How do mountain ranges create rain shadows and deserts?

Rain shadows happen when a mountain range blocks moist winds and causes most of the precipitation to fall on the windward side. As the air rises, it cools, water vapor condenses, and clouds form rain or snow. By the time the air crosses the summit, it has lost much of its moisture and warms as it descends, making the leeward side much drier. This process helps create deserts such as the Patagonian Desert east of the Andes and dry basins in the lee of major ranges around the world. Rain shadows are one reason deserts often sit next to high mountains.