Santa Ana Winds, Fire Tornados, and the California Fires

After months of drought, summer is starting with fires in southern California. The fires are being amplified by adiabatic winds, with smoke cyclone and fire tornadoes spinning out of the flames. Here's the atmospheric science to go along with the breaking-news coverage of evacuations and burned acreage.

Wildfire approaching homes in San Marcos, California. Image credit: AP Photo

The fires in San Diego are dominating the news cycle — people are being evacuated, the I-5 highway was briefly shut down, and and the non-emergency information phone number was jammed with calls. (Trying to call? All information is available online at the county emergency response site.)

Image credit: NASA/MODIS

The MODIS satellite managed to get some rapid-response images of the scene at 3pm this afternoon. Smoke is clearly visible in the true-colour image, but the fire damage is easier to spot as magenta splotches in a sea of bright green in the false-colour image. At the time of writing, over a thousand acres were burning, with less than 25% contained.

Smoke Cyclones and Fire Tornados

Even small temperature differences are enough to spawn a funnel. If a column of hot air rises rapidly inside a pool of cool air, through the conservation of angular momentum any rotation will amplify as the tower of air grows. More hot air gets pulled horizontally along the ground before rising within the column, feeding the funnel. Air cools as it rises, eventually tumbling down the outside of the tower. This cooler air reinforces the structure by stabilizing it and keeping it contained.

If smoke, dust, or fire is anywhere nearby, it will be drawn in, highlighting its structure and making the funnel visible.

Update 15 May: The Vane has a more detailed look at how fire tornadoes form.

If fire is sucked up into the funnel, news media loves to call it a firenado, but the actual name for it is a fire whirl.

Santa Ana Winds

Southern California is subject to adiabatic winds, more commonly known as the Santa Ana winds. They're driven by the pressure differential between the hot, dry interior, and the cooler, moist coast.

Image credit: NOAA

The pressure difference draws air up and over the mountains. As winds flow down-slope, they compress, heating up as they gain speed. The result is very strong, hot, dry winds blasting the coast. During fire season, these winds make a bad situation worse by drying out vegetation into fuel. The strong gusts quite literally fanning the flames, building flare-ups into raging fires.

Right now, the Santa Ana winds are combining with a wriggle in the jet streamproducing already record-breaking high temperatures, making containing the fires that much more difficult.

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Check the CalFire official map and incident updates for the latest information on the California fires. Use their guide on how to reduce your fire hazard, and learn how the hazard can be reduced through controlled burns.

Fire in Carlsbad, California. Photography credit: Associated Press

Learn more about other disasters common in California: earthquakes, landslides, and El Niño.