A rain shadow occurs when prevailing winds force warm moisture rich air up and over a mountain range. As the warm air is forced upwards against the face of the mountain range it begins to cool and condense. (this side of the mountain range is referred to as the windward side of the range) As the moisture in the air condenses it then falls back to the ground as rain. As the air reaches the apex of the mountain range it gradually becomes much drier. This drier air slowly begins to descend down the opposite side of the mountain range. (this side of the mountain range is referred to as the leeward side of the range) As the dry air descends it gradually warms as it mixes with warmer surface air. This warming back up pulls any remaining moisture out of the air, thus causing a zone of aridness to form on the leeward side of the mountain range. The arid zone which is formed is a rain shadow.
Here in the state of Washington we experience two different rain shadows. One which is caused by the Olympic Mountain Range, and affects places on the northwest side of the Olympic Peninsula such as Sequim. This rain shadow helps to explain why Sequim experiences on average only 18 inches of rainfall per year, and Forks, which is approximately 90 miles to the west, has on average over 120 inches of rainfall per year. The other rain shadow in Washington is caused by the Cascade Mountain Range, which affects almost the entire eastern side of the state.