Ask A Scientist© Weather Archive

name        John
status      educator
age         50s

Question -
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
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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, www.jsimmonsalgoma@yahoo.com

Dr. Cook
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