Geography
How Mountains Create Rain Shadows on Leeward Slopes
Quick fact
The leeward side of a mountain can receive less than 20% of the precipitation falling on the windward side, creating rain-shadow deserts like the Mojave in California.
Why this is interesting
Why is one side of a mountain lush and green while the other only a few kilometers away is a dry desert? The mountain itself is the answer—and it's not the rock that blocks rain, but the air.
Read the full explanation
Understanding How Mountains Create Rain Shadows on Leeward Slopes
Imagine the atmosphere as a stack of invisible parcels of air. When prevailing winds push warm, moist air toward a mountain range, the air has nowhere to go but up. As it rises, it expands because the pressure decreases with altitude. This expansion causes the air to cool (adiabatic cooling). Cooler air can hold less water vapor, so the vapor condenses onto tiny particles, forming clouds. If enough cooling occurs, droplets grow and fall as rain or snow—this falls on the windward slope. After crossing the summit, the air descends the other side (leeward). During descent, it is compressed and warms up (adiabatic warming). Because the air already lost most of its moisture, it is now very dry. The warm, dry air then evaporates any existing moisture, creating a dry, sunny region known as a rain shadow.
A deeper explanation
The mechanism is governed by the physics of ideal gases. As an air parcel rises, atmospheric pressure drops, so the parcel expands. This expansion does work against the surrounding air, using internal energy and lowering temperature. The rate of cooling is about 10°C per 1000 meters (dry adiabatic lapse rate) until the air reaches its dew point. Beyond that, condensation releases latent heat, slowing the cooling to about 6°C per 1000 meters (saturated adiabatic lapse rate). The amount of precipitation depends on initial moisture and the height of the barrier—taller mountains create stronger lifting, more cooling, and larger precipitation on the windward side, leaving a more pronounced rain shadow on the leeward side. The descending air warms at the dry adiabatic rate (10°C/1000 m) because no condensation occurs, so it becomes even warmer and drier than it was at the same elevation on the windward side. This process explains arid regions such as the Great Basin, the Atacama Desert, and the Tibetan Plateau.