Santana Wind
Niobe Hennigan
The Santa Ana winds, also sometimes called the devil winds,[1][2] 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.
Santa Ana winds are known for the hot, dry weather that they bring in autumn (often the hottest of the year), but they can also arise at other times of the year.[3] They often bring the lowest relative humidities of the year to coastal Southern California, and "beautifully clear skies".[4] These low humidities, combined with the warm, compressionally-heated air mass, plus high wind speeds, create critical fire weather conditions and fan destructive wildfires.[4]
Typically, about 10 to 25 Santa Ana wind events occur annually.[5] A Santa Ana can blow from one to seven days, with an average wind event lasting three days.[6] The longest recorded Santa Ana event was a 14-day wind in November 1957.[5] Damage from high winds is most common along the Santa Ana River basin in Orange County, the Santa Clara River basin in Ventura and Los Angeles County, through Newhall Pass into the San Fernando Valley of Los Angeles County, and through the Cajon Pass into San Bernardino County near San Bernardino, Fontana, and Chino.[6]
The Santa Anas are katabatic winds (Greek for "flowing downhill") arising in higher altitudes and blowing down towards sea level.[7] The National Weather Service defines Santa Ana winds as "a weather condition [in southern California] in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions".[8]
Santa Ana winds originate from high-pressure airmasses over the Great Basin and upper Mojave Desert. Any low-pressure area over the Pacific Ocean, off the coast of California, can change the stability of the Great Basin High, causing a pressure gradient that turns the synoptic scale winds southward down the eastern side of the Sierra Nevada and into the Southern California region.[9] According to one meteorology journal, "a popular rule of thumb used by forecasters is to measure the difference in pressure between the Los Angeles International Airport and Las Vegas; a difference of 9 millibars (0.27 inches of mercury) is enough to support a Santa Ana event."[5] Dry air flows outward in a clockwise spiral from the high pressure center. This dry airmass sweeps across the deserts of eastern California toward the coast, and encounters the towering Transverse Ranges, which separate coastal Southern California from the deserts.[10] The airmass, flowing from high pressure in the Great Basin to a low pressure center off the coast, takes the path of least resistance by channeling through the mountain passes to the lower coastal elevations, as the low pressure area off the coast pulls the airmass offshore.[11]
Mountain passes which channel these winds include the Soledad Pass, the Cajon Pass, and the San Gorgonio Pass, all well known for exaggerating Santa Anas as they are funneled through.[5] As the wind narrows and is compressed into the passes its velocity increases dramatically, often to near-gale force or above. At the same time, as the air descends from higher elevation to lower, the temperature and barometric pressure increase adiabatically, warming about 5 F for each 1,000 feet it descends (1 C for each 100 m).[12] Relative humidity decreases with the increasing temperature. The air has already been dried by orographic lift before reaching the Great Basin, as well as by subsidence from the upper atmosphere, so this additional warming often causes relative humidity to fall below 10 percent.[13]
The end result is a strong, warm, and very dry wind blowing out of the bottom of mountain passes into the valleys and coastal plain. These warm, dry winds, which can easily exceed 40 miles per hour (64 km/h),[citation needed] can severely exacerbate brush or forest fires, especially under drought conditions.
During Santa Ana conditions it is typically hotter along the coast than in the deserts,[14] with the Southern California coastal region reaching some of its highest annual temperatures in autumn rather than summer. Frigid, dry arctic air from Canada tends to create the most intense Santa Ana winds.[15]
While the Santa Anas are katabatic, they are not Fhn winds. These result from precipitation on the windward side of a mountain range which releases latent heat into the atmosphere which is then warmer on the leeward side (e.g., the Chinook or the original Fhn).
If the Santa Anas are strong, the usual day-time sea breeze may not arise, or develop weak later in the day because the strong offshore desert winds oppose the on-shore sea breeze. At night, the Santa Ana Winds merge with the land breeze blowing from land to sea and strengthen because the inland desert cools more than the ocean due to differences in the heat capacity and because there is no competing sea breeze.[13][16]
Santa Ana winds often bring the lowest relative humidities of the year to coastal Southern California. These low humidities, combined with the warm, compressionally-heated air mass, plus the high wind speeds, create critical fire weather conditions. The combination of wind, heat, and dryness accompanying the Santa Ana winds turns the chaparral into explosive fuel feeding the infamous wildfires for which the region is known.
