West wind drive in2/2/2024 As the wind narrows and is compressed into the passes its velocity increases dramatically, often to near- gale force or above. 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. 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. 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. 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." Warm, dry air flows outward in a clockwise spiral from the high pressure center. 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. Santa Ana winds originate from high-pressure airmasses over the Great Basin and upper Mojave Desert. This map illustration shows a characteristic high-pressure area centered over the Great Basin, with the clockwise anticyclone wind 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. The National Weather Service defines Santa Ana winds as "a weather condition in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions". The Santa Anas are katabatic winds (Greek for "flowing downhill") arising in higher altitudes and blowing down towards sea level. 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. The longest recorded Santa Ana event was a 14-day wind in November 1957. A Santa Ana can blow from one to seven days, with an average wind event lasting three days. There are typically about ten to 25 Santa Ana wind events annually. 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. 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 originate from cool, dry high-pressure air masses in the Great Basin. The Santa Ana winds (sometimes devil winds) are strong, extremely dry downslope winds that originate inland and affect coastal Southern California and northern Baja California. Los Angeles is in the upper left of this image, while San Diego is near the center. The Santa Ana winds sweep down from the deserts and across coastal Southern California, pushing dust and smoke from wildfires far out over the Pacific Ocean.
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