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conditions east of the ranges. On the eastern slopes of this great continental backbone, moisture occasionally
flows upward from the Gulf of Mexico in the currents and eddies of storm systems formed off the Rockies,
but this eastern moist air is swimming upstream and does not provide the level of moisture seen on the
opposite coast in the piedmont reaches of the Appalachian and associated mountain ranges where no
rainshadows are found.
As a result, each western range has a rainy (relatively) western slope and a rainshadow along the eastern slope, with the total precipitation accumulated declining with each successive ridge crossing. The accompanying figure shows the impact of mountain ranges from the Pacific shores to the prairies of Alberta across British Columbia, but similar effects can be seen in American regions as well. The composite result across the continent is manifest with a band of desert and near-desert conditions that extends from the lee of the Sierra Nevada Mountains in Mexico and Arizona northward into southern British Columbia. (In fact, a pocket desert can be found tucked in British Columbia behind the Cascade Range. For more on this unique region, see Canada Has A Desert! by Maureen Fleury ) On the regional scale, the rainshadow impact is quite noticeable on southern Vancouver Island when annual precipitation twenty to thirty kilometres west of downtown Victoria are typically greater than 2000 mm (78.7 inches) while a few kilometres east of the central core, the total is less than 700 mm (27.6 inches). This rainshadow is centred on the Georgian Basin, that body of water that separates Washington, Vancouver Island and mainland British Columbia. The following figure shows the impact on summer rainfall totals.
Copyright 2004, Keith C. Heidorn, All Rights Reserved. (Illustrations, unless otherwise specified, ©Keith C. Heidorn, All Rights Reserved.) Go To Page: 1 2
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