Orographic Precipitation

Mountain ranges act as barriers to the flow of air across the surface of the earth, squeezing the 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.

The phenomenon of orographic lifting can be witnessed on an almost daily basis during the warm summer days in California's Central Valley. East of the foothills, large cumulonimbus clouds form every afternoon as the warm valley air rises upslope on the west side of the Sierra Nevada mountains. Throughout the afternoon, the cumulonimbus clouds form the telltale anvil head, signaling the development of a thunderstorm. The early evenings sometimes bring lightning, showers, and hail. The warm valley airlifts, creating instability in the atmosphere and causes thunderstorms, which squeezes the moisture from the air.

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.