Orographic lift

When a moving air mass encounters a mountain barrier, the terrain can deflect it upward. This terrain-driven ascent is called orographic lift or orographic uplift. Rising air expands under lower atmospheric pressure and cools. If the air contains enough water vapour and cools to saturation, vapour condenses into cloud droplets or ice crystals. Continued cooling and cloud development can produce rain or snow. Orographic lifting can also create extensive cloudiness and increased precipitation over higher ground, but precipitation is not guaranteed: sufficiently dry air may rise without forming cloud or rain.

Precipitation generated by air rising over mountains is called orographic precipitation. Much of it often falls on the upwind, or windward, side of a range, particularly when moist winds blow across the terrain. Some precipitation can pass beyond the ridge. As air descends on the leeward side, it is often drier after losing moisture on the ascent and warms as pressure increases. The resulting contrast can make the lee region relatively dry, a pattern known as a rain shadow. Rain shadows vary in strength and location: atmospheric conditions, mountain shape and orientation, and the air mass's initial moisture all matter.

The amount and placement of orographic precipitation depend on the moisture available in the incoming air, the wind direction and strength, the height and shape of the terrain, and the temperature profile that determines whether precipitation falls as rain or snow. Winds directed toward and across a ridge can produce strong uplift, while winds parallel to a range may produce a different pattern. Orographic effects interact with larger weather systems, so a mountain range does not generate the same precipitation pattern in every storm.

Orographic lift helps explain why nearby mountain slopes can receive sharply different amounts of rain or snow and why some high terrain is frequently cloud-covered. These variations influence local water supply, snow accumulation, ecosystems and hazards such as flooding or heavy snowfall. The related rain-shadow effect is one factor that can contribute to arid conditions downwind of a range; it is not by itself a complete explanation for every desert or dry region.