Orographic lift
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Orographic lift occurs when moisture-carrying clouds floating over lower-elevation terrain move over rising terrain. As the clouds are forced to rise in altitude with the air which is carrying them, they cool. This cooler air cannot hold the moisture as well as warm air and this effectively raises the relative humidity to 100%, creating precipitation.
Orographic Lift is demonstrated in many places in the world, but few examples are as clear as those in the United States. In the U.S. state of Washington, places on the sea-facing side of the mountains see over 100 inches (over 2.5 m) of precipitation per year. These places are on the side of the mountains which is in the way of storm systems, and therefore receives the moisture which is effectively squeezed from the clouds. However, on the other side of the mountains, sometimes as little as 15 miles (25 km) away, annual precipitation can be as low as 8 inches (200 mm) per year.