Explainer
What is a
‘fire cloud’, and how have they worsened wildfires in Europe?
Severe
wildfires can generate their own unique weather systems. One of the most
striking examples of this are dangerous pyrocumulonimbus, or pyroCB, clouds
Kate Lamb
Mon 27
Jul 2026 05.43 BST
As France
and Spain battle some of the worst wildfires in recent history, experts have
warned of an unprecedented phenomenon that could exacerbate the effects of the
blazes: pyrocumulonimbus, or “fire clouds”.
Wildfires
often spread when hot, windy and dry conditions coalesce but severe wildfires
can generate their own unique weather systems. One of the most striking
examples of this are pyrocumulonimbus clouds, or fire-generated thunderstorms,
which are the most severe form of fire clouds.
Here is
what you need to know about them.
What is a
‘fire cloud’?
In a
nutshell, pyrocumulonimbus, or “pyroCb” clouds, are gigantic storm clouds that
are generated by a wildfire.
Massive
wildfires emit large amounts of energy and heat that rapidly rise in a smoke
plume. When the fire plume rises high enough, low atmospheric pressure causes
its air to cool, and the moisture condenses and forms a cloud. Such clouds can
generate lightning strikes and windy conditions.
If a
wildfire is sufficiently large and intense, the smoke plume it generates can
form a pyrocumulonimbus cloud, which can reach altitudes of 10–15km (six to
nine miles) and penetrate the stratosphere. Nasa calls them the “fire-breathing
dragon of clouds”.
“It’s a
full thunderstorm forming within the fire plume,” says Rick McRae, an adjunct
professor from the University of New South Wales’ (UNSW) bushfire research
group. “These things, they get up to roughly 10,000 cubic kilometres of the
atmosphere, so they have a big impact.”
Why are
these pyrocumulonimbus cloud formations so dangerous?
When
pyrocumulonimbus clouds form, they can create winds that make the wildfires
below spread even faster, and sometimes generate their own lightning, which can
in turn ignite more fires further afield.
Experts
say stronger wind gusts can create more unpredictable fire patterns and
therefore make conditions more dangerous for firefighters battling the blazes
on the ground.
“The
cloud is confirmation that the fire is very dangerous,” says UNSW’s McRae, who
worked as a senior fire manager for 30 years. “The cloud is a confirmation that
it’s appropriate that lots of people have been evacuated.”
How
common are fire clouds?
Australia
started experiencing pyrocumulonimbus clouds at the beginning of the century
and experts say it appears they are becoming more common.
They also
formed during massive wildfires in California in 2022, but this could be a new
situation for France.
If
confirmed, this would be the first time a pyroCb has been observed in France.
“It’s the first time France has had to
explicitly consider the concept of a pyroCb,” says McRae. “Whether or not they
had one a few days back or not isn’t really the question,” he says, “the
question is whether there’s one coming on Tuesday, Wednesday this week.”
Why are
these clouds occurring more frequently?
The
climate crisis is creating more dangerous conditions for wildfires globally,
and also the conditions that lead to the formation of pyrocumulonimbus clouds.
“It’s not
just as simple as turning the temperature dial up. It’s full-on climate
change,” says McRae. “It’s changes in the atmosphere, changes in the surface
weather, changes in a lot of things.”
Experts
say climate models indicate these trends will increase in severity in the
future.
The
wildfires in Europe have happened during peak tourist season in a region that
has been hit by successive heatwaves, with woodland areas dried out from a lack
of rain since May.
Weather
forecasters warn another heatwave could hit France from Tuesday, with
temperatures reaching 40C in some areas.
“There is
going to be a blob of high heat that will move from Czechia over a four- or
five-day period,” says McRae.
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