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Earth Sense: Deserts form in common latitudes

POSTED: September 30, 2012 1:30 a.m.

Phoenix, Ariz., is a fascinating city. Completely surrounded by some of the driest, hottest desert in America, it’s a flourishing trade and convention center.

Water is even more precious in Phoenix than in drought-hassled North Georgia. Some front yards are made up desert style, with beds of bare soil and gravel and cacti growing in them. Others try to look like Florida, displaying lush lawns, complete with palm trees and sprinklers running around the clock.

But the most striking feature are the drainage ditches at the edges of the properties. Some are 6 feet wide and 5 feet deep. Flowery Branch gets a lot more rain than Phoenix, so what’s the purpose?

Rain storms are rare in the desert, but when the rain comes, it tends to come down hard. Movie buffs may recall a funny scene in National Lampoon’s “Vacation” from 1983, where the Griswold family pulls into Phoenix during a torrential downpour, with the late Aunt Edna strapped to the roof.

Desert floor has a dense, tough crust on top from weeks of baking in the sun. Unable to percolate into the ground, the water runs off on the surface. In many desert areas, sudden rainstorms can produce torrents of water that overwhelm a dry streambed, and flood human habitats.

This month, Nigeria remained at the top of disaster lists. Flooding that has plagued the central part of the country since August is now occurring farther down the Niger River, inundating 400,000 farmlands, according to allafrica.com.

Most deserts and semi-deserts in the world are the result of high pressure cells.  Winds in the upper atmosphere come together at those latitudes, and because they can’t move much farther toward the poles, the air descends down the “chutes.”

When air sinks it becomes warmer and drier. This is why you find the world’s great deserts around 30 degrees north and south of the equator. The Sonoran Desert out west fits well into that geographic zone. At 33 degrees north, Phoenix also matches the pattern.

The climatic change which the earth is undergoing isn’t showing any clear trends. But a shift of the highs will mean that the weather gets drier somewhere, and another region will get much rainier than it was in the past. Where exactly that will be remains uinknown.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.


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