The Sonoran Desert is so dry because high temperatures evaporate the little rainfall it gets. The Sonoran Desert gets such little rainfall for several reasons:
• Its 30º latitude - a horse latitude. A horse latitude is created because warm, wet air near the equator rises, cools, and drops its moisture in heavy rains (creating tropical rainforest areas near the equator). By the time the air reaches the high altitudes, it is cold and dry and can not rise further, so it spreads out and moves toward the poles and near the 30º north and 30º south, it begins to sink toward the earth's surface., preventing moisture from reaching the area from elsewhere. Many of the worlds deserts are found at these latitudes.
• A double rain shadow. Wet air moving up windward mountain slopes expands and cools, clouds form, and the moisture falls out as rain or snow. By the time the air descends the leeward slopes (contracting and reheating as it goes) it has been drained of moisture. As a result, lands down-wind of mountain ranges tend to be dry.
