For the Australian television series see Rain Shadow (TV series).
A rain shadow or rainshadow, or more accurately, precipitation shadow, is a dry region of land that is leeward of a mountain range or other geographic feature, with respect to prevailing wind direction. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them.
A rainshadow is warm and dry because as moist air masses rise to the top of a mountain range or large mountain, the air cools and its temperature decreases until it reaches its dew point, the point at which the moisture condenses as rain, and then falls either on the windward side or atop the mountain. This is called orographic lifting precipitation. The effect of this phenomenon is the creation of an arid region on the leeward side of the mountains. Also, the warm air absorbs moisture from the already dry and warm air (see Foehn winds). The land gets little precipitation because all the moisture is lost on the mountains. Furthermore, the warm air absorbs moisture from the already dry land.[1]
There are regular patterns of prevailing winds found in bands round the Earth's equatorial region. The zone designated the trade winds is the zone between about 30° N. and 30° S., blowing predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. The westerlies are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. The strongest westerly winds in the middle latitudes can come in the Roaring Forties between 30 and 50 degrees latitude.[citation needed]
Examples of notable rain shadowing include:
Most rainshadows in the western United States are due to mountain ranges, notably the Sierra Nevada and Cascades,[2] that intercept rain and snowfall that would otherwise reach a valley in the lee of the mid-latitude prevailing westerlies.
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