By Jon Keeley
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. 9, and at least five 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.
USGS
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.
This year, however, Southern California has very dry conditions, with very little moisture over the past several months. With these extreme winds, we have the perfect storm for severe fires.
AP Photo/Richard Vogel
It’s very hard to extinguish a fire under these conditions. The firefighters in the area will tell you, if there’s a Santa Ana wind-driven fire, they will evacuate people ahead of the fire front and control the edges – but when the wind is blowing like this, there’s very little chance of stopping it until the wind subsides.
Other states have seen similar fires driven by strong downslope winds. During the Chimney Tops 2 Fire in Tennessee in November 2016, strong downslope winds spread the flames into homes in Gatlinburg, killing 14 people and burning more than 2,500 homes. Boulder County, Colorado, lost about 1,000 homes when powerful winds coming down the mountains there spread the Marshall Fire in December 2021.
Have the Santa Ana winds changed over time?
Santa Ana wind events aren’t new, but we’re seeing them more often this time of year.
My colleagues and I recently published a paper comparing 71 years of Santa Ana wind events, starting in 1948. We found about the same amount of overall Santa Ana wind activity, but the timing is shifting from fewer events in September and more in December and January. Due to well-documented trends in climate change, it is tempting to ascribe this to global warming, but as yet there is no substantial evidence of this.
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.
More people now live in and at the edges of wildland areas, and the power grid has expanded with them. That creates more opportunities for fires to start. In extreme weather, power lines face a higher risk of falling or being hit by tree branches and sparking a fire. The area burnt because of fires related to power lines has greatly expanded; today it is the major ignition source for destructive fires in Southern California.
AP Photo/Damian Dovarganes
The Eaton Fire, which has burned many homes, is at the upper perimeter of the San Gabriel Basin, at the base of the San Gabriel Mountains. Fifty years ago, fewer people lived there. Back then, some parts of the basin were surrounded by citrus orchards, and fires in the mountains would burn out in the orchards before reaching homes.
Today, there is no buffer between homes and the wildland. The point of ignition for the Eaton Fire appears to have been near or within one of those neighborhoods.
Homes are made of dried materials, and when the atmosphere is dry, they combust readily, allowing fires to spread quickly through neighborhoods and creating a great risk of destructive fires.
Jon Keeley is Research Ecologist at USGS and Adjunct Professor at the University of California, Los Angeles.
Ed P says
It’s refreshing to see an article that doesn’t lay the wild fires at the door step of climate change. That excuse is sometimes used as a get out of jail card, or to do nothing.
I’m perplexed at how this much devastation is able to even happen with today’s technology and building codes. If earthquake resistant homes or hurricane resistant home are mandated, why not fire resistant? Local municipalities require upgrades during renovations for older properties in Florida, do they in California? Are the fire codes strong enough? We have Miami-Dade codes and even tougher in some areas of Flagler county.
Additionally, someone should have known that the water tanks could potentially run dry, that the reservoirs should have been filled especially during fire season, and better forest management was required. What about individual landscaping for yards? Will anyone admit it? The cover ones ass and lying, as well as finger pointing by the governing bodies is disturbing. The explanation that the winds prevented fixed wing or helicopter support is in fact true and would have reduced the stress on the fire hydrants but the wind conditions there are a fact of life.
It’s easy to criticize after the fact. However we are all going to pay the price when our homeowners insurance premiums rise because of this catastrophe. Additionally, FEMA’s role will also require a bigger budget.
Laurel says
Unbelievably sad! I can’t even imagine 😯. My heart goes out to the people there.
To make matters worse, certain politicians are blaming others for this disaster, for personal, political gains.
What this article brought up was excessive growth. Excessive growth is a strain on the environment as well as the community.
What it didn’t bring up is the resource of water. The lack of water is not so much the issue here, it’s the lack of water pressure. No municipality has the ability to provide water pressure under such circumstances. They are set up to address a couple of houses at a time, not every home owner turning on the spigot at the same time. The pressure goes down to nill.
Another factor, as stated here, are the winds. The winds gusted up to 100 mph, preventing helicopters from dropping water and retardants.
So, if you see a lot of finger pointing, consider the source.
Skibum says
I was born and raised in L. A., living in the San Fernando Valley for the first 30 years of my life. The Santa Ana winds have always been both a blessing and a curse to Angelenos. Back in the 60s and 70s when L. A.’s infamous smog was much worse than now, the ocean breeze blowing inland from the Pacific Ocean would often send the worst of the smog into both the San Gabriel and San Fernando valleys, only to be trapped up against the surrounding hills, and smog alerts were common with advisories for those with respiratory problems to curtail outdoor activities. Both my twin brother and I had asthma, and many times we could not participate in physical education at school due to wheezing and difficulty breathing in the smoggy air. But during those times of the year when the Santa Ana winds would blow from the east, through the canyons and hills into the valley areas, all of the smog would be forced westward and out over the ocean, and we would have beautiful blue skies and clean air to breathe. But, of course, those winds were always hot winds, and usually in the fall when the dry conditions would spark wildfires. But the fire conditions back then were NOTHING like seen now, with more and more residential development encroaching into the hills and canyons where fire danger has always been significantly higher, and that has allowed really horrendous fires to spread from those areas into the city proper. L. A. and California’s increased danger and property insurance liability due to allowing development in to wildland areas is not unlike Florida’s penchant for allowing homes to be build right on the ocean’s edge when we have a significant threat of destructive hurricanes 6 months out of the year. Something has to change because the west’s wildfires, the historical “tornado alley” in the center of the country, and the hurricanes on the east coast have been getting increasingly worse and less predictable, and the seasons and areas that are vulnerable to these annual natural disasters are expanding. Climate scientists have been warning all of us about these increasing threats for some time now, but there are still those who would rather just close their eyes, bury their heads in the sand to reality, but then only after disaster strikes close to home, start complaining and pointing their fingers every which way trying the blame game but still not fully ready to name the real culprit and address the difficult AND expensive solutions that must be made and funded if we are to get through this ever expanding disaster to come.