A third type of desert is found on the leeward, or back, side of some large mountain ranges, such as the sub-Andean Patagonian Gran Chaco and Pampas of Argentina, Paraguay, and Bolivia. A similar effect is partly responsible for the formation of the Mojave and Sonoran Deserts of the United States. These deserts form because as moist air masses move toward the mountain ranges, they must rise to move over the ranges. As the air rises it cools, and cold air cannot hold as much moisture as warm air. The clouds thus drop much of their moisture on the windward side of the mountains, explaining why places like the western Cascades and western sierras of the united states are extremely wet, as are the western Andes in Peru. But the eastern lee, or back, sides of these mountains are extremely dry. This is because as the air rose over the fronts, or windward, sides of the mountains, it dropped its moisture. As the same air descends on the lee side of the mountains, it warms and can hold more moisture than it has left in the clouds. The result is that the air is dry and it rarely rains. This explains why places like the eastern sub-Andean region of south America and the sonoran and Mojave Deserts of the western united states are extremely dry.
Rain-shadow deserts tend to be mountainous because of the way they form, and they are associated with a number of mass wasting hazards such as landslides, debris flows, and avalanches. occasional rainstorms that make it over the blocking mountain ranges can drop moisture in the highlands, leading to flash floods coming out of mountain canyons into the plains or intermountain basins on the lee side of the mountains.
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