The gap between a flash and its thunder gives a rough sense of distance.

The longer version

Light arrives almost instantly, while sound travels through air at roughly a third of a kilometre per second, or about one kilometre in three seconds. Counting the seconds between the flash and the bang therefore gives a rough idea of how far away the strike was.

The method works because both events are produced together at the same place. Thunder arriving within a few seconds means the storm is close overhead. The count is only approximate, since temperature and wind affect the speed of sound and echoes from hills or buildings can stretch the rumble. It is a rough guide, not a precise measurement.

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.