How do prevailing winds form and where do they flow? Earth is encircled by belts of prevailing winds that develop because the Sun warms Earth most intensely at the equator, causing air to rise there and flow toward the poles. As the air moves toward the poles, it gradually cools and begins to sink. This cool, dense air then flows back along Earth’s surface to the equator. The Coriolis effect, produced by Earth’s rotation, deflects these prevailing winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
How do winds transport and erode sand and finer-grained sediments? Winds can pick up and transport dry particles in a manner similar to flowing water. Air flows are limited, however, in the size of particles they can carry (rarely larger than sand grains) and in their ability to keep particles in suspension. These limitations result from air’s low viscosity and density. Windblown materials include volcanic ash, quartz grains, and other mineral fragments such as clay, as well as organic materials such as pollen and bacteria. Wind can carry great amounts of sand and dust. It moves sand grains primarily by saltation and carries finer-grained dust particles in suspension. Sandblasting and deflation are the primary ways in which winds erode Earth’s surface.
How do winds deposit sand dunes and dust? When winds die down, they deposit sand in dunes of various shapes and sizes. Dunes form in sandy desert regions, behind beaches, and along sandy floodplains, all of which are places with a ready supply of loose sand and moderate to strong winds. Dunes start as sand drifts in the lee of obstacles and may grow to heights of up to 250 meters, though most are tens of meters in height. Dunes migrate downwind as sand grains saltate up their gentler windward slopes and fall over onto their steeper downwind slip faces. The shapes and arrangements of sand dunes are determined by the direction, duration, and strength of the wind and by the abundance of sand. As the velocity of dust-laden winds decreases, the dust settles to form loess, a thick blanket of fine particles. Loess layers have been deposited in many formerly glaciated areas by winds blowing over the floodplains of streams formed by glacial meltwater. Loess can accumulate to great thicknesses downwind of dusty desert regions.
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How do wind and water combine to shape the desert environment and its landscape? Deserts occur in subtropical zones of sinking air, in the rain shadows of mountain ranges, and in the interiors of some continents. In all these places, the air is dry, and rainfall is rare. In deserts, physical weathering is predominant, whereas chemical weathering is minimal because of the lack of water. Most desert soils are thin, and bare rock surfaces are common. Wind plays a larger role in shaping the landscape in deserts than it does elsewhere, but streams are responsible for most erosion in deserts even though they flow only intermittently. Playa lakes, which form in arid mountain valleys or basins, deposit evaporite minerals as they dry up. Among the prominent features of desert landscapes are pediments, which are broad, gently sloping platforms eroded from bedrock as mountains retreat while maintaining the steepness of their slopes.