Here's why:
* Orographic Lift: As moist air masses approach a mountain range, they are forced to rise. As the air rises, it cools, causing the water vapor in the air to condense and form precipitation.
* The Wet Side: The side of the mountain facing the oncoming wind (the windward side) receives the most precipitation and is typically lush and green.
* The Dry Side: The other side of the mountain (the leeward side) is in the rain shadow. The air has already released most of its moisture on the windward side, leaving the leeward side dry and arid.
Example: The Sierra Nevada mountain range in California. The western side (windward) is known for its lush forests, while the eastern side (leeward) is a desert region called the Great Basin Desert.
