The windward side of a mountain faces the wind while the leeward side faces away from the prevailing wind. The climate on different sides of the mountain can vary greatly.
As air passes across the windward side of the mountain, most of the moisture is drawn out of it. This is because the air cools as it rises up the slope of the mountain and condenses, leading to clouds and rain. On the leeward side of the mountain, the climate is warmer and drier; as the air moves down the opposite side of the mountain, it loses its moisture and warms up. Scientists call this the rain shadow effect.