A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. When wind blows up a mountain, the air is moving into an area that has lower air pressure, making the rising air expand and cool. As air continues to rise up the mountain, it cools to the point its at its maximum capacity to hold water vapor. When air is at its maximum capacity, it is at 100% relative humidity. Further cooling will cause excess water vapor to condense or turn into liquid water, forming clouds. If enough condensation occurs, rain or snow will fall out of the clouds. The side of the mountain where wind is blowing up the mountain is called the windward side. Many areas of the world experience large amounts of rain or snow where air is forced up mountains. However, on the leeward side of the mountain, the air descends and warms, causing it to become drier and creating a rain shadow.

Examples of rain shadow areas include:

• The Dungeness Valley around Sequim and Port Angeles, Washington, which lies in the rain shadow of the Olympic Mountains. The area averages 10–15 inches of rain per year. The rain shadow extends to the eastern Olympic Peninsula, Whidbey Island, parts of the San Juan Islands, and Victoria, British Columbia, which receive between 18-24 inches of precipitation each year.

• The Mojave, Black Rock, Sonoran, and Chihuahuan deserts, which are all in regions that are rain shadowed.

• The Owens Valley in the United States, behind the Sierra Nevada range in California.

• Hawaii also has rain shadows, with some areas being desert. Orographic lifting produces the worlds second-highest annual precipitation record, 12.7 meters (500 inches), on the island of Kauai; the leeward side is understandably rain-shadowed. The entire island of Kahoolawe lies in a rain shadow, receiving only about 18 inches of rain per year.