What Is Orographic Rainfall?

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Orographic rainfall is rain that is produced from the lifting of moist air over a mountain. The moist air rises and cools, producing orographic clouds, which are the source of the rain. Most orographic rain falls upwind of the mountain range, with some also falling a short distance downwind. This process can produce any type of precipitation, including snow, sleet, hail or freezing drizzle.

Orographic precipitation, also known as relief precipitation, typically affects mountainous areas near coastlines. When these areas are subjected to consistent winds, a wetter climate is generally present on the windward side and a drier, almost desert-like climate is present on the downwind side. This is due to orographic precipitation, which removes the wetness from the air. This dry region is also referred to as a rain shadow.

Many people do not realize that the Hawaiian Islands are affected significantly by orographic precipitation. Interior Hawaiian uplands may receive more than 100 inches of precipitation due to orographic precipitation, while coastal areas receive 20 to 30 inches of precipitation. The tops of moderately high uplands may receive as many as 475 inches of precipitation, while on the downwind side, rain shadow areas receive fewer than 20 inches of precipitation.