- The Saddleridge Fire has spread through 7,542 acres of land in the Sylmar area of Los Angeles, burning at least 25 buildings and causing evacuations for 100,000 people.
- Fierce Santa Ana winds that blow into the Los Angeles area can make California wildfires tough to fight.
- The winds' strength has to do with where they develop in the mountains and how they travel down to the coast. They are typically fiercest in the fall, before the first rains of the season arrive.
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A bush fire is ripping through the Sylmar area of Los Angeles, prompting mandatory evacuation orders for 100,000 people.
The blaze, dubbed the Saddleridge Fire, had spread across 7,542 acres of the San Fernando Valley and destroyed at least 25 buildings and homes as of Friday afternoon. It's growing at a rate of 800 acres per hour.
Part of the reason the Saddleridge flames have spread so quickly and proved so hard to contain is the Santa Ana winds. These winds blow into Southern California during the fall and winter months. Similar winds are felt in the northern part of the state — those are called Diablo winds, though some residents refer to them as the Santa Anas as well.
"These weather conditions are significant," Los Angeles Fire Department Chief Ralph Terrazas told the LA Times on Friday. "You can imagine the embers from the wind have been traveling at significant distances, which cause other fires to start."
Where the Santa Ana 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 will eventually become Santa Anas and Diablos start out as colder breezes.
In the fall, as the desert lands cool down, the high-altitude basin develops chilly, high-pressure winds. All that air in the basin 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 of the Alps, the Chinook of 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 near 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, on average: The length of the state's 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 the Saddleridge Fire, low relative humidity and warm temperatures increased the risk that dry brush could ignite. Then the powerful Santa Ana wins, with gusts up to 50 mph, gave the blaze a boost when it broke out Thursday night, according to the LA Times.
The Santa Ana winds in pop culture
Angelinos sometimes like to wax poetic about how the dusty, hot 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."