Rain Shadows A Rain Shadow is a Natural Phenomenon that occurs when a Region of Mountains blocks a Rain producing weather system.The Rain Shadow is the effect produced on the Lee or opposite side of the incoming weather system. First winds push the clouds toward the mountain range, then the warm air rises and cools as it is pushed over the mountain. At the crest or Top of the mountain the air condenses and precipitates, causing rainfall on the windward side of the mountain. The air that passes over the mountain loses most of its moisture producing a dry weather system, or Rain Shadow on the opposite side of the mountain.