|thunder (thŭn'dər) Pronunciation Key
The explosive noise that accompanies a stroke of lightning. Thunder is a series of sound waves produced by the rapid expansion of the air through which the lightning passes. Sound travels about 1 km in 3 seconds (about 1 mi in 5 seconds). The distance between an observer and a lightning flash can be calculated by counting the number of seconds between the flash and the thunder. See Note at lightning.
The noise created when air rushes back into a region from which it has been expelled by the passage of lightning.