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Unformatted text preview: The warm, moist air masses in the tropics rise upward in the atmosphere as they heat. The pressure of air rising forces air in the upper atmosphere to flow away north and south. This air at higher elevations is cooler and loses much of its moisture as rainfall. When the air masses begin to descend they heat up and begin to draw moisture from the lands they descend upon, at 30 degrees north and south of the equator. Many of the world's deserts are at approximately 30 degrees latitude, as shown in Figure 11. . Rain shadow deserts also form when cool, dry air masses descend after passing over a tall mountain range, such as the Coast Range and Sierras in California. The Sonoran desert in Arizona (shown in Figure 7) is a doubly caused desert, being at 30 degrees latitude as well as in the rain shadow of California mountains. The Tian Shan desert in China is a typical rain shadow desert, as shown by Figure 12....
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- Fall '10
- Biology, Precipitation