This phenomenon, the rain shadow effect, describes such aridity on the inland side of coastal mountains. Moisture blowing in from the ocean is effectively drained, and the air that moves down the ranges' eastern slopes is usually so dry it cannot produce any more rain.
Since cool air cannot retain as much water vapor as warmer air can, the excess water precipitates.
Moist air moving east from the Pacific Ocean is forced to rise over the Cascades and Sierra Nevada, cooling as it rises. Ringed by mountains that keep the rain away for much of the year, the Sonoran Desert quietly bakes. Why It Doesn't Rain Much in the Sonoran Desert Yet deluges in the past have dumped over four inches (100 mm) in a single day. Yuma, Arizona, for example, is one of the driest places on earth, averaging about 3H inches (89 mm) a year. And while not �normal,�it isn�t unusual for a single storm to produce fifty percent more rain than typically falls in a whole year. Sometimes rainfall over a summer will be recorded in small showery increments, but often the rain falls in a few large storms. The most salient feature of rainfall is not so much its rarity, but its variability, or capriciousness, to put it in terms of human personality. Rainfall here is infrequent and undependable. While there are local variations depending on elevation and proximity to mountains, this pattern basically holds for the entire area. August, September and December are the region�s wettest months May and June are the driest. In the Arizona Upland subdivision of the Sonoran Desert, rain falls about equally in two rainy seasons�a winter one in December and January, and a summer one in July through early September. Generally speaking, the Sonoran Desert averages only three to fifteen inches (76 to 400 mm) of rain a year.
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