MadSci Network: Earth Sciences

Re: Desert rain

Date: Tue Jul 28 17:06:45 1998
Posted By: Nezette Rydell, forecaster,National Weather Service
Area of science: Earth Sciences
ID: 901387215.Es

Your question is a good one.  First a little background information, then 
an answer to your specific questions.

Most climate 
classification systems define three basic arid climates on our planet. 
 The first are tropical dry climates found between 20 and 25 degrees North 
and South Latitudes.  These generally coincide with the location of the 
semipermanent subtropical anticyclones...high pressure areas that spend 
much of the time in the same general area due to the general circulation of 
the global atmosphere.  In the sinking air under these 'highs', the airmass 
warms and dries and little or no precipitation forms.  These areas are too 
close to the equator to be affected by frontal systems, and too far from 
the Intertropical Convergence Zone (ITCZ) for tropical rains.  Examples of 
tropical dry climates include the Sahara, northern Austrailia, Arabia, 
southwestern Africa, and northwestern Mexico.

The second basic type are those associated with what are called the Western 
Littorals.  These unusually cool near-shore winds are found along the 
western edges of continents. These breezes, cooled over cold ocean currents 
that occur on the western side of major continents, contribute to a very 
stable atmosphere where convection is almost non-existent. This air can be 
quite humid, producing the famous San Francisco area fog and a similiar fog 
called Peruvian Mist in the southern hemisphere but produces very little 
rain.  Examples these climates include Southern California, the desert 
coasts of Peru and Chile, and the western coast of Africa.

The third basic type are called mid-latitude dry climates. These are 
usually located in the interior of continents out of the normal reach of 
moist flow from the oceans.  The dryness in these areas can be intensified 
by the blocking effects of north-south mountain chains as in the US and in 
Siberia.  These climates receive some rain on a seasonal basis...but are 
generally dry. 

Now to address your question.  Libya and the Sahara fall into the first 
category, tropical dry climates and Mauritania into the second.  Death 
Valley's climate is a product of both Western Littorals and drying 
mountains...which may help explain it's extreme dryness.       

The remaining point has to do with sea breezes.  Since the soil warms 
faster than the ocean areas, a sea breeze will indeed form on western 
shores, blowing from water to land.  However, this type of breeze is a 
small scale feature, and shallow, not more than 3000-5000 ft deep.  It is 
rarely able to overcome the predominating factors inhibiting rainfall 

For more detailed descriptions of these climates and more information on 
the factors that produce each one of them, consult a basic climate textbook 
in your local library. 

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