a wildfire causes intense upward air movement.
If the fire is particularly large, the atmosphere is unstable and there is enough moisture in higher layers of the atmosphere, the pyrocumulus can develop into a pyrocumulonimbus – a storm system born from the smoke column of a wildfire.
NASA has compared these clouds to “fire-breathing dragons” because they can generate lightning, hail and extremely powerful winds.
How high can they reach?
Fire clouds can develop at heights of 10 to 15 kilometres and, in extreme cases, can break through the tropopause and send smoke directly into the stratosphere.
They act like giant chimneys, carrying soot, particles and gases much higher than a normal wildfire would. As a result, smoke can remain in the atmosphere for months and travel thousands of kilometres.
Why are they so dangerous?
The greatest danger comes from the winds created by the cloud itself.
The intense rise of hot air forces surrounding air masses towards the fire. At the same time, descending currents from the cloud can reach the ground and spread in different directions.
This can cause sudden and unpredictable changes in wind speed and direction. As a result, the fire front can accelerate, change course or create new fire outbreaks, surprising even experienced firefighting teams.
The cloud can also carry burning material over long distances, causing new flare-ups. In extreme cases, fire tornadoes can even form.
The appearance of a pyroCb is a clear sign that a wildfire has entered a phase of extreme and highly dangerous behaviour.
Can they create new fires?
Yes. Like ordinary cumulonimbus clouds, pyrocumulonimbus clouds can develop strong electrical charges and produce lightning.
Lightning strikes can occur several kilometres away from the original fire, starting new fires in areas that have not yet been affected by flames.
This creates a dangerous cycle: the fire creates a storm, and the storm can create more fires.
Do they bring rain?
Although a pyroCb can produce rainfall, it is usually not enough to contain the wildfire.
Smoke particles encourage the formation of many small droplets that struggle to combine into heavy rainfall. In addition, when the air near the ground is extremely dry, much of the rain evaporates before reaching the flames.
This evaporation can rapidly cool the air, strengthening downward currents and creating powerful wind gusts that accelerate the spread of the fire.
Are fire clouds a new phenomenon?
Pyrocumulonimbus clouds are not a new phenomenon. They have been recorded for decades in regions affected by major wildfires, including Australia, Canada, the United States and Russia.
Some of the most dramatic examples occurred during Australia’s devastating 2019-2020 bushfires, when fire storms sent smoke into the stratosphere.
Similar events have also been observed during major wildfires in California, while scientific records have identified hundreds of cases worldwide between 2013 and 2023.
The link with climate change
Climate change does not directly cause pyrocumulonimbus clouds. However, it increases the likelihood of the conditions that support their formation: prolonged heatwaves, severe drought, dry vegetation and larger, hotter wildfires.
The World Meteorological Organization has warned that higher temperatures and drier climate conditions increase wildfire risks and extend fire seasons in many parts of the world.
Scientific studies suggest that as extreme wildfire events become more frequent, episodes involving fire clouds may also increase in the coming decades, making wildfire management even more complex and challenging.
Also read: Spain and France wildfires force over 300,000 evacuations
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