Orographic Precipitation

Orographic precipitation, also called relief precipitation, falls when moist air is forced to rise over rising terrain, either deflected upward by a mountain barrier or lifted by daytime heating along a slope. As the air climbs it expands and cools, and once it reaches its dew point the water vapor condenses into cloud droplets that can grow heavy enough to fall as precipitation; steeper slopes and narrower mountain barriers produce stronger updrafts and heavier rainfall. The effect creates sharp local contrasts, such as the windward uplands of Hawaii receiving over 100 inches of rain a year while leeward Waikiki gets under 20 inches, or the Pennines of England, where the western, windward slopes receive far more rain than the eastern, leeward side around Leeds.