How Santa Ana winds fueled the lethal fires in Southern California

Highly effective Santa Ana winds, close to hurricane power at occasions, swept down the mountains outdoors Los Angeles and pushed wildfires into a number of neighborhoods beginning Jan. 7, 2025. Over 1,000 stuctures, largely houses, had burned and at the very least 5 folks had died by Jan. 8. Officers urged greater than 100,000 residents to evacuate, however with the winds so sturdy, there was little firefighters might do to regulate the flames.

Jon Keeley, a analysis ecologist in California with the U.S. Geological Survey and adjunct professor at UCLA, explains what causes excessive winds like this in Southern California, and why they create such a severe hearth danger.

What causes the Santa Ana winds?

The Santa Ana winds are dry, highly effective winds that blow down the mountains towards the Southern California coast.

The area sees about 10 Santa Ana wind occasions a 12 months on common, usually occurring from fall into January. When situations are dry, as they’re proper now, these winds can turn out to be a extreme hearth hazard.

The Santa Ana winds happen when there may be excessive strain to the east, within the Nice Basin, and a low-pressure system off the coast. Air plenty transfer from excessive strain to low strain, and the extra excessive the distinction within the strain, the sooner the winds blow.

Topography additionally performs a task.

Because the winds rush downslope from the highest of the San Gabriel Mountains, they turn out to be drier and warmer. That’s a operate of the physics of air plenty. By the point the winds get to the purpose the place the Eaton Fireplace broke out in Altadena on Jan. 7, it’s not unusual for them to have lower than 5% relative humidity, which means primarily no moisture in any respect.

Canyons additionally channel the winds. I used to stay within the Altadena space, and we’d get days throughout Santa Ana wind occasions when the wind wasn’t current in any respect the place we lived, however, a number of blocks away, the wind was extraordinarily sturdy.

These sturdy, dry winds are sometimes round 30 to 40 mph. However they are often stronger. The winds in early January 2025 had been reported to have reached 60 to 70 mph.

Why was the hearth danger so excessive this time?

Sometimes, Southern California has sufficient rain by now that the vegetation is moist and doesn’t readily burn. A examine a number of years in the past confirmed that autumn moisture reduces the chance of Santa Ana wind-driven fires.

This 12 months, nonetheless, Southern California has very dry situations, with little or no moisture over the previous a number of months. With these excessive winds, now we have the right storm for extreme fires.

It’s very laborious to extinguish a fireplace beneath these situations. The firefighters within the space will let you know, if there’s a Santa Ana wind-driven hearth, they are going to evacuate folks forward of the hearth entrance and management the perimeters – however when the wind is blowing like this, there’s little or no likelihood of stopping it till the wind subsides.

Different states have seen related fires pushed by sturdy downslope winds. Throughout the Chimney Tops 2 Fireplace in Tennessee in November 2016, sturdy downslope winds unfold the flames into houses in Gatlinburg, killing 14 folks and burning greater than 2,500 houses. Boulder County, Colorado, misplaced about 1,000 houses when highly effective winds coming down the mountains there unfold the Marshall Fireplace in December 2021.

Have the Santa Ana winds modified over time?

Santa Ana wind occasions aren’t new, however we’re seeing them extra typically this time of 12 months.

My colleagues and I not too long ago revealed a paper evaluating 71 years of Santa Ana wind occasions, beginning in 1948. We discovered about the identical quantity of general Santa Ana wind exercise, however the timing is shifting from fewer occasions in September and extra in December and January. As a result of well-documented tendencies in local weather change, it’s tempting to ascribe this to international warming, however as but there isn’t any substantial proof of this.

California is seeing extra damaging fires than we noticed previously. That’s pushed not simply by modifications within the local weather and the winds, but additionally by inhabitants development.

Extra folks now stay in and on the edges of wildland areas, and the ability grid has expanded with them. That creates extra alternatives for fires to start out. In excessive climate, energy strains face the next danger of falling or being hit by tree branches and sparking a fireplace. The world burnt due to fires associated to energy strains has drastically expanded; in the present day it’s the main ignition supply for damaging fires in Southern California.

The Eaton Fireplace, which has burned many houses, is on the higher perimeter of the San Gabriel Basin, on the base of the San Gabriel Mountains. Fifty years in the past, fewer folks lived there. Again then, some elements of the basin had been surrounded by citrus orchards, and fires within the mountains would burn out within the orchards earlier than reaching houses.

Right this moment, there isn’t any buffer between houses and the wildland. The level of ignition for the Eaton Fireplace seems to have been close to or inside a type of neighborhoods.

Houses are product of dried supplies, and when the environment is dry, they combust readily, permitting fires to unfold rapidly by neighborhoods and creating a terrific danger of damaging fires.