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Wind, Dust and Deserts (video) Global Geography 12 Definition of a Desert: Dry most of the year, total rainfall less than 25cm each year. Where and why deserts form: Global Weather Patterns Convection cells of air from the equator lose most of their moisture through heavy rains. When air in the cells descend at the subtropics (30ºN and S latitude), they are warm and very dry. Local Landforms A rain shadow desert forms when prevailing winds force air up over mountains, where condensation and precipitation remove moisture. Air descending on the other side is dry, creating a desert. Distance from Ocean The interior of large continents are far from oceans, so air masses lose moisture before reaching these areas. The Gobi desert in China Desertification (human activity) Vegetation absorbs suns energy, resulting in cooler air, while increasing humidity. Both conditions make rain more likely. Removal of vegetation allows soil to dry out and erode, so new plants cannot take root. If the climate is semi-arid or near a desert, desertification results. Desertification is nearly irreversible, due to difficulty in re-establishing plants. The Sahel Rainy years (1960’s) encouraged farmers to increase herds and grazing lands, while population increased as people moved in to farm. Drought (early 1970’s) caused a loss of herds. Continuing drought (1980’s) complete exposure of soil, dust storms and moving dunes 20 million people affected, Over 100 000 starved, especially in Ethiopia and Sudan Problem partly created by technology – deep drilled wells allowed population increase beyond the capacity of the environment to sustain their other activities, especially when lower rainfall returned. Prevention Water conservation techniques to save groundwater reserves Farming techniques (saving trees, shallow or minimal ploughing) that minimize erosion and water loss Eden Foundation Website http://www.edenfoundation.org/project/slideshow15.html