There are two sides of the mountain i.e windward side and leeward side. The windward side is that side of a mountain subject to the prevailing wind, and is thus the wetter side. When air reaches the mountain, it rises because the mountains are in the way. As the air rises, it cools, and because cool air can carry less moisture than warm air, there is usually snow or rainfall. So windward side cause rain or snow.
Windward side: Area or side where high rainfall occurs. Wind prevailing loose their moisture on this side which leads to high rainfall on this side.
Leeward side : Area or side where low rainfall occurs.
Rain shadow is the dry area on the leeward sides of the mountainous area. As windward side blocks the moisture which prevents the rainfall on the Leeward sides.This effect is known as Rain shadow effect.
