Dear Tom,
Peals of thunder have always fascinated me. Some are brief, like a sudden crash. Sometimes thunder rolls and rumbles for 10 or 20 seconds. What causes the difference?

John Hannessey, Rockford

Dear John,
A bolt of lightning heats the air through which it travels practically instantaneously to about 50,000 degrees F. Such extreme heating causes explosive expansion of the air followed by rapid cooling and contraction. This sets up sound waves that we hear as thunder.

Thunder waves (sound waves) move at about one mile every five seconds. Lightning bolts are often 10 to 20 miles in length, but occasionally much longer than that, and thunder is generated everywhere along the bolt’s path. You will first hear thunder from the part of the lightning bolt that is nearest to you, and then thunder from progressively more distant portions of the bolt.

The kind of thunder that you hear depends on your distance from the lightning bolt. If the bolt is oriented in such a way much of it is at about the same distance from you, thunder arriving at your ears comes from much of the bolt simultaneously, or nearly so, and what you hear will be a short-lived crash.

Lightning oriented at varying distances from you will produce thunder first from the nearest portion of the bolt to you, then thunder from portions of the bolt at progressively greater distances.

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