(c) Anthony K. Grafton 2003

Thunder and Lightning

As spring approaches, so too does the season for thunderstorms. The sound and fury of these sometimes-violent rainstorms have awed humans for millennia. A couple of thousand years ago, thunder and lightning might have been attributed to various gods throwing lightning bolts at one another. Even in modern times, the only explanation for thunder that may come to mind is that two clouds are “bumping heads”. But the cause of thunder is lightning, and the cause of lightning is not very hard to understand.

Lightning is caused by a buildup of regions of opposite static electric charge. In the same way that you can build up static electricity when you shuffle your feet across a rug during the winter, raindrops bustling about in and falling out of clouds can do the same thing. These raindrops can carry their charge away from the cloud and down to the ground when they fall. Large regions of separated, opposite charge will naturally seek to recombine, and we see this happen in the form of lightning bolts. Lightning can occur between the ground and a cloud, between two clouds, or even within a single cloud depending on where the regions of charge are located.

One lightning bolt releases a tremendous amount of energy and can heat the surrounding air up to temperatures five times that of the surface of the sun. But lightning bolts only last a tiny fraction of a second, and so the air that is heated quickly expands and then quickly contracts again when the bolt is gone. This rapid expansion and contraction of a large volume of air is what creates the thunder that we hear.

Here’s a question for you to answer: If sound travels 350 meters per second, how far away is a thunderstorm if you hear the thunder 5 seconds after you see the lightning?