In the Golden State, these winds are most common from October through March, according to a 2017 climatology study, but can occur anytime from September into June.
However, for both the Santa Ana and the Diablo winds, the most dangerous time of the year is during the fall season when vegetation is at its driest. These winds can whip an existing wildfire, just-developed small brush fire or smoldering campfire into an inferno within minutes or hours.
The Santa Ana winds of Southern California
The Santa Ana winds are strong, extremely dry downslope winds originating from cool, dry high-pressure air masses in the Great Basin of the United States. They affect coastal Southern California and northern Baja California.
Not only do these winds bring the hottest dry weather of the year, but they also bring the lowest relative humidities of the year to coastal Southern California. Combining low humidities with a warm, compressionally-heated air mass, plus high wind speeds and you have a recipe for critical fire weather conditions.
We mentioned the wind originating from high-pressure air masses in the Great Basin. The Great Basin spans nearly all of Nevada, much of Oregon and Utah, and portions of California, Idaho, and Wyoming. The problem is this – Any low-pressure area over the Pacific Ocean, off the coast of California, can change the stability of the Great Basin high-pressure air mess.
If this happens, it causes a pressure gradient that turns the downward-flowing winds southward down the eastern side of the Sierra Nevada and into the Southern California region. The airmass, flowing from high pressure in the Great Basin to a low-pressure center off the coast, takes the path of least resistance by storming through the mountain passes to the lower coastal elevations and out to sea.
Howling through Santa Ana Canyon, the same winds sweep through Malibu and the Cajon Pass – but those areas weren’t as populated as the Santa Ana region in the 1870s when newspapers coined the name.
We all know about the extreme wildfire danger the Santa Ana winds can spark, and southern California has had more than its fair share of wildfires to date. In October 2003, the biggest fire in California state history, fanned by Santa Ana winds, burned 721,791 acres in about two weeks.
In 2007, fueled by the Santa Ana winds – wildfires charred 497,963 acres (778 square miles) in southern California, causing 85 injuries and at least seven deaths.
With Diablo winds in the north, and Santa Ana winds in the south, much of California became scorched earth in a day pic.twitter.com/ruErSdYkGI
— CBS News (@CBSNews) October 11, 2017
The Diablo winds of northern California
Diablo wind is a name that has been occasionally used for the hot, dry wind from the northeast that typically occurs in the San Francisco Bay Area of Northern California, during the spring and fall.
The term was first used shortly after the 1991 Oakland firestorm, perhaps to distinguish it from the comparable, and more familiar, hot dry wind in Southern California known as the Santa Ana winds. “Diablo wind” refers to the fact that the wind blows into the inner Bay Area from the direction of Mount Diablo in adjacent Contra Costa County.
The Diablo wind is created by the combination of strong inland high pressure at the surface, strongly sinking air aloft, and lower pressure off the California coast. Sinking air from aloft, combined with sinking air from the Coast Ranges compresses at sea level. The compression causes the air to warm as much as 20 degrees Fahrenheit and it loses relative humidity.
#KincadeFire Evacuation Press Conference October 26, 2019
Keep in mind that while both the Diablo and Santa Ana winds can pose a fire risk, the dynamics of the two winds are different. Santa Ana winds are gravity-driven winds, draining air off the high deserts. Diablo winds originate mainly from strongly sinking air from aloft, pushed toward the coast by higher inland pressure.
This difference makes Santa Anas the strongest in canyons, while Diablo winds are usually first noted and blow strongest atop and on the western slopes of the various mountain peaks and ridges around the Bay Area. The one thing both winds have in common? With both, the air sinks, it heats up by compression and relative humidity drops, creating an extreme wildfire hazard.
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