A Santa Ana wind event is fueling a wildfire in Los Angeles that has destroyed homes and forced the evacuation of thousands of people. Here’s what to know about the winds.
How do the Santa Ana winds work?
The Santa Ana winds are the strong, dry and often warm winds that blow west from Nevada and Utah to Southern California.
The winds are most common in the colder months, when low- and high-pressure weather systems pepper across the mid-latitudes of the northern hemisphere, said Alex Hall, director of the Institute of the Environment and Sustainability at the University of California, Los Angeles. Sometimes, an area of high pressure can become locked in place over the arid, mountain-bound area known as the Great Basin, which covers much of Nevada, part of Utah and portions of other states, because of its unique geography.
To understand how the Santa Ana winds form, Mr. Hall said, imagine the Great Basin as “a big bowl,” topographically speaking, its sides lined with holes representing mounting passes. “If you have a lot of heavy, high pressure sitting there in the bowl, you’re going to be pushing air out through the holes,” he said.
When air rushes through the mountain passes — the bowl’s holes — the winds gain speed, moving toward the lower-pressure area of the Southern California coast.
The air of the Santa Ana winds is unusually dry because it originates in the desert environment of the Great Basin. And although it can begin cool for the same reason, it often warms as it becomes compressed while tumbling down the mountainside. (You might have noticed your bicycle tires warming when you fill them with air; the same principle is at work here, Mr. Hall said.)
How does climate change affect the Santa Ana winds?
Scientists don’t yet know for sure, Mr. Hall said.
On one hand, it has “been pretty well demonstrated that warmer temperatures create larger fires,” he said. And as climate change shortens California’s rainy season, the winds could increasingly coincide with drier environmental conditions more prone to wildfire outbreaks.
At the same time, he said, research suggests “if the desert is warming more than the ocean, then there’s going to be less of a difference in pressure between the desert and the ocean,” which could actually weaken the Santa Ana winds. “It’s that pressure difference that drives those winds,” he said.
Still, Mr. Hall said, there is no strong consensus on the matter.
Are the Santa Ana winds dangerous?
The Santa Ana winds have the potential to create life-threatening weather conditions. They can lift embers from an existing fire into an unburned area, where they may ignite a new fire, said Jesse Torres, a battalion chief for the California Department of Forestry and Fire Protection. This “creates almost two fires. And those fires burn together rapidly as well,” he said.
Damage caused by the strength of the Santa Ana winds can inhibit firefighting operations, Mr. Torres added, because dangerous winds can temporarily ground fire department aircraft. Even if the aircraft can operate, “powerful wind affects the drops that are made with retardant and water” and how effectively they are able to reach directly onto the fire, Mr. Torres said. The high winds can also topple trees, which put people at risk of harm and can obstruct emergency vehicles and evacuations from areas in danger, he said.
“One of our bigger concerns for not only the public but ourselves as firefighters is downed power lines and downed power poles that fall down because of these strong winds and block roadways,” Mr. Torres added.
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