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Climate - The warm moist air masses in the tropics rise...

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Climate, Altitude and Terrestrial Biomes Climate controls biome distribution by an altitudinal gradient and a latitudinal gradient. With increases of either altitude or latitude, cooler and drier conditions occur. Cooler conditions can cause aridity since cooler air can hold less water vapor than can warmer air. This is shown by Figure 10. Deserts can occur in warm areas due to a blockage of air circulation patterns that form a rain shadow, or from atmospheric circulation patters as shown in Figure 11. Warm air rises, producing low pressure areas. Cooler air sinks, producing high pressure areas. The tropics tend to be atmospheric low pressure zones the arctic areas atmospheric highs. Relative humidity is a measure of how much water an air mass at a given temperature can hold. In short, warm air can hold more moisture than can cold air. This basic physical feature of air helps explain the distribution of some of the world's great deserts.
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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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