Where The Winds Begin
Strange as it seems, hot Santa Ana winds start out as cold air hundreds of miles away from Los Angeles. The warm, dry winds that plague Southern Californians in the fall- stinging eyes and drying out sinuses- suck moisture out of the air and fan the flames of wildfires. Those winds are the result of several meteorological events and are part of a complex weather system that extends throughout the Western states.
It’s no coincidence that these yearly wind events occur during brush fire season. Santa Ana winds dry out the shrubbery and fine grasses that fuel wildfires. Low humidity and high winds have spread many fires in Southern California, including last month’s Calabasas / Malibu fire and the Old Topanga firestorm in 1993. Fire officials estimate that fires pushed by Santa Anas spread five to six times faster than other brush fires.
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How It Gets Hot, Fast and Dry
1. Hot: The sinking movement of the high- pressure- system wind causes it to warm- temperatures increases 5.5 degrees for every 1,000 feet it descends. Moving across the deserts of Nevada and eastern California adds to its warming.
2. Fast: Wind gains speed as it is funneled through the narrow mountain passes and canyons of the San Gabriel and San Bernardino mountains.
3. Dry: The sinking air motion can cause humidity to go down to single digits.
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Low Pressure: A low-pressure area with counterclockwise winds developes off or along the California coast.
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Jet Stream: Powerful Santa Ana winds need a strong jet stream occurring over Southern California. The jet stream is what guides storms, and its position and strength varies.
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High- Pressure: The cold, dense air from an Alaskan storm helps create a high-pressure system centering roughly over the northern Utah- Nevada border. For strong Santa Anas, the high- pressure system must exist for a greater vertical depth over California, extending from about 18,000 feet above sea level to the ground.
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Cold Front: Cool air from a cold front moves from the Gulf of Alaska into the Great Basin states.
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Air Circulation: Air circulating clockwise in the high- pressure system moves west toward Southern California, becoming the northeasterly winds called Santa Anas.
Sources: James Murakami, meteorologist at UCLA Atmospheric Science Department; National Weather Service. Researched by JULIE SHEER / Los Angeles Times
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