Strong Santa Ana winds, at times almost hurricane-force, swept across the mountains outside Los Angeles beginning January 7, 2025, sparking wildfires in several neighborhoods. By January 8, well over 1,000 houses and several schools had burned down, including at least five people who had died. Officials urged more than 100,000 residents to evacuate at the height of the fires, but with winds so strong, firefighters could do little to control the flames.

Jon Keeley, California research ecologist with the US Geological Survey and associate professor at UCLA, explains what causes such extreme winds in Southern California and why they pose such a dangerous fire threat.

What causes the Santa Ana winds?

The Santa Ana winds are dry, strong winds that blow over the mountains toward the Southern California coast.

The region averages about 10 Santa Ana wind events per year, typically occurring from fall to January. In dry conditions such as those currently prevailing, these winds can become a serious fire hazard.

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The Santa Ana winds occur when there is a high pressure area to the east, in the Great Basin, and a low pressure area offshore. Air masses move from high pressure to low pressure, and the greater the pressure difference, the faster the winds blow.

Topography also plays a role.

As the winds from the top of the San Gabriel Mountains move downslope, they become drier and hotter. This is a function of the physics of air masses. When winds reach the point where the Eaton Fire broke out in Altadena on Jan. 7, it is not unusual to have less than 5 percent relative humidity, meaning there is virtually no moisture at all.

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

These strong, dry winds often have speeds of around 30 to 40 miles per hour. But they can be stronger. By early January 2025, wind speeds are expected to have reached 60 to 70 miles per hour.

Why was the fire risk so high this time?

Typically, Southern California now has enough rain that the vegetation is moist and doesn’t burn as easily. A study conducted several years ago showed that fall moisture reduces the risk of wind-driven fires in Santa Ana.

However, Southern California is experiencing very dry conditions this year and there has been very little moisture in recent months. With these extreme winds we have the perfect storm for severe fires.

It is very difficult to put out a fire under these conditions. Firefighters in the area will tell you that if there’s a wind-driven fire in Santa Ana, they’ll evacuate people ahead of the fire front and control the edges – but when the wind blows like that, there’s little chance of stopping it by then subsides.

Similar fires caused by strong hillside winds occurred in other states. In the Chimney Tops 2 fire in Tennessee in November 2016, strong hillside winds spread flames to homes in Gatlinburg, killing 14 people and burning more than 2,500 homes. Boulder County, Colorado, lost about 1,000 homes when strong winds coming down from the mountains there spread the Marshall Fire in December 2021.

Have Santa Ana winds changed over time?

Wind events in Santa Ana are nothing new, but we see them more often this time of year.

My colleagues and I recently published a paper comparing 71 years of wind events in Santa Ana, starting in 1948. We found about the same amount of wind activity in Santa Ana overall, but the timing shifts from fewer events in September more in December and January. Due to well-documented climate change trends, it is tempting to attribute this to global warming, but there is no substantial evidence to support this yet.

California is experiencing more devastating fires than in the past. Not only climate and wind changes are responsible for this, but also population growth.

More people now live in and on the edges of wild areas, and with them the power grid has expanded. This creates more opportunities for fires to occur. During extreme weather conditions, power lines are at greater risk of falling or being struck by tree branches and starting a fire. The area burned by fires related to power lines has expanded greatly; Today it is the primary source of ignition for devastating fires in Southern California.

The Eaton Fire, which burned many homes, is located at the top of the San Gabriel Basin, at the foot of the San Gabriel Mountains. Fifty years ago, fewer people lived there. At that time, some parts of the basin were surrounded by citrus orchards, and fires in the mountains burned out the orchards before reaching homes.

Today there is no buffer between homes and the wilderness. The ignition point of the Eaton Fire appears to have been near or within one of these neighborhoods.

Homes are made of dried materials, and when the atmosphere is dry, they burn easily, causing fires to spread quickly through neighborhoods and creating a great risk of destructive fires.

This article was republished from The Conversation. Read the original article.