Here's how it works:

1. Moist air rises: As moist air moves over a mountain range, it is forced upwards.

2. Cooling and condensation: As the air rises, it cools. Cool air holds less moisture, so the water vapor in the air condenses, forming clouds and precipitation on the windward side of the mountain (the side facing the wind).

3. Dry air descends: The air that has lost its moisture then descends on the leeward side of the mountain (the side sheltered from the wind). As it descends, it warms and becomes drier.

4. Little precipitation: The dry air on the leeward side results in a rain shadow, characterized by significantly less precipitation than the windward side.

This effect is responsible for the differences in climate and vegetation on either side of many mountain ranges around the world.