Charles Kuhn, age 11, of Golden Eagle, I11., for his question:

WHAT IS HEAT LIGHTNING?

Lightning, that bright flash you see in the sky, is caused by an electrical current which may flow between parts of the same cloud, between different clouds or between clouds and earth.

Sheet lightning has no particular form but is usually a bright flash that spreads over the entire horizon and illuminates much of the sky. It is usually the result of chain lightning that takes place beyond the horizon.

Heat lightning is very much like sheet lightning, except that the flash is much fainter. You'll see heat lightning most often on summer evenings and it usually occurs too far away for thunder to be heard.