Last month, I wrote about gap winds, winds that accelerate as they push between high topographical features. This month, I'll take a look at one consequence when those winds that flow over high mountain ridges instead: the rainshadow.
Through most of this article, I will refer to my home territory in the Pacific Northwest where the rainshadow effect is especially expressed, but rainshadows can be found on the lee of most mountain ranges and high individual peaks, such as the volcanic peaks that make up the Hawaiian Island chain. (In the temperate zone of the Northern Hemisphere, the rainshadow is usually, but not always, found along the eastern slopes of the ranges because the prevailing winds are westerly.)
The process starts on the windward side of the range where moist air is forced to rise over the mountain ridges. In the ascent, the air cools and its moisture condenses, forming extensive clouds and rain/snow. When ranges are high, like the coastal of western North America, much of the moisture in the air falls out on the windward slope. In the Pacific Northwest, this provides the conditions for the extensive temperate rainforests for which the region is known.
After crossing the ridge lines, the moisture-depleted air begins to descend and, in the descent, warms through compression. The downward motion has two effects on the air mass. First, by warming, the air re-evaporates a good portion of the liquid moisture remaining as raindrops/snowflakes and clouds. Second, the downward flow inhibits the formation of thick, precipitation-rich clouds, and thus the potential for precipitation decreases. The region of descending air and decreased precipitation is what we term the rainshadow.
Where rainshadows regularly form, the differences in precipitation can be extreme over rather short distances, in places a factor of ten in annual precipitation over a hundred kilometres or so. Some examples from the Pacific Northwest point this out ably. Spokane, in Washington's eastern arid zone received less than half of Seattle's annual precipitation total (16.7 inches (420 mm) to 37.1 inches (942 mm)); Kamloops in interior British Columbia receives less than a quarter of Vancouver's precipitation (279 mm (11.0 inches) to 1199 mm (47.2 inches)).
Storm systems moving out of the Gulf of Alaska (see my October 2003 discussion) push moist air along the western American coast from southern Alaska through British Columbia and down into Washington and Oregon and northern California. The air not only meets the coastal mountains but several parallel ranges before reaching the plains and prairies of the central continental interior. Each range robs the air of a little more moisture, producing arid
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