The temperature of the air in the lightning channel may reach as high as 50,000 degrees Fahrenheit, 5 times hotter than the surface of the sun. Immediately after the flash, the air cools and contracts quickly. This rapid expansion and contraction creates the sound wave that we hear as thunder.

The initial sound reaches the ear with a loud bang, crack, or snap. As shock waves propagate away from the path of the lightning bolt, they are distorted, becoming stretched and elongated. The sound is more muted. Other shock waves from more distant locations arrive at the listener.

A bolt of lightning heats the air along its path causing it to expand rapidly. Thunder is the sound caused by the rapidly expanding air.

a loud, explosive, resounding noise produced by the explosive expansion of air heated by a lightning discharge. any loud, resounding noise: the thunder of applause. a threatening or startling utterance, denunciation, or the like.