A Rainshadow is an area which is unusally dry due to nearby geographic features. A rainshadow is usually caused by a mountain range, but can they can be caused by other things as well, although to a much lesser degree.

Rainshadows can be observed in the western United States and also in southern central South America. These particular rainshadows are caused by mountain ranges, notably the Sierra Nevada and Cascades in the former, and the Andes in the latter. A rainshadow caused by mountain ranges works in a simple way. As a storm approaches the mountain, the air in the storm undergoes orographic lift, which causes all of the rain to be wrung out of the storm system as it crests the mountains, leaving no moisture to fall on the other side, this area has become a rainshadow.

There are other situations in which rainshadows occur, and those which occur in central Asia are a prime example. In this case, storms sweep in on a eastward jet stream, usually coming off of the Mediterranean Sea or some other area in the vicinity of Europe. As these storms cross dry land, they gradually lose their moisture due to constant precipitation as they cross vast continental expanses. By the time these storm systems reach deep central Asia, they have little or no water left. In the United States, these storms could pick up additional moisture from the Gulf of Mexico, but the lack of a major body of water extending inland leaves the Asian storms dry, the central areas have become rainshadows.