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.

With nearby lightning strikes the thunder will sound like a loud bang, crack or snap and its duration will be very short. As the shock wave propagates away from the strike center, it stretches, diminishes, and becomes elongated. Then other shock waves from more distance locations arrive at the listener.

Thunder is created by a rapid expansion of air. Think of a balloon popping. There is a sudden release of noise due to the rapid expansion of air. When lightning moves through the air it increases the temperature of the air dramatically in a very short period of time.

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.