[Barnes]

Originally Posted by Cowperson

Rain Shadow . . . . .The mountains are a physical barrier to weather advancing past that point . . . . hence it can be quite wet or snowy in the mountains but very arid and dry on the plains.

Not quite describing our situation but you get the idea:

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)).

My favourite rainshadow place is Maui . . . . . . if you have the balls to circle Haleakala in a rental vehicle, past Hana, you go from rain forest to desert in as little as five miles. Remarkable.

Cowperson

Thanks smarty pants. University Physical Geography is coming back to me now.