Why thunder rumbles instead of a single crack

Settled in 1969 by Arthur A. Few.

Why thunder from a single bolt of lightning sounds like an extended rumble rather than a single sharp crack was a natural question since antiquity, with early guesses invoking echoes. In 1969, atmospheric physicist Arthur A. Few published a quantitative model in the Journal of Geophysical Research showing that because a lightning channel is long, irregular, and tortuous, different segments are at different distances from an observer; sound from the nearest point arrives first and sound from more distant, oblique segments arrives progressively later, and this geometric spreading of arrival times - not echoing - produces the characteristic rolling rumble, with the rumble's frequency content also predictable from the channel's energy-per-unit-length.

A single, near-instantaneous lightning bolt gets stretched into several seconds of rolling thunder purely because sound from its far end has so much farther to travel than sound from its near end.

Explanations that circulated

• A common lay explanation is that thunder rumbles simply because it echoes off clouds, mountains, or buildings; while echoes can contribute in some settings, the primary cause of prolonged rumbling is geometric - the lightning channel is typically kilometers long and irregularly shaped, so sound generated at different points reaches a distant listener at different times, spreading a near-instantaneous event into a rumble even with no reflecting surfaces.
• Some assume the rumbling is caused by multiple separate strokes in a single flash; that does happen and can contribute to flickering, but it is distinct from and secondary to the geometric path-length explanation for the sustained rumble of even a single return stroke.

References

• Power spectrum of thunder, Arthur A. Few, Journal of Geophysical Research, 1969