In the western half of the United States, the Rocky Mountains play a major role in the environment. Warm, moist air blowing off of the Pacific Ocean must rise in altitude to cross the mountain range.
As the air rises, it cools. Because cooler air cannot hold as much moisture, or precipitation, as warm air, precipitation falls on the western slopes of the Rocky Mountains.
Once the air crosses the peaks of the mountains, it has lost much of its moisture, so relatively little rain and snow fall on the eastern slopes of the mountains.