Answer:
The windward side is a lot rainier than the leeward side of a mountain. This is because the prevailing winds come up and the cool air tries to hold that much moisture. But since cool air can't hold as much moisture as warm air, it begins to precipitate on the windward side (the side of mountain that is facing the prevailing winds). The dry air moves on to the leeward side (the side of a mountain that is not facing prevailing winds) and stays there, usually causing deserts to form. A good example is eastern Oregon and Washington.
You COULD say: Their deserts are the result of the rain-shadow caused by the mountain ranges.