What Are California's Santa Ana Winds?
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So you may have been hearing about how the wildfires in Los Angeles are connected to the Santa Ana winds. These winds blow every year. But meteorologists with the National Weather Service in Los Angeles anticipated this week's windstorm would, quote, likely be the most destructive in over a decade.

The winds picked up Tuesday morning, sparking several fires across L.A. County. Especially in canyon slopes, where the wind dropped the humidity. Basically, the moisture and rain gets squeezed out of these systems right on those same slopes where...

The Santa Ana winds blow the hardest. We talked to Alexander Gershnov about this. He's a research meteorologist at the Scripps Institution of Oceanography, UC San Diego.

And he says that last winter was really wet and vegetation flourished, but this year the L.A. area has received very little rain for months. Specifically in that region where the wildfires are right now, you have a lot of vegetation dry and ready to burn. And that's exactly where you get the strongest Santa Ana wind. The largest and earliest of the fires was in the Pacific Palisades neighborhood,

Within hours, other fires began, including the Eaton, Hearst, Lydia and Sunset fires. The battle against the deadly Eaton fire is now an around-the-clock fight. The wind-driven blaze exploded to more than 10,000 acres by morning. As of this taping Thursday afternoon, this patchwork of fires has claimed the lives of five people and leveled the homes and businesses of multiple communities.

Nearly 180,000 people have been ordered to evacuate and nearly 30,000 acres have burned. So today on the show, we talk about the chain of environmental and weather events that led to these fires. We get into the science behind the winds and the role of human-caused climate change. You're listening to Shorewave, the science podcast from NPR. ♪

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This message comes from Warby Parker. What makes a great pair of glasses? At Warby Parker, it's all the invisible extras without the extra cost, like free adjustments for life. Find your pair at warbyparker.com or visit one of their hundreds of stores around the country. All right, Alexander, let's talk about the wins themselves, because I think a lot of people outside of California don't know what the Santa Ana wins are. What are they, and where do they originate?

Santa Ana winds are a type of downslope wind. They accelerate down the slopes of the coastal topography here. And basically, they originate in an elevated basin at about 1,200 meters behind the Sierra Nevada mountains and goes all the way to the Salt Lake. And you have...

cool air masses settling into the Great Basin. And when you have the pressure gradients, basically high pressure around the four corners area, you get this air mass to come over and through the passes and through the gaps in the surrounding topography, basically getting pushed towards the ocean. And it basically increases

accelerates downhill under the force of gravity. And you get these dry, gusty, downslope winds that we call Santa Ana's here. Does the geography of the region make these winds unique to Southern California? Does this effect happen anywhere else in the world?

Downslope winds happen everywhere there are mountains, but these winds are very special because of the geography of the southwestern United States and specifically the fact that we have this elevated great basin where these winds are really rooted.

They're basically the breath of the desert. So these Santa Ana winds, they blow every year, these downslope gusts, the breath of the desert, as you called them, they become drier and warmer as they descend. And yet fire officials are calling this the most destructive windstorm in over a decade. Do you know why the Santa Ana winds are blowing so hard right now, this month?

The one unusual thing about this particular event is that there's a lot of what meteorologists call upper air support. And basically, it's the jet stream that's blowing in the high troposphere that happens to be meandering in such a way that it just intensifies that flow from the northeast coming right into Southern California. Now, of course, you know, winds...

Don't create fires. They're not matchsticks. What sparks fires? And particularly right now in California, how is the wind contributing to the fires that we're seeing now?

All the ignitions are human-caused, and there are plenty of sources of ignition out there because population is expanding into that wildland-urban interface, you know, that sloping topography where the Santa Ana winds blow. And a lot of the sources of wildfire ignitions are going down, things like

cigarette butts and campfires, but ignitions due to arson and electrical power infrastructure are actually going up. So the more the population spreads into those fire-prone areas, the more sources of ignition there are. How does the addition of the wind cause this fire to spread? Because it carries the fire? Like, what's happening there? Well, these are very dry, gusty winds.

And they can be strong enough to impact infrastructure, like, for example, cause power lines to sway and contact each other when they're

You know, sparks fly. They can break trees or branches of trees that then land on top of power lines, for example, and that causes shorts and sparks to fly. These are very common causes of ignition. And, you know, our power utilities are operating a lot more safely lately.

They have what they call public safety power shutoffs when the weather conditions are predicted to be very fire prone, specifically Santa Ana winds in this region. And when the vegetation is very dry and the National Weather Service calls a red flag warning, then that triggers fire.

a public safety power shutoff. Right. Of course, key to all of this is that California is already very dry to begin with. You have vegetation dried to a crisp from a summer that broke heat records and very little rainfall at the start of the rainy season. Was the dryness leading up to this winter of concern to you? Did you anticipate something like this could happen? You know, we've seen this before, but with climate change, we do expect

the wet season to start later. Basically, we expect the wet season to contract into the winter and start later and finish earlier. That, together with warmer conditions, basically spells a longer and more intense fire season. And I say more intense because when these fires occur in Santa Ana winds,

in December and January, they're occurring during the peak season for Santa Ana winds. So then wildfires that start far enough away from the coast and have room to burn, they can become huge. This is what happened in 2017-18 with the Thomas fire that is the biggest wildfire on record in Southern California.

I think of the Thomas Fire as the poster fire for future fires in a warmer climate. You know, here we have a repeat of what happened seven years ago. Wildfire season typically runs from like the early start of the summer into the fall, but it's January now.

What can Californians expect in this region of their weather? Well, in most places, wildfire season is in the summer, like you say, Emily. But here in coastal California, and especially in Southern California, the season for the biggest wildfires is October, because that's when the fuels are dry after the dry, hot summer.

That's when you have the confluence of the winds that are capable of spreading huge wildfires and the dry fuels. But later on in November, December, we typically get enough rain so that the fuels are not flammable or that flammable anymore. But this is an exceptional year.

Well, Alexander, thank you so much for talking to me about the science of what's happening in the environment that's driving all this. It was really good to chat with you. My pleasure, Emily. Thank you for reaching out. This episode was produced by Jessica Young. It was edited by Rebecca Ramirez. Kweisi Lee was the audio engineer. Beth Donovan is our senior director. And Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening to ShoreWave.

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