Here's why:
* Orographic Lift: As moist air rises up the windward side of a mountain (the side facing the wind), it cools. This cooling causes the air to lose its capacity to hold moisture, leading to condensation and precipitation.
* Rain Shadow: The air that has already released its moisture descends on the leeward side of the mountain. This descending air warms and becomes drier, creating a rain shadow where precipitation is significantly lower.
This phenomenon is a major factor in regional climate differences. For example, the eastern slopes of the Sierra Nevada mountains in California are lush and green, while the western slopes (in the rain shadow) are much drier.
