TThere are a lot of variables at play in the catastrophic wildfires currently sweeping through Southern California: dry vegetation, lack of rain, dense housing development, and errant sparks potentially from cigarette butts, campfires, power lines, or even arsonists. And then of course there is gravity. Of all the factors involved, none may be so fundamental or powerful as the tendency of a body with mass to roll, slide, or slide downward under the force of the ground beneath it.
In the case of wildfires, the massive body is air, specifically cold air, that swirls and flows at an altitude of 1,200 meters (4,260 feet) in the California region. Sierra Nevada and white Mountains, and Klamath Basin In southern Oregon and northern California – a swirl of atmospheric dervishes creates the characteristic Santa Ana winds. The warmer, lower-pressure air at sea level is no match for the cooler, denser air at high altitudes and what’s at the top collapses. Fire absolutely loves it when that happens.
“We call these winds slope winds,” says Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography at the University of San Francisco. “It works like water does in a dead-end stream – it heads up against the terrain, and as it rushes over the peaks and through gaps in the terrain, it accelerates down the lee slopes and forms something like a waterfall – or an air drop from the downslope wind.”
It is those airflows that have done as much as anything else to fan the flames that have so far claimed dozens of lives, displaced tens of thousands and destroyed thousands of buildings across the Los Angeles area. Last week, isolated wind gusts were witnessed, especially in the Santa Monica Mountains. It reached 100 mph. The weekend saw some decline, but Wednesday. January 15 and Thursday. January 16 It is expected to see Winds up to 65 mph. Category 1 hurricanes, by comparison, Start at 74 mph.
As shocked as Los Angeles was, some of this was completely expected. In all, Gershunov says, the Los Angeles area absorbs five hits from Santa Ana winds every December and four or five every January. Most of these disturbances are caused by seasonal changes in pressure gradients and the jet stream over the Great Basin and the Four Corners region, causing dense high air flow over its banks.
That’s not to say these are the only two months when Santa Anas can cause damage. Wind events occur in October as well Usually the end of the fire seasonwhich usually begins in June or July, as the winds arrive before the first winter rains. The problem in late 2024 and early 2025: There were winter winds but no winter rains — the latest start to a rainy season in 150 years — and there is nothing in the forecast; This is bad news given the possibility of Santa Anse reappearing at the same time the Earth continues to burn.
“We’re now starting to see another Santa Ana wind event, and there will be another early next week, and we may see two or three Santa Ana winds before the first rains hit,” Gershunov says. “This is what happened in 2017 and 2018, when Thomas Fire It burned for most of December and the smoldering remains were extinguished by a Pacific River weather storm that occurred on January 9, 2018. But [the fire] It was intense enough over Montecito, in Santa Barbara County, that it caused a debris flow from the fire scar that killed 22 people.
Containing, let alone extinguishing, wildfires in dry and windy conditions can be extremely difficult. Dry vegetation is rocket fuel for fires and wind fuels the flames and spreads the embers. “The wind is like blowing air into a heater, except it’s not contained, and it’s on a much larger scale,” Gershunov says.
What’s more, while the ability of weather forecasters to forecast the return of Santa Ana winds as they do this week inspires some confidence that Los Angelenos can plan for what’s coming, the forecast is only reliable until it isn’t. Surface features—trees, hills, mountains, and buildings—are agents of chaos, causing all sorts of unpredictable disturbances when the wind blows. Higher up in the atmosphere, things can be more dangerous, and on a larger scale.
“Last week, the jet stream did this massive loop and backed up into the upper troposphere and was blowing in the opposite direction than it normally does,” Gershunov says. “It was pointed directly into the Los Angeles basin.”
Humidity – or more specifically, the lack of it – is another problem. Hot, arid winds blowing from the Sierra Nevada, White Mountains and Klamath Basin remove any moisture that may be present in the Los Angeles air, further drying out foliage, shrubs and other fuels. “The single digits in terms of relative humidity are a recipe for uncontrollable wildfires if the fuel is dry,” Gershunov says. What less humid winds often blow from the ocean, pushing fires that might actually roll downhill to extinguish themselves in the water up the hill.
If there’s a faint bright spot in the current disaster, it’s that the Santa Anas were blowing against a clear blue sky, meaning there was no lightning to serve as an additional ignition source. But despite this, regardless of the cause, once a fire is ignited in a firebox environment such as the stormy city of Los Angeles, the process of extinguishing it requires a massive expenditure of public effort and money – all in the face of massive loss of life and property. The current crisis will end, but at the moment no one can say when.