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Rain Shadows, Orographic Lifting and Orographic Precipitation

Squeezing the Moisture Out of the Air

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Mountain ranges acts as barriers to the flow of air across the surface of the earth. They act to squeeze moisture out of the air. When a parcel of warm air reaches a mountain range, it is lifted up the mountain slope, cooling as it rises. This process is known as orographic lifting and the cooling of the air often results in large clouds, precipitation, and even thunderstorms.

I witnessed the phenomenon of orographic lifting on an almost daily basis during the warm summer days in California's Central Valley. East of our foothill home, large cumulonimbus clouds would form every afternoon as the warm valley air rose upslope on the west side of the Sierra Nevada mountains. Throughout the afternoon, the cumulonimbus clouds would form the telltale anvil head, signaling the development of a thunderstorm. The early evenings would sometimes bring lightning, showers, and hail. The warm valley air had been lifted, created instability in the atmosphere, and caused thunderstorms, which squeezed the moisture from the air.

Rain Shadow Effect

As a parcel of air rises up the windward side of a mountain range, it has its moisture squeezed out. Thus, when the air begins to descend the leeward side of the mountain, it is dry. As the cool air descends, it warms and expands, reducing its possibility of precipitation. This is known as the rain shadow effect and is the primary cause of leeward deserts of mountain ranges, such as California's Death Valley.

Orographic lifting is a fascinating process that keeps the windward sides of mountain ranges moist and filled with vegetation but the leeward sides dry and barren.

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