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Unformatted text preview: 1 Deserts and Winds I- Definition : Originally, the word desert means a place that is deserted, or uninhabited because of its inability to sustain plant or animal life due to inadequate water supply. However, modern technological advances have allowed human beings to inhabit some deserts. Therefore, a desert is best defined as an area with very low rainfall or precipitation. Areas receiving < 25 cm/year of rainfall are considered "arid lands" and qualify for the definition of "desert", whereas areas receiving between 25 and 50 cm/year are "semi-arid", and may at times be considered "deserts". It should be noted that not all deserts are characterized by hot climates; some areas in polar regions are considered deserts as these areas also receive a limited amount of precipitation. Nevertheless, most deserts are characterized by temperatures in the range of ~30 to ~55°C (with the highest temperatures having been recorded in Death valley, California and the Libyan desert at ~ 57°C). II- Area and distribution of deserts: Arid lands constitute ~ 19% of total land area on earth, whereas semi-arid regions (also known as "Steppes") constitute another 13%. A quick look at a map of the world (Fig. 1) shows that most deserts occur within two belts; one between latitudes 20 and 30 N and another between latitudes 20 and 30 S. This distribution is a function of the global directions of wind flows; these two belts are always characterized by high atmospheric pressures, where subsiding air is warmed up as it sinks producing low humidity. However, not all deserts lie within these two belts (Fig. 1), which in turn leads us to identify different types of deserts. III- Types of deserts: (a) Subtropical deserts: These are the deserts located in belts of descending dry air between latitudes 20 and 30 N and S of the equator. Examples include the Sahara, the Arabian desert, the Great Australian desert and the Kalahari desert. (b) Continental deserts: These are deserts that occur in the continental interiors, far from sources of moisture. Examples include the Gobi desert in China. (c) Rain Shadow deserts: Are deserts located on the lee sides (sides protected from wind) of mountain chains, which act as barriers between such deserts and oceans. Moist air flowing inland therefore does not reach these areas as it is "blocked" by the mountain causing precipitation on the side of the mountain facing the ocean. Examples include deserts behind the Sierra Nevada and Cascade Mountains in the western U.S.A. Fig. 2 illustrates how a rain shadow desert forms. (d) Polar deserts: These form in polar regions where the cold air is extremely low in water vapour. Examples include the dry valley region of Southern Victoria Land in Antarctica. 2 IV- Agents of weathering, erosion and deposition in deserts: In order to understand the processes that take place in deserts and their geomorphological features, we must first identify the various agents of weathering, erosion and deposition in deserts. These include:in deserts....
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This note was uploaded on 02/27/2012 for the course GEOLOGY 110 taught by Professor Staff during the Fall '08 term at Marshall.
- Fall '08