Another local motion in the atmosphere is caused by the presence of a mountain range and the resulting precipitation pattern that occurs when prevailing winds are consistently pushed over a specific side of the mountain barrier. In the midlatitudes, when low-pressure cells move eastward they interact with the topography. This is common in the western United States when air masses encounter the massive Rockies. Air flow eastward from over the Pacific Ocean carries huge amounts of water vapor. When this air encounters mountain ranges, its forward movement is blocked. The air mass is then forced to rise to higher elevations to get over the mountain range.
As the air rises, it cools. Water vapor then condenses from the cooling air and produces heavy rainfall on the side of the mountain that the air was forced up—the windward side. This is called orographic precipitation. Once the air reaches the crest of the mountain and begins moving down the other side, most of the water vapor has been depleted, making the air very dry. As the air sinks down the mountainside, it is compressed and warmed. At this point, the air can store more water vapor without condensation occurring. Because of this, the lee or rain shadow side of mountain ranges is an area of lower precipitation and can look much different than the windward side. This is one of the factors that causes the interior areas of the United States to be dry. If global warming alters the dynamics of moisture availability and wind systems, this could cause further dryness.
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