Complex sentence: In some parts of the world, deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture.
Split into simpler sentences: In some parts of the world, deserts are created by a rain shadow effect. Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.