Thundering along

It may sometimes sound like the end of the world when a loud clap of thunder rocks the sky. However, thunder is only an acoustic shock wave generated by the extreme heat associated with a lightning flash.
Thunder is caused by the rapid heating and cooling of air near the lightning. The air “explodes” during the process, causing the sounds of thunder.
One trick you may have learned as a child is to count the seconds between when you see the lightning flash and hear the thunder – multiply the seconds by 1,000 feet and that's how far away the lightning is!