Dear Tom,
Why can you hear thunder so long after a lightning bolt? I have timed it to go on for more than 40 seconds.
— Dave Coleman, Naperville
Dear Dave,
The phenomenal heat (about 50,000 degrees) brought to bear on a column of air exposed to a lightning spark produces an explosive expansion and subsequent contraction of the column — a process that sets up the shock waves we hear as thunder.
Rather than originating from a single point, these sound waves, propagating at about 1,100 feet per second, are generated along the entire length of a lightning stroke. That length is often in excess of 10 miles. If a lightning bolt were oriented such that the distance between its nearest and farthest points from you were 10 miles, you would hear thunder from that bolt for 48 seconds.
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
Weather Report is prepared by the WGN-TV Weather Center, where Tom Skilling is chief meteorologist. His forecasts can be seen Monday through Friday on WGN News at noon, 5:55 p.m. and 9 p.m.
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