Thunder is the sound of air expanding rapidly after lightning heats it to extreme temperatures.

The longer version

The air along a lightning channel is heated almost instantly to temperatures far hotter than the surface of the sun. Such abrupt heating makes the air expand violently, and that expansion pushes a shock wave outward through the air around it.

The wave weakens as it spreads and eventually reaches us as ordinary sound. A nearby strike arrives as a sharp crack, while a distant one rumbles because sound from different parts of a long channel arrives at slightly different times and is reflected by terrain and buildings. Sound also travels more slowly in cold air, which changes how far away a strike seems.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.