Fires are ripping through California with the wind at their backs.
In Los Angeles, the Getty Fire has spread across more than 500 acres near the Getty Center, and prompting mandatory evacuation orders for thousands of people. Nearby, 4,600 acres of the Santa Clarita Valley have burned in the Tick Fire. That blaze has destroyed 22 structures and damaged another 27.
Meanwhile, the Kincade Fire continues to grow in Sonoma County. It’s swallowed 66,200 acres so far and forced nearly 200,000 people to evacuate their homes.
At least 11 other fires affected the state on Monday as well, according to the California Department of Forestry and Fire Protection (Cal Fire).
A big reason these blazes have spread so rapidly and proven so difficult to control is the Santa Ana and Diablo winds, which blow during the fall and winter months. The Santa Anas hit southern California, while the Diablo winds are felt in the northern part of the state (though some residents refer to them as the Santa Anas as well).
In recent days, wind gusts as powerful as 102 mph helped the Kincade Fire spread. Near LA,gusts reached 66 mph and, in some places, 74 mph.
“These weather conditions are significant,” Los Angeles Fire Department Chief Ralph Terrazas told the LA Times earlier this month, as another fire was spreading across the San Fernando Valley in similar weather conditions. “You can imagine the embers from the wind have been traveling at significant distances, which cause other fires to start.”
Where the Santa Ana and Diablo winds come from
High above the California coast, in the Great Basin that extends across Nevada, Utah, and other Rocky Mountain states, the winds that eventually become Santa Anas and Diablos start out as cold breezes.
In the fall, as the desert lands cool down, the high-altitude basin develops chilly, high-pressure winds. All that air looks for a breezy, convenient escape route.
As the winds funnel through narrow mountain passes, they gather speed and heat, becoming stronger and warmer as they descend towards the coast. The phenomenon is similar to what happens when air is compressed inside a bike tire: It quickly heats up.
A similar process happens with the Fohen winds in the Alps, the Chinook in the Pacific Northwest, and the Zonda winds in Argentina. These are all called katabatic winds because they move high-density air downslope.
According to UCLA meteorology professor Robert Fovell, the Santa Anas warm at rate of nearly 30 degrees Fahrenheit per mile.
“That means if you take a piece of air located a only mile above your head, and brought it down to your feet, it would wind up 30 degrees Fahrenheit warmer than when it started,” he wrote.
Last year, the Santa Ana winds played a key role in the Woolsey Fire, which burned nearly 100,000 acres in Los Angeles and Ventura counties and killed three people.
Rain and the Santa Ana winds
The Santa Ana winds are most strong and able to stoke wildfires in the fall. California has distinct wet and dry seasons, so if the winds arrive before the first rain, that can lead to a high risk of fire.
California’s dry periods have been getting longer and more severe in recent years: The length of the state’s average fire season has increased an estimated 75 days in the past decade, according to Cal Fire. That makes the Santa Anas even more dangerous.
“Realistically, a lot of the large Santa Ana fire years on record end up being years where we have a delay in the onset of fall rain,” atmospheric scientist John Abatzoglou told Business Insider last year.
In the case of these recent fires, low relative humidity and warm temperatures increased the risk that dry brush could ignite. Then the powerful Santa Ana and Diablo winds gave the blazes a boost and helped them spread faster than firefighters could put them out.
“It’s a dangerous season right now,” Terrazas said in a press conference on Monday. “We have not had any significant rainfall for a period of time. So that’s why we’re very, very concerned about these weather conditions.”
The Santa Ana winds in pop culture
Angelinos sometimes like to wax poetic about how the Santa Ana winds can change people. Legend has it the warm gusts make people moody, violent, and prone to migraines or fights.
“I have neither heard nor read that a Santa Ana is due, but I know it, and almost everyone I have seen today knows it too,” Joan Didion wrote in her essay “Los Angeles Notebook.” “We know it because we feel it. The baby frets. The maid sulks. I rekindle a waning argument with the telephone company, then cut my losses and lie down, given over to whatever it is in the air. To live with the Santa Ana is to accept, consciously or unconsciously, a deeply mechanistic view of human behavior.”
The television show “Crazy Ex-Girlfriend” also featured the Santa Ana winds.
“I’m a hot, hot breeze that originates from high-pressure air masses” the male chorus version of Santa Ana winds sings in an episode in the show’s second season. “Technically, I’m known as a katabatic wind, that’s science for: a pain in your asses.”