Glossary

Orographic lift causes precipitation to fall on the windward side of mountains and ranges (shown here on the left), while the leeward side (right) remains dry. Arrows transitioning from red to blue, and vice versa, indicate cooling of rising air and warming of descending air.

Orographic lift takes place when a moving mass of air encounters a physical barrier such as a mountain range and is forced upwards. The rising air cools, condenses and forms clouds. With sufficient lift and moisture, precipitation generally falls on windward slopes and/or near mountain peaks. On the leeward side of mountains (downwind), air descends, warms, becomes drier, and creates an area of lower precipitation, commonly called a rain shadow.

Since the predominant wind direction in the mid-latitudes of North America is from west to east, orographic lift is a key component of precipitation patterns and snowpack depth. Consequently, western-facing (windward) slopes experience higher annual precipitation accumulations than eastern-facing (lee) slopes. Pacific frontal systems will also wring out a larger percentage of their moisture content over the Coast Mountains, with less moisture available to fall over the Columbia and Rocky Mountain ranges.

Orographic Lift