Thomas Gallup, age 10, of Sparland, Ill., for his question:

WHAT IS HEAT LIGHTNING?

Almost all lightning strokes are about the same, but they appear to have different forms, depending on the position of the observer.

Sheet lightning has no particular form. It is usually a bright flash that spreads all over the horizon and lights up the sky. Sheet lightning is actually light from a flash of chain lightning that takes place beyond the horizon.

Heat lightning, which can often be seen on summer evenings, is the same as sheet lightning but the flashes are fainter. Thunder usually does not accompany the flashes because the lightning occurs too far away for the thunder to be heard.

When lightning flashes, electrical current may flow between parts of the same cloud, between different clouds or between clouds and the earth.