Compression

Diablo wind is a relatively recent term for a hot, dry offshore wind from the northeast that typically occurs in the San Francisco Bay Area of Northern California during the Autumn. The same wind pattern also affects other parts of California’s coastal ranges.

The winds are created by the combination of strong high pressure at the surface over the Great Basin, 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, 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 mph 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.

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