Here's why:

* Orographic Lift: As moist air masses approach a mountain range, they are forced to rise over the slopes. This upward motion is called orographic lift.

* Cooling and Condensation: As air rises, it cools. Cool air holds less moisture, so the water vapor in the air condenses, forming clouds and precipitation.

* Windward Side Precipitation: The side of the mountain facing the prevailing winds (the windward side) receives the brunt of this precipitation.

* Leeward Side (Rain Shadow): The air that descends on the other side of the mountain (the leeward side) has already released much of its moisture. This drier side experiences a "rain shadow" effect, often with less precipitation.

Example: In the US, the Sierra Nevada mountains are a classic example. The western slopes receive ample rainfall, while the eastern slopes (the rain shadow) are much drier.