California Fires: How Santa Ana "Devil Winds" Form

Santa Ana "Devil Winds" quickly whipped up wildfires in Southern California this weekend. (See Map of California Wildfires.) These images, from NASA satellites Oct. 21 show how quickly the fires spread around Los Angeles in the space of just three hours and fifteen minutes.

NASA image courtesy the MODIS Rapid Response Team, Goddard Space Flight Center

This is how NASA described the situation on the ground:

Santa Ana winds are a California firefighter’s nightmare. These blustery, dry, and often hot winds blow out of the desert and race through canyons and passes in the mountains on their way toward the coast.

The air is hot not because it is bringing heat from the desert, but because it is flowing downslope from higher elevations. As fall progresses, cold air begins to sink into the Great Basin deserts to the east of California. As the air piles up at the surface, high pressure builds, and the air begins to flow downslope toward the coast. When winds blow downslope, the air gets compressed, which causes it to warm and dry out.

In fact, the air can warm at a rate of 29 degrees Fahrenheit per mile. Canyons and passes funnel the winds, which increases their speed.

Not only do the winds spread the fire, but they also dry out vegetation, making it even more flammable.

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