Few weather phenomena are as emblematic of Los Angeles as the notorious Santa Ana winds, which have become a defining aspect of the city’s climatic personality.
One of the earliest documented observations of these winds can be traced back to the diary of Commodore Robert Stockton dated January 6, 1847; shortly thereafter, on January 7, his troops seized control of Los Angeles, marking a significant event in American history.
As Los Angeles has established itself as a cultural icon, the Santa Ana winds have further gained international fame, being referenced by renowned figures such as Raymond Chandler, film director Nancy Meyers, and the legendary musical group the Beach Boys.
These winds are infamous for their hot, dry, and dusty characteristics — earning them the foreboding moniker of “devil winds.” However, what truly sets them apart is their unique direction of flow.
Unlike the common winds in Southern California, which typically carry cooler air from the Pacific in a west-to-east direction, the Santa Ana winds originate from the northeast, coursing southwestward from the arid Mojave Desert. What triggers this unusual flow, and why does it have such perilous consequences?
To understand the formation of the Santa Ana winds, one must first consider a brisk autumn day in the high desert of southern Nevada, which serves as the initial catalyst.
The initial chill creates heavy, cold air, which is subsequently compressed from above by a robust high-pressure system. Normally, this surface air would remain trapped within the Great Basin, bounded by the Sierra Nevada and the Rocky Mountains; however, a secondary low pressure system situated off the California coast acts as a catalyst, generating sufficient gravitational potential to push the air westward toward the Pacific.
As the air makes its descent, it encounters compression due to the weight of the atmosphere above it. According to the ideal gas law (PV=nRT), increasing pressure results in rising temperature. Thus, the descending air can warm by nearly 30 degrees Fahrenheit for every mile it plunges.
The parched desert air, now heated, races toward the coast, but encounters obstacles from the Transverse Ranges, compelling it to navigate through the Cajon and San Gorgonio passes. In a manner akin to water surging through a constricted opening, these winds gain strength as they funnel through the canyons, often achieving gale-force speeds by the time they sweep across Los Angeles and San Bernardino.
The mild iteration of Santa Ana winds may simply be a minor annoyance, causing nosebleeds and blowing sand into people’s eyes, yet the more intense manifestations can lead to catastrophic outcomes. For instance, during a particularly vigorous episode in December 2011, wind gusts exceeding 50 mph knocked down trees, wreaked havoc on countless buildings, and left hundreds of thousands without power.
This atypical wind direction also introduces specific hazards for maritime activities, as harbors that typically enjoy protection from the predominant winds suddenly find themselves vulnerable to powerful gusts and tumultuous waves.
The heated, arid air dramatically depletes moisture from vegetation, a process exacerbated by the relentless strong winds. Santa Ana winds frequently usher in scorching temperatures in the triple digits along with a humidity level plummeting below 10%, creating tinderbox conditions that facilitate rapid ignition. In addition, the strong winds enable fires to expand and spread rapidly, supplying a continuous stream of oxygen and propelling sparks, even coaxing flames closer to unburned fuel ahead of the blaze.
In recent decades, these winds have been linked to a series of devastating wildfire clusters, including the devastating Witch Creek fire in 2007, the Sayre fire in 2008, and the 2017 Thomas fire, which was then the largest wildfire in California’s recorded history.
Until recently, the Santa Ana winds were considered one of the few potential benefits in the context of climate change; a 2019 study predicted a reduction in the frequency of these winds, particularly during September and October, attributing this change to a projected northerly shift of the “Great Basin high” system over Nevada.
However, a more recent study, published two years later by the same researchers, indicated that this downward trend primarily pertains to a specific subtype of Santa Ana winds that, while sharing their origins, are caused by different mechanisms and instead bring intense cold to Southern California rather than warmth.
Although these “cold Santa Anas” can still inflict wind-related damage, they do not typically correlate with increased wildfire risk; thus, a reduction in their occurrence would have minimal influence on the potential for fires. Regrettably, it appears that those hot, dry days when the winds whip through neighborhoods, leaving irritation and embers in their wake, will remain a fixture of Southern California life.
Ned Kleiner is a scientist and catastrophe modeler at Verisk. He has a doctorate in atmospheric science from Harvard University.
