Lightning heats the air along its path to extreme temperatures in a fraction of a second. The air expands explosively, creating a shock wave that travels outward as sound. That sound is thunder.

Close to the lightning the thunder is a sharp crack. At greater distances it becomes a prolonged rumble as sound from different parts of the lightning channel arrives at different times. The speed of sound is much slower than the speed of light, so the flash is seen before the thunder is heard.

Counting the seconds between flash and thunder gives a rough estimate of distance: roughly three seconds per kilometer or five seconds per mile. Thunder is rarely heard more than about 20 kilometers from the lightning. The presence of thunder confirms that a thunderstorm is nearby and that lightning is occurring.

• Sound produced by lightning
• Caused by rapid heating and expansion of air
• Heard after the flash
• Indicates nearby thunderstorm