Pyrocumulonimbus cloud forms over Bordeaux wildfires as 42,000 hectares burn and 200,000 flee

Scale of the blaze

The fire started in Gironde on Wednesday 22 July and has since consumed more than 42,000 hectares (420 km²). Within four days it swept to within just 7 miles (11 km) of Bordeaux. At one stage the flames threatened sensitive military and industrial sites, including facilities linked to ballistic missiles, rocket motors, and hazardous chemicals.

Evacuations and response

More than 200,000 people in southwest France were ordered to leave, with totals across France and Spain reaching 360,000 according to one account. President Emmanuel Macron met his cabinet on Monday but said Bordeaux itself would not be evacuated. About 2,500 firefighters, 1,500 military personnel, and 1,200 police and gendarmes were deployed. Aircraft such as the Canadair scooped 6,000 litres of water in 12 seconds and dropped it directly on the flames.

The fire cloud

On 27 July, the blaze generated a pyrocumulonimbus, a towering thunderstorm cloud fed by the fire’s own heat and moisture. Intense heat forced air, ash, and water vapour upward; as the plume rose and cooled, water vapour condensed into a cloud that grew into a cumulonimbus, the same type linked to ordinary thunderstorms.

It's a full thunderstorm forming within the fire plume.

These clouds can reach 10–15 km altitude, penetrate the stratosphere, and produce lightning strikes and violent downdrafts that scatter embers far from the main front.

Unpredictable fire behaviour

A spokesperson for France’s National Firefighters Federation (FNSPF) described the difficulty of battling a fire that no longer follows a predictable pattern.

[It is] unlike a standard fire that spreads in a conical pattern. The result is that it spreads in every direction and develops multiple fire fronts. We don't know how the blaze will spread; the fire front is constantly shifting. We cannot fight it directly. It's a David-versus-Goliath scenario.

PyroCb clouds generate erratic wind gusts and dry lightning, igniting new spot fires across parched ground and making direct attack impossible.

First for France and climate context

If confirmed, this would be the first pyrocumulonimbus officially recorded in France. Rick McRae, an adjunct professor from the University of New South Wales, noted that Australia has faced such clouds since the early 2000s but that the phenomenon is now appearing in regions where it was previously unknown.

It's the first time France has had to explicitly consider the concept of a pyroCb.

The event follows a summer of record-breaking heat across Europe, with temperatures above 40°C in many places. Scientists link the more frequent formation of fire clouds to the climate crisis, which creates hotter, drier conditions favourable to intense wildfires.