|MadSci Network: Earth Sciences|
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 production. 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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