Dear Tom,
Sometimes thunder rolls, and sometimes it makes a sharp crash. What causes the difference?
Sam Aletti
Dear Sam,
Literally, thunder is “the sound of lightning.”
A lightning spark heats the channel of air through which it passes to 54,000 (degrees) F, and such extreme heating causes the air to expand explosively, initiating a shock wave that we hear as thunder.
University of Florida lightning expert Dr. Martin Uman says, “If a section of the lightning channel is perpendicular to your line of sight, all points on that section will produce sound that arrives simultaneously at your ears, and the result is a high-intensity sound–a clap. If the channel is along your line of sight, the sound arrival times from various points on the channel will be spread out, resulting in a rumble or roll.”
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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.
