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 same wind pattern also affects other parts of California's coastal ranges and the western slopes of Sierra Nevada, with many media and government groups using the term Diablo winds for strong, dry downslope wind over northern and central California. [1]
The term first appeared 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. 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, [2] such as the one that was associated with the 1923 Berkeley Fire. [3]
The name "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 and evoking the fiery, sensationalist connotation inherent in "devil wind".
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. [4]
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". [5] While hydraulic jumps can occur with Santa Ana winds, the same thermodynamic structure that occurs with them typically favors "gap" flow [6] more frequently. Santa Ana winds are katabatic, 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 pressure aloft. 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 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.
The dry offshore wind, already strong because of the offshore pressure gradient, 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. 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. [7]
Effects tend to be halved at farther southern coasts, and even can spawn Sundowner winds in Santa Barbara. Although Santa Ana winds may recur, Diablo events in autumn tend to precede Santa Ana wind events by a few weeks with a lull in between as solar incidence declines. Southern Californian coasts from Ventura County south are generally unaffected by Diablo events.
The Santa Ana winds are strong, extremely dry downslope winds that originate inland and affect coastal Southern California and northern Baja California. They originate from cool, dry high-pressure air masses in the Great Basin.
A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase of the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
A katabatic wind is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds; the spelling catabatic winds is also used. Katabatic winds can rush down elevated slopes at hurricane speeds, but most are not that intense and many are 10 knots or less.
A Foehn or Föhn, is a type of dry, relatively warm, downslope wind that occurs in the lee of a mountain range. It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windward slopes. As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes.
Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.
In meteorology, lee waves are atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Giant Mountains. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.
June Gloom is a mainly Southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. While it is most common in the month of June, it can occur in surrounding months, giving rise to other colloquialisms, such as “Graypril,” "May Gray," "No-Sky July," and "Fogust." Low-altitude stratus clouds form over the cool water of the California Current, and spread overnight into the coastal regions of California.
The Santa Ynez Mountains are a portion of the Transverse Ranges, part of the Pacific Coast Ranges of the west coast of North America. It is the westernmost range in the Transverse Ranges.
A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.
The Berkeley Hills are a range of the Pacific Coast Ranges that overlook the northeast side of the valley that encompasses San Francisco Bay. They were previously called the "Contra Costa Range/Hills", but with the establishment of Berkeley and the University of California, the current usage was applied by geographers and gazetteers.
The climate of San Diego, California is classified as a hot-summer Mediterranean climate. The basic climate features hot, sunny, and dry summers, and cooler, wetter winters. However, San Diego is much more arid than typical Mediterranean climates, and winters are still dry compared with most other zones with this type of climate. The climate at San Diego International Airport, the location for official weather reports for San Diego, as well as the climate at most beach areas, straddles the border between BSh and BSk due to the mild winters and cool summers in these locations.
A sundowner is a northerly offshore wind in California along the southern Pacific slope of Santa Ynez Mountains, in communities along the Gaviota Coast and Santa Barbara towards but not including Ventura County.
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, in the Sahara, in the Kalahari, over north-west Argentina, in South America, over the Kimberley region of north-west Australia, over the Iberian peninsula, and over the Tibetan plateau.
The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, centered roughly on the Rio Grande Valley, and typically occurring between June and mid-September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon and early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when much drier conditions are reestablished over the region. Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua.
The 1978 Agoura-Malibu firestorm was a firestorm fueled by at least eight significant wildfires in the Los Angeles area on October 23, 1978. At around noon that day, an arsonist started a fire that eventually burned 25,000 acres (10,000 ha) from Cornell to Broad Beach in Malibu. The first fire alarm in Agoura was reported at 12:11 pm, and by 2:30 pm, the fire had reached the Pacific Ocean 13 miles (21 km) south in Malibu. It had been declared a Level 2 fire at 1:57 pm.
The alpine planetary boundary layer is the planetary boundary layer (PBL) associated with mountainous regions. Due to its high spatial and temporal variability, its behavior is more complex than over a flat terrain. The fast changing local wind system directly linked to topography and the variable land cover that goes from snow to vegetation have a significant effect on the growth of the PBL and make it much harder to predict.
The Sherpa Fire was a wildfire that burned in the Santa Ynez Mountains along the Gaviota Coast in the southwestern part of Santa Barbara County, California in June 2016. In a matter of hours the fire spread to over 1,400 acres (570 ha) as the fire was propelled by downslope sundowner winds. This offshore northerly wind contrasts with the more typical onshore flow and sent the fire down the canyons towards the ocean with gusts of over 35 miles per hour (56 km/h). The wildfire resulted in evacuations at two state beach campgrounds and some residences together with intermittent interruption of traffic on a state transportation route.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that usually occurs on the coastal plain of southern New South Wales, and as well as in southeastern Victoria and eastern Tasmania, on the leeward side of the Great Dividing Range. Ranging from cool to hot, the effect occurs when westerly winds descend steeply from the Great Dividing Range onto the coastal slopes, whereby causing major adiabatic compression and a substantial loss of moisture.