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Name: John
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2000-2001

What causes the Santa Anna winds ?

Dear John-

The Santa Anna winds are caused by an atmospheric pressure pattern that sometimes develops over the southwestern U.S. High pressure builds over the intermountain region to the east of southern California, as low pressure deepens off the coast. As the winds flow clockwise around the high towards the area of low pressure to the west, they descend from higher altitudes inland to near sea-level in Southern California. This descent causes temperature warming in the air due to the pressure increase with the descent. As the air warms during descent it becomes much drier, because no moisture is added to the air mass at this time.

Depending on the pressure gradient between the high pressure center and the low pressure off the coast, very strong winds with an easterly component can develop...winds sometimes as strong as 100 mph. It is a very hot and dry wind, as described above. The Santa Anna winds are a type of chinook winds that occur in many parts of the world near mountain ranges.

Wendell Bechtold, Meteorologist

The Santa Ana (one "n") winds occur in Autumn, when a high pressure system forms over the Pacific northwest and low pressure occurs over Mexico. The north-south pressure gradient causes strong winds that spill over the Sierra Nevada mountains from the northeast. Just as with the Chinook winds that sometimes affect Colorado's east slope, the air descends from the mountains and compresses, causing it to warm and decrease in relative humidity. Since the air is already fairly dry (in the absolute sense) because of it's origin in the dry desert of Nevada, the warming produces very low relative humidities, downwards to single digits. The Santa Ana has a great capacity to produce large amounts of evaporation and vegetative transpiration that can dessicate vegetation and dry the soil. This often results in high fire dangers in California and the resultant fires that we often see on the TV news.

sent by John A. Simons,

Dr. Cook

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