Thunder sounds after lightning is seen because light travels much faster than sound.

The longer version

Light crosses the gap from a storm to an observer in a tiny fraction of a second, while sound crawls along far more slowly through the air. Both begin at the same instant, so the flash arrives almost immediately and the rumble follows later once the pressure wave has covered the same distance.

Counting the seconds between them gives a rough idea of distance, since sound covers a predictable stretch of ground each second. The rumble also lasts longer than the flash because sound from different parts of a long lightning channel reaches the ear at different times, and echoes from hills and buildings stretch it further.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.