French Fire Crews Now Battle Thunderclouds

A wildfire in southwest France grew so powerful that its smoke column became a thunderstorm, generating lightning that struck the ground and ignited new fires beyond the original blaze, French officials said. Put another way, the inferno was no longer simply being propelled by the weather, it was making its own. The thundercloud also produced violent, erratic winds capable of driving flames in new directions, the AP reports. The phenomenon, called a pyrocumulonimbus, or pyroCb, is rare in Europe and more commonly documented in North America and Australia, where it occurs only over some of the most extreme fires. The national firefighters federation said it had never before recorded one in France.

• The origins: The blaze erupted near Saumos on July 22 and has burned more than 162 square miles of forests and scrubland, damaged or destroyed more than 240 homes, and forced 220,000 people to evacuate in Gironde. The departmental fire and rescue service soon reported that it had generated a pyrocumulonimbus. The cloud weakened overnight as humidity rose, then formed again several times. Repeatedly, heat and smoke have surged upward into a vast black thundercloud, electrified and lit from within. "Imagine a campfire so large and hot that the smoke rising off it turns into a storm-like cloud," said Theodore M. Giannaros, a fire meteorologist at the National Observatory of Athens. "That is a pyroCb—a storm cloud born from the intense heat that a wildfire produces."

• Conducive conditions: There is no set temperature at which one forms, Giannaros said; what is needed is the kind of atmosphere that breeds dry thunderstorms—very hot, very dry air near the ground, with cooler and moister air above it. Hot air loaded with smoke and moisture then rises rapidly from the flames. As it climbs and cools, water vapor condenses around particles of ash. Above the freezing level, droplets become ice crystals whose collisions separate electrical charges, as in an ordinary thunderstorm.
• The results: Beneath this storm lies a landscape already primed to burn. The cloud then amplifies the disaster that created it. Rising air pulls powerful winds toward the fire, while downdrafts punch gusts back to the surface. Those shifts can turn the flames, divide them into several fronts, or send them racing in a new direction. "It's a feedback loop, not a one-way effect," Giannaros said. "Once it forms, the cloud becomes its own weather system, sitting on top of the fire and making it harder to predict." The systems can also spin up whirling columns of flame—usually a fire whirl, which Giannaros describes as a dust devil made of fire and heat instead of dust.