What Causes Thunder?

Answer:

The temperature of a streak of lightning reaches 27,000° F or more — hotter than the surface of the sun. This blast of heat can explode trees by vaporizing their sap, and it sometimes melts sand into shards of glass.

Such temperatures also heat up the air along a stroke of lightning. The heated air expands so violently that it generates the shock waves we hear as thunder. A nearby stroke of lightning produces an explosive thunderclap. A more distant stroke produces a drawn-out rumbling, because the sound waves are refracted by the atmosphere and bounced off hills and other landforms.

Since light travels faster than sound, it is possible to determine just how far away lightning has struck. Counting the seconds between the visible flash and the audible blast, then dividing by five, yields the approximate distance in miles from the lightning stroke.