PARIS—The wildfire sweeping through the pine forests of southwest France became so ferocious last week that it created its own weather system, generating thunderstorm clouds that hurled lightning to the ground and ignited fresh fires beyond the fire line, French authorities said.
The phenomenon, called a pyrocumulonimbus, is rare in Europe and is chiefly documented during the most extreme blazes in North America and Australia.
The fire broke out on July 22 near Saumos, a village west of Bordeaux, and has since burned some 104,000 acres (42,000 hectares) of forest and scrubland, damaged or destroyed more than 240 homes, and forced 220,000 people to evacuate in the Gironde department, some 55,000 of them overnight into Sunday alone, as flames advanced toward the city.
The departmental fire service reported the first pyrocumulonimbus on the evening of July 24, two days after ignition. The cloud weakened overnight as humidity rose, then re-formed several times. On Saturday night, the prefecture said, the fire “redoubled in violence” beneath a pyrocumulonimbus crackling with lightning.
How a Fire Builds Its Own Thunderstorm
“When a fire zone stretches over several hundred meters, even a kilometer, it sends hot air up into the atmosphere. That hot air forms a convection column,” Patrick Malière, a meteorologist and the director general of the French weather consultancy Météo Climat Pro, told The Epoch Times. “It is the principle of a pan brought to a boil: the hot air above rises.”
That rising air gives birth to a cloud identical to a thunderstorm cell, the cumulonimbus, with “pyro” marking its fiery origin. Laden with ash, it can spawn fire tornadoes and ash showers, Malière said, and it catapults burning embers “several hundred meters, even several kilometers,” seeding fresh outbreaks more than a mile from the flames.
The fire’s power alone is not enough, according to Jean-Christophe Vincendon, the national forest fire coordinator at Météo-France, the state weather service.
“For a pyrocumulonimbus to develop properly, the atmosphere must generate vertical movements easily,” he told The Epoch Times, noting that the air over Gironde was unstable at the moment the cloud appeared.
Once born, such a system takes on a life of its own, Vincendon warned.
“There is such power, such energy in there that flows set in inside and around these systems,” he said.
“Those flows generate winds that can prove particularly dangerous, because they are very capricious, very gusty, without a fixed direction,” catching fire crews off guard.
Firefighters experienced exactly that. At nightfall on Saturday, Interior Minister Laurent Nuñez said, the blaze turned “extremely virulent and unpredictable again, generating its own wind” as it lurched toward the Bordeaux metropolitan area.
A First for France?
“We are used to seeing it in action in Australia, in North America, or in Canada, regions regularly struck by major fires,” Malière said. “It is the first time we have observed it here with such intensity.”
The current blazes, he added, exceed everything France has known, including the 1949 Landes fire that killed 82 people, long the national benchmark.
Vincendon said that cloud formation above a fire, or pyroconvection, was documented in France as recently as last summer over the Ribaute blaze in the south, and appeared in far weaker form this month above the Fontainebleau forest fire near Paris.
What may be unprecedented is the sheer force at work. “We have no instrument that can measure the power of a pyrocumulonimbus,” he said, but forestry service data suggest the fire’s intensity has no known equivalent in the country.
‘The Rule of the Three 30s’
Neither expert attributed the cloud itself to high temperatures. What warming does, Malière argued, is multiply the weather situations in which the slightest spark turns into a blaze.
He pointed to the rule of the three 30s: temperatures above 30 degrees Celsius (86 degrees Fahrenheit), humidity below 30 percent, and wind gusts above 30 kilometers per hour (19 mph).
“When all of these parameters come together, the risk is at its maximum,” he said.
Virtually all of France, except a narrow strip along the English Channel, is now crossing the temperature threshold, with peaks near 104 degrees Fahrenheit, in the summer’s fourth pairing of heat wave and strong winds.
Malière said the arriving El Niño, in his assessment among the most powerful ever measured, will affect the Atlantic and Western Europe by year’s end and into 2027.
Vincendon described “a vicious circle.”
“The more the periods of intense heat and drought multiply, the more the vegetation dries out,” he said.
“The more flammable the available fuel, the higher the power of an eventual fire. And the higher that power, the greater the risk of encountering this type of phenomenon.”
Questions Over Preparedness
Malière also criticized decades of governments of all stripes. After the 2022 fires that scorched the same corner of Gironde, he asked, “What has really been done since to guard against another event of this kind?” adding that “everyone takes refuge behind difficult decisions, postponed or deferred investments.”
He faulted environmental groups for opposing brush clearing in the name of preserving forests, and recalled that nine out of 10 fires in France are started by humans, whether accidentally or deliberately.
Since the beginning of July, 142 people have been taken into custody in connection with the various wildfires that have erupted across France.
President Emmanuel Macron, visiting the crisis center in Bordeaux on Monday, called the fire unprecedented and pledged to rebuild a different kind of forest, warning that “the coming weeks will be tough.”
Authorities said 88 firefighters have been injured since the fires began.
By Monday, after light overnight rain, officials said the blaze had finally stopped growing, though it had not yet been contained.
Whether the fire-born storms return will depend less on the sky than on what burns beneath it, Vincendon suggested. “What is much less frequent is the development of a fire of such power.”
