There are a lot of variables at play in the catastrophic wildfires currently clawing through southern California: dry vegetation, lack of rain, dense housing development, errant sparks potentially from cigarette butts or campfires or power lines or even arsonists. And then of course there’s gravity. Of all of the factors involved, there may be nothing as basic or as powerful as the tendency of an object with mass to roll or slide or plunge downward under the pull of the Earth below it.
In the case of the wildfires, the massive object is air—specifically cold air, swirling and flowing 1,200 meters (4,260 ft.) high in California’s Sierra Nevada and White mountains, and the Klamath Basin in southern Oregon and northern California—a whirling of atmospheric dervishes that creates the signature Santa Ana winds. The warmer, less compacted air down at sea level is no match for the colder, denser air in the high elevations and what’s at the top comes crashing down. Fire absolutely loves it when that happens.
“We call those downslope winds,” says Alexander Gershunov, a research meteorologist at Scripps Institution of Oceanography at the University of San Francisco. “They act as water does in an obstructed stream—they pull up against topography, and as they rush over the crests and through the gaps in the topography, they accelerate down the lee slopes and form something of a waterfall—or an airfall of downslope winds.”
It is those atmospheric gushers that did as much as anything else to fan the flames that have so far killed dozens, displaced tens of thousands and destroyed thousands of structures across the Los Angeles area. Last week, isolated gusts, especially in the Santa Monica Mountains, reached 100 mph. The weekend saw some easing back, but Wed. Jan. 15 and Thurs. Jan. 16 are expected to see winds back up to 65 mph. Category One hurricanes, by comparison, start out at 74 mph.
Blindsided as Los Angeles has been, some of this was entirely foreseeable. On the whole, Gershunov says, the Los Angeles area absorbs five batterings by Santa Ana winds every December and four or five every January. Most of that turbulence is stirred by seasonal changes in pressure gradients and the jet stream sitting over the Great Basin and the Four Corners region that cause the high, dense air to spill its banks.
That’s not to say those are the only two months that the Santa Anas can do their damage. Wind events occur in October as well and that is typically the end of the fire season, which usually begins in June or July, since the gusts arrive before the first winter rains do. The problem in late 2024 and early 2025: there’s been winter wind but no winter rain—the latest start to the wet season in 150 years—and there’s none in the forecast; that’s bad news given the likely resurgence of the Santa Anas at the same time the land continues to burn.
“We are beginning to experience another Santa Ana wind event now, and there’ll be another one early next week, and we may have two to three Santa Ana winds before we get the first rains,” says Gershunov. “That’s what happened in 2017 and 2018, when the Thomas fire burned through most of December and the smoldering remains were put out by an atmospheric river Pacific storm that occurred on Jan. 9, 2018. But [the fire] was extreme enough over Montecito, in Santa Barbara County, that it caused debris flows from the fire scar that killed 22 people.”
Containing wildfires—never mind extinguishing them—in parched and windy conditions can be monstrously difficult. Dry vegetation is rocket fuel for blazes and wind both feeds the flames and spreads the embers. “The winds are like pointing an air blower at a fireplace,” says Gershunov, “except it’s not contained and it’s on a much larger scale.”
What’s more, while the ability of weather forecasters to predict a resurgence of the Santa Ana winds as they’re doing this week inspires some confidence that Los Angelenos can plan for what’s coming, the fortune telling is reliable only until it isn’t. Surface features—trees, hills, mountains, buildings—are agents of chaos, causing all manner of unpredictable turbulence when the winds touch down. And higher in the atmosphere things can be even screwier—and on a larger scale.
“Last week the jet stream did this tremendous loop and retrograde up in the upper troposphere and was blowing in the opposite direction from what it normally does,” says Gershunov. “It was pointed right into the L.A. basin.”
Humidity—or, specifically, the lack of it—is another problem. Hot, arid winds blowing in from the Sierra Nevadas, White Mountains, and Klamath Basin strip whatever moisture there might be in the Los Angeles air, and that further dries out leaves and underbrush and other fuel. “Single digits in terms of relative humidity are a prescription for uncontrollable wildfires if the fuels are dry,” says Gershunov. What less humid wind there is often blows in from the ocean, which unhandily pushes fires that might actually be rolling down to extinguish themselves in the water back up hill.
If there’s even a faint bright spot in the current disaster, it’s that the Santa Anas have been gusting in clear blue skies, meaning no lightning to act as an additional ignition source. But even so, regardless of the cause, once a fire is lit in a tinder box environment like a windy Los Angeles, the business of extinguishing it takes a massive expenditure of public effort and treasure—all in the face of a massive loss of life and property. The current crisis will end, though for now no one can say when.
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