Why thunder arrives after the flash

Thunder arrives after the flash because light from the lightning reaches you almost at once and the sound does not. Both start together in the channel, and the gap you notice is the travel time of the sound.

The delay is not the storm deciding to rumble after it has finished striking, and the sound is not manufactured slowly. The shock begins when the channel heats, and your eyes simply get the news before your ears.

The mechanism is more particular than the slogan. The lightning current heats a narrow path of air so violently that the air expands as a shock, which soon becomes the sound called thunder. Sound in ordinary air covers about a kilometer in three seconds, so a quiet gap of a few seconds means the channel was still nearby.

Follow it one step further. A long, crooked channel is not all at one distance from you. The near part of the same flash can arrive first and the far part later, and reflections add more arrivals, which turns one sudden expansion into a rumble instead of a single crack.

The case worth keeping in mind is specific. A strike on a nearby tree is followed by a sharp crack with almost no gap. A flash on the horizon can be followed by a growl many seconds later, from the same kind of heating, only farther along the path the sound has to travel.

That case has a boundary. Counting about three seconds to a kilometer is a rough clock for ordinary air, not a survey. Temperature, wind, and the chance that you heard a branch of the channel rather than the brightest fork all move the estimate.