The Weather Notebook: Rain Shadows

Rain Shadows
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Washington's Olympic Peninsula is one of the wettest parts of the country, with rainfall topping 200 inches a year in some places. Yet, few people realize that this same area of young mountains and deep saltwater inlets also holds pockets of downright arid terrain. Hi, I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook.

Just 60 miles from the moss-shrouded hemlocks of the Hoh Rain Forest, the resort town of Sequim basks under a paltry 16.75 inches of annual rain - about the same as Santa Barbara, California. The explanation for this dramatic climate contrast goes by the poetic name "rain shadow." Here's how it works.

When moist Pacific air hits the wall of the Olympic Mountain range, it rises and cools, and the cooling causes the water vapor to condense and fall as rain...lots and lots of rain, especially in the winter months when the active jet stream funnels in one storm after another. But as soon as these juicy air masses cross the 5,000 to 8,000 foot Olympic crest, mostly from southwest to northeast, the process goes into reverse: the air warms as it descends, clouds disperse, sun peeks through, and the landscape looks thirsty.

Rain shadows exist wherever mountains intercept a flow of moist air: the Cascade and Sierra ranges cast a rain shadow on the three Pacific states, and the Great Plains parch in the rain shadow of the Rockies. But for sheer starkness of contrast, the Olympic Mountains cast the driest shadow of all.

Thanks to today's contributing writer, David Laskin author of Braving the Elements, a look at the weathers roll in US history. Checkout www.weathernotebook.org Our show is underwritten by Subaru, the beauty of all wheel drive.