What causes thunder after lightning?

Thunder is caused by the rapid expansion of air heated by a lightning discharge. When lightning strikes, it heats the air in its path to temperatures as high as 30,000 °C (54,000 °F). This intense heat causes the air to expand rapidly, creating a shock wave that travels outward. As this shock wave moves through the atmosphere, it slows down to the speed of sound and is heard as thunder. The return stroke of lightning, rising from the ground back along the path of the initial stroke, produces particularly loud thunder because it carries a larger and faster-rising electric current compared to the initial stroke.

The path of a lightning channel is often long, branched, and tortuous. This complex geometry causes sound waves from different parts of the channel to reach an observer at different times. As a result, thunder is not just a single sound but a series of claps, rolls, and rumbles. The duration and character of these sounds depend on the distance and the specific path the lightning takes. The farther away the lightning is, the longer and more rumbling the thunder will sound.

The time delay between seeing lightning and hearing thunder can be used to estimate the distance of the lightning strike from an observer’s location. Every three seconds of delay between the flash and the thunder correspond to one kilometer, and every five seconds correspond to one mile.