Diablo wind is a name that has been occasionally used for the hot, dry offshore wind from the northeast that typically occurs in the San Francisco Bay Area of Northern California, during the spring and fall. The same wind pattern also affects other parts of California's coastal ranges. The term was first used by local news media during and after the 1991 Oakland firestorm, to distinguish it from the comparable, and more familiar, hot dry wind in Southern California known as the Santa Ana wind. In fact, in decades previous to the 1991 fire, the term "Santa Ana" was occasionally used as well for the Bay Area dry northeasterly wind, such as the one that was associated with the 1923 Berkeley Fire.
The name "Diablo wind" may have been coined from the observation that the wind blows into the inner Bay Area from the direction of the Diablo Valley in adjacent Contra Costa County, and mindful of the fiery, sensationalist connotation inherent in "devil wind".
The Diablo winds are created by the combination of strong inland high pressure at the surface, strongly sinking air aloft, and lower pressure off the California coast. The air descending from aloft as well as from the Coast Ranges compresses at sea level where it warms as much as 20 °F (11 °C), and loses humidity.
Unlike the Santa Ana wind which drains surface air off the high deserts, the so-called Diablo wind mainly originates from areas of strongly sinking air aloft, associated with the development of high atmospheric pressure inland following the passage of storms just north and east of California. The similar, though distinctive mechanisms can be distinguished by where the strongest winds in each type of event occur. Typically, Santa Anas are strongest in canyons whereas a Diablo wind is first noted and blows strongest atop the various mountain peaks and ridges around the Bay Area. In both cases, as the air sinks, it heats up by compression and its humidity drops. This heat is in addition to, and usually greater than, any heat picked up by the wind as it crosses the Central Valley and the Diablo Valley. This is the reverse of the normal summertime weather pattern in which a trough of low rather than high pressure lies east of the Bay Area, drawing in cooler, more humid air from the ocean. If the pressure gradient is large enough, the dry offshore wind can become quite strong with gusts reaching speeds of 40 miles per hour (64 km/h) or higher, particularly along and in the lee of the ridges of the Coast Range where the higher wind speed aloft acts like a pump, drawing warm, dry surface air from the windward eastern side up and over the ridgelines. This effect is especially dangerous with respect to wildfires as it can enhance the updraft generated by the heat in such fires.
While the Diablo occurs in both the spring and fall, it is most dangerous in the fall when vegetation is at its driest.
- extract from the Report on the Berkeley, California Conflagration of September 17, 1923, issued by the National Board of Fire Underwriters’ Committee on Fire Prevention and Engineering Standards, reprinted in the Virtual Museum of the City of San Francisco
- WEATHER CORNER, San Jose Mercury News, Jan Null, October 26, 1999