Why does thunder occur?
Thunder is the sound produced by pressure waves as air in a lightning channel heats and expands rapidly. Light travels much faster than sound, so the flash is seen before thunder is heard. The delay can provide a rough distance estimate.
Why can thunder rumble?
A lightning channel can be long and branched. Sound from different sections arrives at different times, while reflections and atmospheric refraction modify the result. Nearby discharges may sound sharp, while distant ones produce a rolling rumble. Sound speed depends on temperature, and wind and atmospheric structure influence propagation.
Limits of estimating distance
A delay of roughly three seconds corresponds to about one kilometer of sound travel, but this is approximate. It estimates distance to a heard part of the channel, not the storm boundary or the next strike. Audible thunder indicates that lightning risk deserves attention. Fading sound alone does not establish safety; official guidance and appropriate waiting periods remain important.