No weather pattern is better associated with Los Angeles than the Santa Ana winds.
One of the earliest written descriptions of Santa Anas comes from the diary of Commodore Robert Stockton on the night of January 6, 1847; the next day his troops captured Los Angeles on behalf of the United States.
And as the city has grown to assume an important place in American pop culture, it has given global fame to this local phenomenon, name-dropped by Raymond Chandler, Nancy Meyers and the Beach Boys.
Santa Ana winds are known for being hot, dry, and dusty — traits that have earned them the nickname “devil’s wind” — but their defining quality is their direction.
Unlike the prevailing winds in Southern California, which generally flow from west to east, bringing warm air from the Pacific, the Santa Anas flow from northeast to southwest out of the Mojave Desert. What caused this reversal, and why did it produce such an obnoxious result?
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To form the Santa Ana wind, the typical first ingredient is a cold autumn day in the desert of southern Nevada.
Cold creates cold, dense air, which is squeezed from high up by high pressure systems. Usually the surface air will be in the Great Basin formed by the Sierra Nevada and the Rocky Mountains, but the second ingredient is a low pressure system off the coast of California, which creates enough gravitational potential to force the air out of the basin and pull it. west to the Pacific.
As it flows down, the air is compressed due to the weight of the higher atmospheric column above. The ideal gas law (PV = nRT, if high school chemistry is just a hazy memory) tells you that when the pressure in a gas increases, the temperature also increases. The result is that the descending air heats up by nearly 30 degrees Fahrenheit for every vertical mile it sinks.
The dry desert air, warmed by the descent, rushes toward the coast. But the Transverse Ranges stand in the way, so the air seeks the path of least resistance through Cajon and San Gorgonio. Like spraying water through a narrow nozzle, the wind speeds up as it enters the canyon, often reaching gale force as it exits toward Los Angeles and San Bernardino.
Light Santa Ana winds can be annoying, giving people nosebleeds and blowing sand in their eyes, but more severe events can have deadly consequences. The most obvious risk is high winds – during a particularly intense episode in December 2011, gusts in excess of 50 mph toppled trees, damaged hundreds of buildings and knocked out power for hundreds of thousands of people.
Atypical wind directions can pose particular risks to vessels and maritime infrastructure, as normally well-protected harbors on the leeward side of the Channel Islands are suddenly exposed to strong winds and waves.
Strong Santa Ana offshore winds blow waves into Huntington Beach in October 2018.
(Allen J. Schaben/Los Angeles Times)
A greater danger comes from the potential for wildfires. Hot, dry air can quickly strip moisture from vegetation, especially if the air is constantly replenished by strong desert winds. Santa Anas often bring triple digit temperatures and relative humidity below 10%, leading to dry fuel that can ignite more easily. In addition, strong winds cause the fire to grow and spread faster, because the wind provides a constant supply of oxygen, carries sparks and even bends the fire closer to the material that did not burn before the fire.
In the last few decades, the Santa Ana wind has been associated with several large wildfire groups, including the 2007 Witch Creek fire, the 2008 Sayre fire and the 2017 Thomas fire, which was the largest wildfire in the history of the country at the time.
Firefighters battle a Silverado fire amid Santa Ana winds in Irvine in October 2020.
(Allen J. Schaben/Los Angeles Times)
Until recently, the Santa Ana wind was considered one of the bright spots in climate change; a paper from 2019 predicts a decrease in the frequency of Santa Ana winds in the future, especially in September and October. The authors suggested that this is because the northward migration is projected from the “Great Basin high” that tends to form through Nevada.
However, a new analysis published two years later by the same authors suggests that the downward trend is mostly limited to the “flavor” of the Santa Ana wind which, while originating from the same location, is caused by a different mechanism and causes intense cold. for Southern California instead of summer.
Although this “Cold Santa Anas” can still cause wind damage, they are not usually associated with wildfire activity, and the reduction in frequency will have little effect on fire risk. Unfortunately, it seems that hot and dry days when the wind stings your eyes and sparks fly here to stay.
Ned Kleiner is a disaster scientist and modeler at Verisk. He holds a doctorate in atmospheric science from Harvard University.
