After the warm air picks up moisture and rises and then drops this moisture once it passes the crest of a mountain range, the air then decends again and becomes warmer. Its water holding capacity increases and it actually picks up what moisture is in the valley area and takes it up the next western slope. So we can see that the western slopes are abundant in rainfall and the eastern slopes are scarce in rainfall. This phenomena of the eastern slope being scarce in rainfall is known as a rain shadow. We say then that the eastern slope of the mountain is in a rain shadow. That is, very little precipitation falls on the eastern slope because most of it has fallen out of the air on the western slope.

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