Deserts are found on the leeward side of mountains due to the what is called the rain shadow effect. When moist marine air moves inland and meets a mountain range, it is forced upward. As it rises, it cools, forming clouds and producing precipitation that falls on the windward side of the mountains.
The air that descends on the leeward side of the mountain range is much warmer and drier, producing what is termed as a rain-shadow desert. Examples of a rain-shadow desert are the deserts of the Southwestern United states.