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Rainshadow
Wed Aug 17, 2005
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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