Description
The condition exists because warm moist air rises through "orographic lifting" to the top of a mountain range. Because of decreasing atmospheric pressure with increasing altitude, the air has expanded and adiabatically cooled to the point that the air reaches its adiabatic dew point (which is not the same as its constant pressure dew point commonly reported in weather forecasts). At the adiabatic dew point, moisture condenses onto the mountain and it precipitates on the top and windward sides of the mountain. The air descends on the leeward side, but because of the process of precipitation, it has lost much of its initial moisture. Typically, descending air also gets warmer because of adiabatic compression (see Foehn winds) down the leeward side of the mountain, creating an arid region.
Read more about this topic: Rain Shadow
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