WILDEFIRESHow Santa Ana Winds Fueled the Deadly Fires in Southern California

By Jon Keeley

Published 14 January 2025

California is seeing more destructive fires than we saw in the past. That’s driven not just by changes in the climate and the winds, but also by population growth.

Powerful Santa Ana winds, with gusts reaching hurricane strength, swept down the mountains outside Los Angeles and spread wildfires into several neighborhoods starting Jan. 7, 2025, creating a terrifying scene.

Thousands of homes and other structures, including several schools, had burned by Jan. 10, and at least 10 people had died. Officials urged more than 180,000 residents to evacuate at the height of the fires. With the winds so strong, there was little firefighters could do to control the flames.

Jon Keeley, a research ecologist in California with the U.S. Geological Survey and adjunct professor at UCLA, explains what causes extreme winds like this in Southern California, and why they create such a dangerous fire risk.

What causes the Santa Ana winds?
The Santa Ana winds are dry, powerful winds that blow down the mountains toward the Southern California coast. The region sees about 10 Santa Ana wind events a year on average, typically occurring from fall into January.

When conditions are dry, as they are right now, these winds can become a severe fire hazard.

The Santa Ana winds occur when there is high pressure to the east, in the Great Basin, and a low-pressure system off the coast. Air masses move from high pressure to low pressure, and the more extreme the difference in the pressure, the faster the winds blow.

Topography also plays a role.

As the winds rush downslope from the top of the San Gabriel Mountains, they become drier and hotter. That’s a function of the physics of air masses. By the time the winds get to the point where the Eaton Fire broke out in Altadena on Jan. 7, it’s not uncommon for them to have less than 5% relative humidity, meaning essentially no moisture at all.

Canyons also channel the winds. I used to live in the Altadena area, and we would get days during Santa Ana wind events when the wind wasn’t present at all where we lived, but, a few blocks away, the wind was extremely strong.

These strong, dry winds are often around 30 to 40 mph. But they can be stronger. The wind gusts in early January 2025 were reported to have exceeded 80 mph.

Why was the fire risk so high this time?
Typically, Southern California has enough rain by now that the vegetation is moist and doesn’t readily burn. A study a few years ago showed that autumn moisture reduces the risk of Santa Ana wind-driven fires.