What sound accompanies a lightning strike?

The sound that accompanies a lightning strike is called thunder.

Thunder happens because lightning heats the air around its channel extremely quickly. The heated air expands explosively, producing a shock wave that travels outward as sound. A nearby strike may produce a sharp crack or clap, while more distant lightning often sounds like a prolonged rumble.

Lightning is seen before thunder is heard because light travels vastly faster than sound. In air near room temperature, sound travels at about 343 meters per second, so counting the seconds between the flash and the sound can provide a rough estimate of distance. About three seconds corresponds to one kilometer, although terrain, wind, and temperature can affect the result.

Thunder can also vary because a lightning channel is long and irregular, and different parts of it reach the listener at different times. Multiple return strokes can add separate cracks and rumbles to the same event. Thunder is therefore not the lightning itself, nor is it caused by rain, hail, or wind; it is the acoustic result of lightning’s rapid heating and expansion of air.