The wildfires raging in the hills north of Los Angeles this week burnt so intensely that they generated their own weather system. Extraordinary time-lapse footage taken by a local cameraman, Brandon Riza, shows a seething thundercloud billowing up over the station fire in the Angeles National Forest.
The smoke from the blaze released moisture from the burning vegetation that condensed into a special type of cloud called pyrocumulus. In the right conditions, pyrocumulus can grow into a full-blown thundercloud, which is a mixed blessing for fighting fires. While the rain from a thundercloud can help to extinguish a small fire or cool off a larger blaze, it can also unleash lightning that sets off new fires some distance away. The biggest threat from a large pyrocumulus is the violent gusts of winds it generates, which can fan the flames in all directions.
In the worst-case scenario, these gusting winds stoke up a conflagration that can suck in so much air that it creates a firestorm with gale-force winds. This happened in 1871 when the Peshtigo forest north of Chicago burst into flames and enveloped the town of Peshtigo in a firestorm. Winds reached about 100mph, and an estimated 1,180 people were killed. It was the worst fire recorded in the history of the US.
Footage of the Los Angeles pyrocumulus can be seen at: brandonriza.com/Video/HTML/ZeroPercentContained.html