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Rain Shadow Effect

A rain shadow is alandscape-induced, long-term relative lack ofprecipitationof

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 direc-

tion 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 vaporrisesand cools. The air

cools until it becomes saturated, clouds are formed, and then precipitation pro-

cesses cause some of the water in clouds to fall towardEarth. One type of rain

shadow occurs because of the strongdescentof air. If stable air has been topo-

graphically lifted to the crest of a mountain, it will attempt to sink back to its origi-

nal, 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 precipita-

tion 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

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