The highest parts of the mountains probably do not get much water, but there may be some showers or snow. Warm air continues to rise from lower elevations, pushing the high cool air upward. Eventually this air rises above the mountain tops and begins to flow downward. By now this air at the mountain tops is quite cold. As this air flows downward, it warms up somewhat.
Warm air picks up moisture .
Well, there's not a lot of moisture on this side of the mountains, but there is probably some in the soil. The air will pick up some of the soil moisture and carry it away. This side of the mountain is dry. There may be grass, semi-arid shrubs, and desert on this side of the mountains. Dry areas like this are called the rain shadows.