Orographic precipitation - The second precipitation producing mechanism is described as orographic, which means, 'related to mountains.' Prevailing winds or other moving masses of air may be forced to flow over mountain ranges. As the air rises on the windward side of the range, it is cooled at the adiabatic rate. If cooling is sufficient, precipitation will result. After passing over the mountain summit, the air will begin to descend the lee side of the range. Now it will undergo a warming through the same adiabatic process and, having no source from which to draw up moisture, will become very dry. A belt of dry climate, often called a rain shadow, may exist on the lee side of the range. Several of the important dry deserts of the earth are of this type.
Dry, warm foehn winds (Europe) and Chinook winds (northwestern North America), which occur on the lee side of a mountain range, may cause extremely rapid evaporation of snow or soil moisture. These winds result from turbulent mixing of lower and upper air in the lee of the range. The upper air, has little moisture to begin with, is greatly dried and heated when swept down to low levels.
Much orographic rainfall is actually of the convectional type, in that it takes the form of heavy convectional showers and thunderstorms. The storms are induced, however, by the forced ascent of unstable air as it passes over the mountain barrier.
Cyclonic precipitation - A third type of precipitation is cyclonic. This topic cannot be fully understood until the entire subject of cyclonic storms and fronts has been developed. It will suffice here to note that in middle and high latitudes much of the precipitation occurs in cyclonic storms or eastward-moving centres of low pressure into which air is converging and being forced to rise.
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