Orographic rainfall
Orographic rainfall occurs when a topographic barrier like a mountain blocks the path of a movement of air horizontally. The air mass is forced to move upward. As the air cools rainfall is more likely. Rainfall is more likely to occur on the slope of the mountain that is forcing the air to move upwards. This wetter side is called the windward side of the slope. In the Southern Hemisphere the windward slope usually faces south.
When the air reaches the top of the slope it usually starts to descend down the other side of the mountain. Rain is unlikely on this side of the mountain. This dry side of the slope is called the leeward side of the slope and usually faces north. Occasionally the air will continue to rise when it reaches the top of the mountain and rain continues. This is not common. The term rain shadow is used to describe the drying influence affecting the leeward slope and the area beyond.
When the air reaches the top of the slope it usually starts to descend down the other side of the mountain. Rain is unlikely on this side of the mountain. This dry side of the slope is called the leeward side of the slope and usually faces north. Occasionally the air will continue to rise when it reaches the top of the mountain and rain continues. This is not common. The term rain shadow is used to describe the drying influence affecting the leeward slope and the area beyond.
