Every year around late autumn, the Santa Ana Winds in Southern California and Baja California begin to dominate the news of the western states. This is because they often bring warm, dry weather to the region which causes regional wildfires to grow faster than normal. During periods when the Santa Ana Winds blow, Southern California also experiences its hottest weather of the year.
The Santa Ana Winds are very dry offshore winds (winds moving east to west) that move into Southern California and Baja California in late fall but can last from October until March after a build-up of air pressure in the Mojave Desert and the Great Basin.
Formation of the Santa Ana Winds
Wind is defined as the movement of air across the Earth’s surface caused by differences in air pressure between two places. In the case of the Santa Ana Winds, air pressure builds up in the colder temperatures at high elevations between the Sierra Nevada and the Rocky Mountains in Utah, Nevada and Arizona. Then, when upper level winds form, the high altitude air mass pours out of the Great Basin (due to gravity) and toward Southern California (diagram).
As the air mass moves toward the Southern California coast, it becomes heated and loses its moisture. This is not because it passes through desert landscapes. however, instead the Santa Ana Winds are warmed and dried due to adiabatic heating that occurs as it descends from high elevations. During adiabatic heating, the mass of air descends in the atmosphere where it meets increasing atmospheric pressure. This compresses the air mass and increases its temperature. In addition, as the air heats and descends, its relative humidity is decreased, thus making it very dry (about 10% RH) by the time it reaches Southern California.
Although the Santa Ana Winds can occur at any time of the year, they usually form in autumn because the surface air in the higher elevations of the Great Basin and Mojave Desert begins to cool from their summer highs. This causes the change in air pressure needed to develop the drainage wind which forms as the Santa Ana Winds.
Impact of the Santa Ana Winds
When the Santa Ana Winds blow, they can often cause Southern California to become extremely warm. They can also cause more dust and particulate matter to be in the air (image). More importantly however, the Santa Ana Winds are responsible for feeding wildfires that commonly start in Southern California’s chaparral.
The chaparral landscape which dominates most of the undeveloped areas of Southern California is very dry and flammable. The combination of the warm, dry Santa Ana winds, along with the already fire-prone landscape often causes very large, intense wildfires to burn thousands of acres of land. For example, in October 2003, 721,791 acres (2,921 sq km) of land burned and was fueled by the Santa Ana Winds. In addition, in October 2007, 426,000 acres (1,720 sq km) burned. Fires such as these are common in the region and they are very often pushed along by the Santa Ana Winds.
Santa Ana Fog
In addition to the Santa Ana Winds, Southern California also experiences Santa Ana Fog. This occurs after or at the end of a Santa Ana Wind event. This fog forms as a result fast cooling of the air once the warm, dry Santa Ana winds cease. When this happens, a cool, moist marine layer forms and moves over the Southern California coastal areas. As the marine layer becomes moister, Santa Ana Fog forms.
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