Dear Tom,
Growing up in Chicago, weather forecasts mentioned the THI or Temperature Humidity Index. Was that the same as the dew point?
Joe Traxler
Dear Joe,
The National Weather Service began to use the THI to quantify discomfort from heat and humidity in the 1960s. The formula used temperature and dew point to derive an index that gauged discomfort. An index of 70 or below meant that everyone was considered to be comfortable. At 71, 10% of the populace was deemed uncomfortable, 50% at 75 and 100% at 80. When the THI reached 84, government workers in non-air conditioned offices were sent home. The dew point was a part of the THI computation, but it was not the same thing. In the early 80s the current Heat Index replaced the THI because scientists felt the old THI was not responsive enough to rapid humidity changes and failed to incorporate air movement.
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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.
