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The 100th Meridian
Mon Aug 30, 2004
Listen in RealAudio 
"Beyond the 100th Meridian" might seem like a strange title for Wallace Stegner to have
chosen for his biography of Western explorer John Wesley Powell, but in fact this
phrase captures the essence of the book, its theme, setting, even its insights into
climate.
Hi, I'm Bryan Yeaton and this is The Weather Notebook.
The 100th meridian refers literally to the arc of longitude that slices through America's
Great Plains. But symbolically, this immense imaginary line marks the divide between
two regions of distinctly different climate. Beyond the 100th meridian, the real West
begins. As annual rainfall drops off sharply west of the meridian, the tall waves of
eastern prairie grasses give way to the low-growing thatch of blue gramma and buffalo
grass, and then to sagebrush and cactus. Buttes and arroyos replace the smooth
undulating tableland. Green sears to tan. Ranches take over where grain farms leave
off.
Spread out a map of the U.S. and you'll see why this vivid climatic boundary exists. The
Pacific Ocean and the Gulf of Mexico are the primary sources of moisture for the
Western U.S. and both tend to bypass the Western plains. The Rocky Mountains
squeeze out precipitation from eastbound Pacific storm systems and leave a "rain
shadow" in their lee. Gulf moisture, especially in the humid summer months, travels
on southwesterly winds, rarely straying beyond the 100th meridian. Which is why in
August, Sioux Falls, in eastern South Dakota, averages nearly double the rain of Rapid
City in the state's southwestern sector.
Topographically, the Great Plains may seem rather monotonous, but the climatic shift
as you cross the 100th meridian maintains a drama all its own.
Thanks today to writer, David Laskin. The Weather Notebook is a production of the
Mount Washington Observatory and is supported generously by Subaru of America and
the National Science Foundation.
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