Although the winds often have a destructive nature, they have some benefits as well. They cause cold water to rise from below the surface layer of the ocean, bringing with it many nutrients that ultimately benefit local fisheries. As the winds blow over the ocean, sea surface temperatures drop about 4C (7F), indicating the upwelling. Chlorophyll concentrations in the surface water go from negligible, in the absence of winds, to very active at more than 1.5 milligrams per cubic meter in the presence of the winds.[5]
During the Santa Ana winds, large ocean waves can develop. These waves come from a northeasterly direction toward the normally sheltered sides of the Channel Islands, including commonly visited Catalina and Santa Cruz islands. Normally well-sheltered harbors and anchorages such as Avalon and Two Harbors can develop high surf and strong winds that can tear boats from their moorings. During Santa Ana conditions, it is advised that boaters moor on the Southern side of affected islands or return to the mainland.[18]
A Santa Ana fog is a derivative phenomenon in which a ground fog settles in coastal Southern California at the end of a Santa Ana wind episode. When Santa Ana conditions prevail, with winds in the lower two to three kilometers (1.25-1.8 miles) of the atmosphere from the north through east, the air over the coastal basin is extremely dry, and this dry air extends out over offshore waters of the Pacific Ocean. When the Santa Ana winds cease, the cool and moist marine layer may re-form rapidly over the ocean if conditions are right. The air in the marine layer becomes very moist and very low clouds or fog occurs.[19][20] If wind gradients turn on-shore with enough strength, this sea fog is blown onto the coastal areas. This marks a sudden and surprising transition from the hot, dry Santa Ana conditions to cool, moist, and gray marine weather, as the Santa Ana fog can blow onshore and envelop cities in as quickly as fifteen minutes. However, a true Santa Ana fog is rare, because it requires conditions conducive to rapid re-forming of the marine layer, plus a rapid and strong reversal in wind gradients from off-shore to on-shore winds. More often, the high pressure system over the Great Basin, which caused the Santa Ana conditions in the first place, is slow to weaken or move east across the United States. In this more usual case, the Santa Ana winds cease, but warm, dry conditions under a stationary air mass continue for days or even weeks after the Santa Ana wind event ends.
A related phenomenon occurs when the Santa Ana condition is present but weak, allowing hot dry air to accumulate in the inland valleys that may not push all the way to sea level. Under these conditions auto commuters can drive from the San Fernando Valley where conditions are sunny and warm, over the low Santa Monica Mountains, to plunge into the cool cloudy air, low clouds, and fog characteristic of the marine air mass. This and the "Santa Ana fog" above constitute examples of an air inversion.
The similar winds in the Santa Barbara and Goleta area occur most frequently in the late spring to early summer, and are strongest at sunset, or "sundown"; hence their name: sundowner. Because high pressure areas usually migrate east, changing the pressure gradient in Southern California to the northeast, it is common for "sundowner" wind events to precede Santa Ana events by a day or two.[21]
In early December 2011, the Santa Ana winds were the strongest yet recorded. An atmospheric set-up occurred that allowed the towns of Pasadena and Altadena in the San Gabriel Valley to get whipped by sustained winds at 97 mph (156 km/h), and gusts up to 167 mph (269 km/h).[23] The winds toppled thousands of trees, knocking out power for over a week. Schools were closed, and a "state of emergency" was declared. The winds grounded planes at LAX, destroyed homes, and were even strong enough to snap a concrete stop light from its foundation.[24] The winds also ripped through Mammoth Mountain and parts of Utah. Mammoth Mountain experienced a near-record wind gust of 175 mph (282 km/h), on December 1, 2011.[23]
The winds have been implicated in some of the area's (and even the state's) largest and deadliest wildfires, including the Thomas Fire, and Cedar Fire, as well as the Laguna Fire, Old Fire, Esperanza Fire, and the Witch Creek Fire. Other major wildfires fueled by Santa Ana winds include:
The winds carry Coccidioides immitis and Coccidioides posadasii spores into nonendemic areas,[26][27] a pathogenic fungus that causes Coccidioidomycosis ("Valley Fever"). Symptomatic infection (40 percent of cases) usually presents as an influenza-like illness with fever, cough, headaches, rash, and myalgia (muscle pain).[28] Serious complications include severe pneumonia, lung nodules, and disseminated disease, where the fungus spreads throughout the body. The disseminated form of Coccidioidomycosis can devastate the body, causing skin ulcers, abscesses, bone lesions, severe joint pain, heart inflammation, urinary tract problems, meningitis, and often death.[29]
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