Rain Shadow Effect

A rain shadow is a landscape-induced, long-term relative lack of precipitation of an area compared to its geographic surroundings. Rain shadows exist because the landscape “gets in the way” of the horizontal flow of air. Air cannot reverse direction when it encounters significant topography. Thus, it must stop, flow over, or flow around the barrier; this has a significant influence on patterns of precipitation. The orographic effect is an enhancement of precipitation because of the rise of air over the topography. A rain shadow can be viewed as the muting of precipitation because of the landscape. Frequently, the orographic effect and rain shadows occur in tandem.

Precipitation occurs because air containing water vapor rises and cools. The air cools until it becomes saturated, clouds are formed, and then precipitation processes cause some of the water in clouds to fall toward Earth. One type of rain shadow occurs because of the strong descent of air. If stable air has been topographically lifted to the crest of a mountain, it will attempt to sink back to its original, unlifted altitude after it crests the mountain. It will undergo adiabatic warming at 10°C per every kilometer of descent because, even if originally saturated at the mountain crest, the air quickly becomes unsaturated. Adiabatic warming increases the air’s capacity to hold water vapor and decreases its relative humidity sharply away from saturation. Without a significant rise of air, the potential for precipitation is smaller and the area’s dryness enhanced by greater solar radiation due to the relative lack of clouds.

A second type of rain shadow is formed as moist air encounters topographic barriers but does not have the dynamics to rise over the barrier or divert around it. In this case the air pools in front of the barrier and the land in the lee of the barrier does not receive the moist air.

A third type of rain shadow is formed when air encounters higher terrain but is able to divert around it at low elevations. For instance, Pacific air encounters the Olympic Mountains in Washington State and is able to flow around them through the Strait of Juan de Fuca to the north and a near-sea-level lowland to the south. The result is that east of the Olympics—near the western reaches of Puget Sound—the precipitation is significantly less than in the mountains. Embedded within a regional climate of many cloudy days with light precipitation, the locals jokingly label this part ofWashington “the banana belt.” Port Townsend directly downwind of the OlympicMountains receives a bit over 65 cm (26 in) of precipitation. To the east of Puget Sound, Seattle receives nearly 100 cm (40 in) of precipitation because the air streams that have flowed around the Olympic Mountains are able to converge and cause the rise of air.