What are Southern California's Santa Ana Winds?
Hot, Dry Winds Originate in Great Basin and Increase Wildfire Risk
Sep 22, 2009
Santa Ana winds are a fact of life in Southern California in the fall and winter. Bringing hot temperatures, low humidity and strong winds at the end of the dry season, the Santa Ana winds increase wildfire danger and have inspired cultural references from literature to music.
While the Santa Ana winds may occur anytime between September and March, the ones in October are most infamous, as this is also the peak of fire season. Many of Southern California’s most devastating wildfires have been exacerbated by Santa Ana winds, especially during times of drought when the coastal scrubland or chaparral is brittle dry.
Great Basin High Pressure Systems
In the northern hemisphere, air flows in a clockwise direction around high pressure systems. In general, an area to the west of a high pressure system will therefore be dominated by warmer winds coming from the east and south, while an area to the east of a high pressure system will experience cooler northerly winds.
When a high pressure system develops over the Great Basin to the north and east of Southern California, prevailing winds push the air in a clockwise direction around the high pressure. Santa Ana winds are caused by this phenomenon, especially when lower pressure to the west actually pulls the air across Southern California in strong offshore winds.
Hot Santa Ana Winds Originate in Cooler High Elevations
Santa Ana winds originate in the desert, including the Great Basin, but it is not the desert itself that makes the winds hot. In fact, Santa Ana winds are usually accompanied by desert cooling, when the Great Basin can be cooler than the Los Angeles basin.
Although the Santa Ana winds originate as colder air over the high desert, they arrive in Southern California as hot air because of the difference in elevation. Air temperature is strongly correlated to elevation, and if there was not a dramatic difference in elevation between the Great Basin and the Los Angeles Basin, the Santa Ana winds would reach Southern California as cool winds. When air drops in elevation, however, it becomes compressed and warmer. This is called adiabatic warming. Because the air has to descend from the Great Basin, the Santa Ana winds are usually hot.
When the hot air is blown through the many passes and canyons characteristic of Southern California topography, the wind’s speed increases. This is why the Santa Ana winds are frequently described as strong or fast winds by the time they reach Southern California. It is not uncommon for Santa Ana winds to exceed 25 knots (46 kilometers/hour).
Dry Winds Increase Southern California Fire Danger
Santa Ana winds are usually dry winds, which further exacerbates the potential danger of wildfires. As the air descends and warms due to compression, its relative humidity decreases. Relative humidity can drop below 10% during a Santa Ana wind event. Because of the low relative humidity, this hot, fast-moving air can literally strip already dry vegetation of whatever little moisture it may have, further increasing fire danger.
The wind also picks up dust, and during Santa Ana wind events, dust plumes carried by strong offshore winds can be seen over the ocean from space.
Red Flag Warnings
Southern California forecasters typically issue a red flag warning in advance of a Santa Ana Wind event. The red flag warning generally covers the mountains of Los Angeles and Ventura counties, the Santa Monica Mountains, and the San Fernando and San Gabriel valleys. According to the National Weather Service, a red flag warning means that “critical fire weather conditions are either occurring now…or will shortly.”
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