Dear Tom,
Why are temperatures taken in the shade?
Jim McGill
Dear Jim,
Official temperatures are always taken in the shade so that a true air temperature can be obtained. Clear, clean air is transparent to sunlight, letting sunshine pass through it without capturing the sunOs heat energy and being warmed by it. However, when sunlight hits an object that is not transparent, like a thermometer, the energy is converted into heat and the temperature of the object rises. A thermometer placed directly in the sun would warm excessively and reveal its temperature and not that of the air. On July 15, 1972, when the official temperature at Death Valley, Calif., was 128 (degrees), a thermometer placed on the ground in direct sunlight read an incredible 201 (degrees). That is why official thermometers are exposed to freely circulating air, but sheltered from direct sunlight.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
