NWS Forecast Office San Diego, CA

Do you know what causes Santa Ana Winds? When a trough and cold front moves through the Western US, cold air may settle into the Great Basin (NV/UT). Because cold air is more dense than warmer air, it creates an area of surface high pressure in this region. The difference in pressure, often referred to as the pressure gradient, between the Great Basin High and relatively lower pressure over Southern California generates wind that moves from higher to lower pressure. In this case, the wind would be coming from the north or northeast. The greater the pressure difference between over a given distance, the stronger the wind. As the air flows between the local mountain passes and canyons, it is “squeezed” and speeds up, much like when you put your thumb over the nozzle of a hose. Under the right conditions, air moving over the tops of the mountains is forced down the coastal slopes (referred to as downslope winds), also causing the winds to strengthen. Now why are they usually hot and dry? As the air flows down from the higher elevation mountains and passes towards the lower elevation coastal areas, the air is compressed and heats up – at roughly the rate of 5.5 degrees per 1,000 ft elevation loss. The dew point also drops at a rate of about 1 degree per 1,000 ft loss. With heating and drying going on, the humidity drops rapidly, leaving us with warm, dry, and windy weather. In some cases (when the marine layer is gone and winds are strong enough to make it to the coast), the beaches end up being the hottest location because they are at the lowest elevation. More elevation loss = more heating! Overall, how hot it gets depends on how warm the desert air is before it heads over the mountains and into the valleys. During the Fall, the air is still warm, allowing temperatures in the coasts/valleys to easily climb into the 90-100 degree range. Once we start to head into late December and January, however, the air over the deserts can be much colder – meaning even after the air flows down to lower elevations and the temperature increases it can still be quite cool.

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