The leeward side of a mountain is dry because of a phenomenon called the rain shadow effect. Here's how it works:

1. Moist Air Rises: As moist air from the prevailing wind approaches a mountain, it is forced to rise.

2. Cooling and Condensation: As the air rises, it cools due to lower atmospheric pressure at higher altitudes. This cooling causes the water vapor in the air to condense, forming clouds and leading to precipitation on the windward side of the mountain.

3. Dry Air Descends: The air, now relatively dry, descends on the leeward side of the mountain.

4. Adiabatic Warming: As the dry air descends, it warms due to increasing atmospheric pressure. This warming makes the air even drier as its capacity to hold moisture increases.

Result: The windward side of the mountain experiences abundant rainfall, while the leeward side remains dry, creating a "rain shadow" where precipitation is significantly reduced.

Example: The eastern slopes of the Sierra Nevada mountains in California receive copious rainfall, while the western slopes (the leeward side) are much drier, leading to the development of the Mojave Desert.