Description

The Santa Ana winds, also sometimes called the devil winds, are strong, extremely dry downslope winds that originate inland and affect coastal Southern California and northern Baja California. They originate from cool, dry 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. They often bring the lowest relative humidities of the year to coastal Southern California, and "beautifully clear skies". These low humidities, combined with the warm, compressionally-heated air mass, plus high wind speeds, create critical fire weather conditions and fan destructive wildfires. Typically, about 10 to 25 Santa Ana wind events occur annually. A Santa Ana can blow from one to seven days, with an average wind event lasting three days. The longest recorded Santa Ana event was a 14-day wind in November 1957. 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.

Formation

The Santa Anas are katabatic winds arising in higher altitudes and blowing down towards sea level - strong, hot, dust-bearing winds descending to the Pacific Coast around Los Angeles from inland desert regions.

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. A popular rule of thumb used by forecasters is to measure the difference in pressure between Los Angeles International Airport (KLAX) and Las Vegas (Nevada) Harry Reid International Airport (KLAS); a difference of 9 millibars (0.27 inches) is enough to support a Santa Ana event. 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. 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.

Mountain passes which channel these winds include the Soledad Pas, the Cajon Pass and the San Gorgonio Pass, all well known for exaggerating Santa Anas as they are funneled through. 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 1 °C for each 100 m. 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.

This map illustration shows a characteristic High pressure area centered over the Great Basin, with the clockwise anticyclonewind flow out of the high-pressure center giving rise to a Santa Ana wind event as the airmass flows through the passes and canyons of Southern California, manifesting as a dry northeasterly wind. [Source: Wikicommons, Author: NOAA's National Weather Service]

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 kts, 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, 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.

While the Santa Anas are katabatic, they are not Fohn 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 Föhn).

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.

Santa Ana winds are associated in the public mind with dry hot weather, but cold Santa Anas not only exist but have a strong correlation with the highest "regionally averaged" wind speeds.

Flight safety considerations

Strong low level winds may cause low level turbulence especially around mountain ridge lines. Lower visibility due to desert dust may be exacerbated by smoke from associated wildfires. See the separate advice in the article Wildfires: Guidance for Flight Crews.

Although rare, when the Santa Ana winds cease and if there is a rapid and strong reversal in wind gradients from off-shore to on-shore winds, then thick sea fog can rapidly envelope coastal cities in as quickly as 15 minutes bring low visibility conditions to airports like KLAX.