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Rainshadow
08/28/2002
Listen in RealAudio 
Think of the Pacific Northwest, and images of dripping temperate rainforests and flocks of urban
umbrellas emerge. However, in a few areas such as within the British Columbia capital of
Victoria, annual rainfall is low, even semiarid, accumulating less than most major American
cities.
Hi, I'm Bryan Yeaton and you're listening to The Weather Notebook.
A large region of precipitation minimum, called the rainshadow, covers the sunken valley of the
Strait of Georgia between the coastal and continental ranges of Washington and British
Columbia. This rainshadow reaches from Victoria on Vancouver Island across several BC Gulf
Islands and the Washington State islands of Orcas, San Juan and Whidbey.
When Pacific maritime air ascends to cross the formidable coastal mountains of Washington's
Olympic Peninsula and southern Vancouver Island, it drops rain on the western slopes by the
buckets. In the Olympic Mountains, totals can reach 200 inches annually. Further north on
Vancouver Island as much as 250 inches may fall.
Much moisture is lost from the airmass in the crossing, however, and on the leeward side, the air
descending into the Strait of Georgia basin warms through compression, and cloud droplets in
the airstream evaporate. Thus, rain formation is inhibited, and often prevented, within the rainshadow region.
Clear skies may prevail all day around the basin's center while a few miles west, heavy rain
showers fall.
As a result, in Victoria's southeastern corner, annual rainfall totals around 24 inches while about
20 miles west of downtown as much as 58 inches accumulates annually. On Whidbey Island, the
total is only 18 inches. In comparison, just outside the rainshadow zone, Seattle receives 37
inches and Vancouver, British Columbia, 46 inches.
Thanks to contributing writer Keith Heidorn of Victoria, British Columbia. The Weather Notebook is a production of The Mount Washington Observatory and is supported generously by Subaru of America and The National Science Foundation. Thanks today to marketing manager, Melody Nester.
Today's Links
How Mountains Influence Rainfall Patterns
http://www.usatoday.com/weather/tg/wrnshdw/wrnshdw.htm
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