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The Earthly Winds
| Article
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16329 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1989 |
1,680 Words |
| Author
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Patrick J. Kennedy
Patrick J. Kennedy is a physicist at the National Center for
Atmospheric Research (NCAR) in Boulder, Colorado. He also
teaches science as a volunteer both in Boulder and in Mexico. |
In Homer's epic poem the Odyssey, Ulysses prudently asks Aeolus, the god of the winds, for guidance before embarking on his first voyage. Aeolus, who has kept the winds imprisoned in a cave, gives Ulysses a bag containing the winds, with instructions that he not open it without uttering certain words that would release only favorable winds. Enroute, as Ulysses sleeps, some of his crew, curious about the potent purse, open it. Without the special invocations, all the winds escape, unchained and free. It takes ten long years for Ulysses to return, having endured the full fury of the unleashed storms and winds.
To this day, the winds have not returned to their cave; nor will they ever, until the sun ceases to shine. For it is the sun, with its uneven heating of the earth, that makes the winds. Our planet receives far more heat in the tropics, where the sun's rays are direct, than in the Polar Regions, where the rays strike obliquely or not at all during the winter months. A massive amount of air, warmed in the tropical regions, rises. Cold polar air, on the other hand, is heavier because the air molecules, lacking the vibrational energy that heat provides, pack more densely together. And so it sinks. The tropical and polar air masses replace each other in a giant "conveyor belt" of winds--equator-to-poles at high altitudes, and poles-to-equator at the earth's surface.
The rotation of the earth changes the north-south direction of these global winds, so that east-west components arise. Also, instead of two giant conveyor loops from the equator to the poles, there are six smaller ones--three for the Northern Hemisphere and three for the Southern Hemisphere--that act roughly like meshed gears to perform the transfer in sections. Wrenches are thrown into the gears, however, in the form of continents, which heat and cool rapidly as the sun's direct rays advance and recede with the seasons. Oceans heat and cool more slowly and have their own circulations transferring heat away from the equatorial regions. Another impediment to the orderly transfer of air masses is the presence of mountain ranges. Nonetheless, the exchange of heated air from the tropics and chilled air from the poles occurs, in spite of the deflections caused by the earth's rotation, continents, oceans, and mountains.
Kitchen demonstrations prove that temperature differences alone can cause air movement or wind. Open an oven door, and the heated air whooshes upward into your face. Open a freezer, and the heavy cold air spills downward onto your toes. Nature is doing the same things, but on a grander planetary
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