What makes thunder last longer than a flash of lightning?
This is easiest to understand if you think of a long vertical flash of lightning with you standing on the ground. This is basically a long vertical explosion of air, driven by electricity, that produces light and sound.
Light travels very quickly (about 186,000 miles per second) so the light from the whole flash reaches your eyes pretty much at the same time. That is to say, the delay between light from the part near the top gets to you only a very little bit delayed with respect to the light from the bottom.
The sound of thunder travels much more slowly (about 1,000 feet per second), so there would be a quite noticeable difference - for instance, one second in the arrival time of sound from nearby and sound from 1,000 feet farther away.
The net result is that while you see the lightning flash pretty much all at once, you hear the sound spread out over time as sound from farther away takes longer to arrive. The sound that travels farther also is a bit fainter and the net result is the rumble of thunder later than the flash of lightning.
You can use the sound of the thunder to estimate the size of the flash and how far away it was. Each second of delay from the flash to the start of the thunder corresponds to about 1,000 feet of distance to the closest part, and each second of rumble to an additional 1,000 of distance to somewhere farther along the flash.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
