Diablo winds, explained
Diablo winds are the San Francisco Bay Area's version of the Santa Ana: dry air from high pressure over the Great Basin flows toward the coast, sinks down the hills and warms on the way, and arrives hot, gusty and extremely dry.
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How they work
When high pressure builds over Nevada and low pressure sits off the coast, air flows from the interior toward the ocean, the reverse of the Bay Area's usual cool sea breeze. That air comes down the slopes of the Sierra Nevada and the coastal ranges, including the area around Mount Diablo the wind is named after. Sinking air is compressed and warms by about 5.5°F for every 1,000 feet it drops, with no water added, so its relative humidity falls to very low values.
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Why they're dangerous
Diablo winds are strongest over ridges and in gaps, often at night, and most common in fall, when vegetation is driest after the long rainless summer. That combination of wind, heat and dryness is why several of California's most destructive wildfires spread during Diablo wind events, and why utilities sometimes shut off power ahead of them.
Diablo vs Santa Ana
Same mechanism, different place. Santa Anas blow through Southern California's passes and canyons toward Los Angeles, Orange County and San Diego. Diablo winds hit the Bay Area and Northern California. Both turn the usual onshore flow around and both are driven by the same kind of high over the Great Basin.
Questions people ask
- When do Diablo winds happen?
- Mostly in fall, from about September through November, with occasional events in winter and spring.
- Why are Diablo winds hot at night?
- The strong wind keeps stirring warmer air from above down to the ground, so the cool, calm layer that normally forms after sunset can't settle in.