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. Cold air sinks. 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 the air will pick up some of it. There is not enough to make a good, wet rain. 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.