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Rain Shadows and Tropical Deserts

Topographic heterogeneity also contributes to the formation of deserts, especially of those that occur outside the mid-latitude belts. In the tropics, for example, when the moisture-laden tropical trade winds reach continental mountain ranges they cool as they ascend, condensing fog and drizzle that feed montane cloud forests. Once the winds pass the mountain divide, they start compressing and warming-up again in their descent, but, having left behind their original moisture, they become hot and dry. Thus, while the windward slopes of most tropical mountain ranges are covered by cloud forests, the leeward part, known as the "rain shadow" of the mountains, is covered by arid scrub.

The rain shadow effect is largely responsible for many tropical arid lands that seem to defy the rule that deserts are only found in the earth's midlatitudinal reaches, such as the Sechura Desert in Peru and Ecuador, the Caatinga scrub in equatorial Brazil, or the Tehuacán Valley desert in southern Mexico, a hotspot for cactus biodiversity. They are also responsible for some high-latitude cold deserts, such as the Great Basin, Patagonia, and the deserts of Central Asia.

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