What is orographic rainfall?

Orographic rainfall is rain that forms when moist air is forced to rise over mountains, cools, and condenses on the windward side.

Moist winds blow toward a hill or mountain. The terrain forces the air to climb. As air rises, it expands and cools. Cool air holds less water vapor, so vapor condenses into clouds and then rain. Most rain falls on the windward slope. The descending air on the other side becomes warm and dry, creating a rain shadow.

Steps

1. Moist maritime or monsoon air approaches high ground.
2. Air rises along the slope (orographic uplift).
3. Cooling to dew point produces cloud formation.
4. Condensation releases latent heat and rainfall occurs.
5. Leeward side receives much less rain; air warms adiabatically.

Examples

• Western Ghats of India: heavy rain on the west coast during the southwest monsoon; drier leeward interior.
• Himalayan foothills and Meghalaya: Cherrapunji and Mawsynram get very high orographic rain.
• Andes, Rockies, Southern Alps: classic rain-shadow landscapes.

Moist windUplift & coolingWindward rainLeeward dry