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Mach number: In aviation, the ratio of the speed of an object in a fluid (like air) to the speed of sound in that fluid. Mach one is the speed of sound; mach two, twice the speed of sound.
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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.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
