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What causes the sound of thunder?

April 15, 1997
Web posted at: 12:12 p.m. EDT (1612 GMT)

By Kathy Wollard

What causes the sound of thunder? asks Deepthi Rao, a student in Brooklyn, New York.

Cracks, rumbles and rolls: As you read this, some 2,000 thunderstorms are in booming progress around the Earth, with lightning striking the ground about 100 times each second. It's no wonder so many cultures invented thunder gods to explain the terrible sounds.

In Europe there was red-bearded Thor, who wielded a hammer. The Chinese had Lei Kung, a creature with a blue body, claws and bat wings that beat out thunder with a mallet and drum. In ancient Sumer, it was Ninhar, the roaring bull. And North American Indians revered the mythical "thunderbird." Lightning flashed from the bird's beak; thunder was the reverberation of its great beating wings.

In old England, thunder on Thursday meant your sheep were healthy, but thunder on Sunday foretold the death of judges and men of learning.

Today we know that thunder is the sound lightning makes in the air. Just as a tiny spark of electrons can jump across a rustling blanket in a dry room, a tremendous spark -- a lightning bolt -- can connect a storm cloud and the ground. And, like the little crackle you hear when sparks jump across a blanket, a clap of thunder shakes the Earth.

Here's how it works. Out of the bottom of the cloud emerges a dimly glowing bolt, called a leader. The leader zig-zags down toward the ground in a fraction of a second, creating a channel through the air about 2 centimeters wide.

The bolt carries an electric current of about 200 amperes (normal household current is 15 or 20). But when the bolt gets to within about 60 feet of the Earth, a spark suddenly jumps out of the ground to join it. When the two sparks connect, the current races back up the channel to the cloud, increasing to more than 10,000 amps as it does.

Another leader snakes out and slams down the channel created by the upward stroke; another spark shoots back up to the cloud. Temperatures in the air channel quickly reach 50,000 degrees Fahrenheit. These lightning strokes -- firing back and forth many times in less than a second -- are what we see as a single bolt.

The superheated air in the channel expands violently, as air molecules get a tremendous burst of energy and fly apart. The air expands outward from the channel at supersonic speed (greater than the speed of sound). We call such a disturbance a shock wave. After a few feet the wave slows to about 1,100 feet a second, the speed of sound; when it reaches our ears we hear the BOOM.

The boom, or thunderclap, comes from the main channel of lightning. The return stroke of the lightning flash actually makes the loudest sound, because it carries the most powerful current and heats the air most. The crackling sound we sometimes hear is made by branches of lightning off the trunk.

After the clap and roll, the rumbles are the thunder's echoes from clouds, mountains and buildings.

Kathy Wollard is a free-lance writer. Send your questions to How Come? P.O. Box 4564, Grand Central Station, New York, N.Y. 10163. If your question is answered, you'll receive the book "How Come" by Kathy Wollard and Debra Solomon (Workman Publishing)

(c) 1997, Kathy Wollard. Distributed by Los Angeles Times Syndicate


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