Richard Brill, a professor at Honolulu Community College, explains:
Thunder is caused by lightning, which is essentially a stream of electrons flowing between or within clouds, or between a cloud and the ground. The air surrounding the electron stream is heated to as hot as 50,000 degrees Farhenheit, which is three times hotter than the surface of the sun. As the superheated air cools it produces a resonating tube of partial vacuum surrounding the lightning's path. The nearby air rapidly expands and contracts. This causes the column to vibrate like a tubular drum head and produces a tremendous crack. As the vibrations gradually die out, the sound echoes and reverberates, generating the rumbling we call thunder. We can hear the thundering booms 10 miles or more distant from the lightning that caused it.
Image: American Red Cross
When the lightning is within sight, however, we see it first because the speed of sound in air is considerably slower that that of the electron flow. Thus, the sound behaves more like a shock wave than an ordinary sound wave. The shock wave follows the path of the electrons like a fist in a sock. The speed of sound is even more insignificant when compared to the speed of light. The light from the flash reaches us in a fraction of a second, whereas the sound lags along like a snail following an interplanetary rocket.
The audiovisual spectacle of thunder and lightning is a combination of the dynamics of the vibration of air molecules and their disturbance by electrical forces. It is an awesome show--and one that reminds all of us of the powers of nature and our own insignificance in relation to them.
Answer originally posted January 17, 2000.