Dear Tom,
Why is dew point a better measure than relative humidity for describing our comfort level?
Ronald Neamer, Mt. Prospect
Dear Ronald,
Dew point, a direct measure of atmospheric moisture, is a much better indicator of discomfort than relative humidity. It is a conservative feature, meaning it remains reasonably constant during the day barring a change of air mass. In contrast, relative humidity can vary tremendously during the course of a day, often by 70 percent or more. Most people begin to feel some discomfort when the dew point approaches 65, and by the time it climbs to 75, almost everyone is uncomfortable. Chicago’s record-high dew point, a steamy 83, was recorded on July 30, 1999. The world’s highest dew points have reached the lower 90s and have been reported near the very warm waters of the Red Sea and Persian Gulf.
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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.)
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