The region on the lee side of a mountain where the precipitation is noticeably less than on the windward side. An example of the rain shadow effect is in the Pacific Northwest where wind blowing off the Pacific Ocean is forced to rise over coastal mountains. As the wind rises on the western or windward side of the mountain, it cools making it unable to hold moisture, so it rains. The wind reaches the summit of the mountain range and begins it's descent along the leeward side of the mountain. It is now dry and warming as it descends.

The rain shadow effect is one of the factors causing the western interior of the United States to be so arid. Other mountain ranges that produce a rain shadow effect are the Andes Mountains in South America and the Atlas Mountains in northern Africa.

The formation of lenticular clouds is also common in rain shadow areas.