Our Window on Nature

. . . exploring the world around us

Seeing the Wind

Filed under: Weather — Lowell Christie -- March 22, 2007 @ 1:55 pm

Dust DevilYou can’t actually see the wind. It’s as invisible as sound waves or thermal energy. Yet wind manifests itself in tangible, sometimes startling ways.

The other day as we walked out on the desert we heard the sound of water swirling down a manhole. Well, that’s what it sounded like. We noticed pebbles tumbling almost at our feet, and bushes trembling and grasses bending — and yet we felt no movement. It was a desert whirlwind, acting upon things we could see.

Clouds speeding across the sky give more evidence of wind, and when we see two levels of clouds traveling in opposing directions we realize that winds aloft can blow from different points of the compass. Other times we watch clouds torn into fragments and strewn across the sky, all done by the wind.

A hurricane manifests the power of wind over water. Basically, it’s a giant heat pump pulling in moist air from the ocean’s surface. The winds accelerate as they approach the center, reaching speeds of 150 miles an hour or more. When the hurricane reaches land we see the effects of winds of that magnitude. Picture palm trees bent flat, with 10- to 15-foot waves crashing onshore, sweeping away buildings and cars and seawalls as though they were toys.

Tornadoes are more violent and more common than hurricanes. On the average, 124 tornadoes strike in the United States every year, most of them in the lower Mississippi River Valley. They’re born out of thunderstorms whose energy creates a funnel of wind whirling at a speed of 100 to 300 miles per hour. And like any efficient vacuum cleaner it sucks up everything in its path.

Fortunately, most tornadoes last but a few minutes, and touch down only briefly. But there have been spectacular exceptions. On May 26, 1917, parts of Illinois and Indiana lay along the one-mile wide, 293-mile path laid waste by a tornado that stayed alive for seven hours and 20 minutes.

A tornado sweeping through a town may leave one side of the street intact and the other in ruins. It can park a tractor in the top of a tree or drive a piece of straw right through a telephone pole. It has other, more peculiar manifestations.

One Nebraska tornado stripped the top half of a grove of cottonwoods bare, yet left the bottom half in leaf. Elsewhere the wind plucked the feathers off a flock of chickens, leaving them squawking in indignation but otherwise unharmed. Such is the power, and capricious nature, of a tornado.

Out on the desert you don’t see many tornadoes, but there is plenty of wind. Dust devils (whirlwinds) are similar in appearance to tornadoes, but they are actually quite different.

Instead of sweeping down out of a cloud, these winds result from extreme heating of the ground surface. They’re quite common on hot, still days, and you often see several at a time making their way across the desert.

The aforementioned whirlwind was invisible because it was small, and because it traveled over an area covered with coarse gravel rather than sand or dirt. These tiny dust devils pass unnoticed unless man or animal happens to be close by.

More visible dust devils reach as much as 100 feet in the air and carry a hefty load of dirt and debris – paper, discarded cans, the odd folding chair that lay in its path. Heavier items seldom travel far before being tossed aside.

Dust storms are another expression of desert wind. Common only near sand dune areas and where agricultural lands lay fallow, they may rise hundreds of feet in the air and spread for miles. Fortunately they’re rarely dangerous, but they do cause traffic accidents and sandblast auto windshields and paint.

The wind sculpts sand dunes too, of course, as anyone who has visited White Sands, New Mexico, knows. And it creates pristine art forms along the coastline where trees grow in grotesquely beautiful shapes. With branches swept back and foliage growing just on the downwind side of the trunk, such trees show how much tenacity is necessary to survive against the indomitable force of the wind.

FMC485

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