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 The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Go ...

of Northern California, during the spring and fall. The same wind pattern also affects other parts of California's coastal ranges and the western slopes of

Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...

, with many media and government groups using the term Diablo winds for strong, dry downslope wind over northern and central California.

Name

The term first appeared shortly after the

1991 Oakland firestorm : The Oakland firestorm of 1991 was a large suburban wildland–urban interface conflagration that occurred on the hillsides of northern Oakland, California, and southeastern Berkeley, California, Berkeley over the weekend of October 19–20, ...

, perhaps to distinguish it from the comparable, and more familiar, hot dry wind in Southern California known as the

Santa Ana winds The Santa Ana winds (sometimes devil winds) "Scholars who have looked into the name's origins generally agree that it derives from Santa Ana Canyon, the portal where the Santa Ana River -- as well as a congested Riverside (CA-91) Freeway -- leav ...

. 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 __NOTOC__ The 1923 Berkeley Fire was a conflagration that consumed some 640 structures, including 584 homes in the densely-built neighborhoods north of the campus of the University of California in Berkeley, California on September 17, 1923. Alth ...

. The name "Diablo wind" refers to the fact that the wind blows into the inner Bay Area from the direction of

Mount Diablo Mount Diablo is a mountain of the Diablo Range, in Contra Costa County, California, Contra Costa County of the eastern San Francisco Bay Area in Northern California. It is south of Clayton, California, Clayton and northeast of Danville, Califo ...

in adjacent

Contra Costa County ) of the San Francisco Bay , subdivision_type = Country , subdivision_name = United States , subdivision_type1 = State , subdivision_name1 = California , subdivision_type2 ...

. 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 and the Diablo Range in adjacent Contra Costa County, and evoking the fiery, sensationalist connotation inherent in "devil wind".

Formation

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. The air descending from aloft as well as from the Coast Ranges compresses as it sinks to sea level where it warms as much as 20 °F (11 °C), and loses relative humidity. Because of the elevation of the coastal ranges in north-central California, the thermodynamic structure that occurs with the Diablo wind pattern favors the development of strong ridge-top and lee-side downslope winds associated with a phenomenon called the "

hydraulic jump A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in th ...

". While hydraulic jumps can occur with Santa Ana winds, the same thermodynamic structure that occurs with them typically favors "gap" flow more frequently. Santa Ana winds are

katabatic A katabasis or catabasis ( grc, κατάβασις, from "down" and "go") is a journey to the underworld. Its original sense is usually associated with Greek mythology and Classical mythology more broadly, where the protagonist visits the Gre ...

, gravity-driven winds, draining air off the high deserts, while the Diablo-type wind originates mainly from strongly sinking air from aloft, pushed toward the coast by higher inland pressure. Thus, Santa Anas are strongest in canyons, whereas a Diablo wind is first noted and blows strongest atop and on the western slopes of the various mountain peaks and ridges around the Bay Area. In both cases, as the air sinks, it heats up by compression and its

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

drops. This warming is in addition to, and usually greater than, any contact heating that occurs as the air stream crosses the Central Valley and the Diablo Valley. This is the reverse of the normal summertime weather pattern in which an area of low pressure (called the California Thermal Low) rather than high pressure lies east of the Bay Area, drawing in cooler, more humid air from the ocean.

Impacts

The dry offshore wind, already strong because of the offshore pressure gradient, can become quite strong with gusts reaching speeds of or higher, particularly along and in the lee of the ridges of the Coast Range. 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 wind pattern occurs in both the spring and fall, it is most dangerous in the fall, when vegetation is at its driest. California's predominantly Mediterranean climate has an extended dry period from May through October. Effects tend to be halved at farther southern coasts, and even can spawn Sundowner winds in Santa Barbara. Although

may recur, Diablo events in autumn tend to precede

Santa Ana wind The Santa Ana winds (sometimes devil winds) "Scholars who have looked into the name's origins generally agree that it derives from Santa Ana Canyon, the portal where the Santa Ana River -- as well as a congested Riverside (CA-91) Freeway -- leav ...

events by a few weeks with a lull in between as

solar incidence Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...

declines. Southern Californian coasts from

Ventura County Ventura County () is a county in the southern part of the U.S. state of California. As of the 2020 census, the population was 843,843. The largest city is Oxnard, and the county seat is the city of Ventura. Ventura County comprises the Oxnar ...

south are generally unaffected by Diablo events.

References

{{reflist Winds Climate of California Weather events in the United States

Name

Formation

Impacts

See also

References