Original film · Weather and sound
How far away was that lightning? The flash-to-bang count, explained
Light arrives at once; sound takes about 3 seconds per kilometer, 5 per mile. Count from flash to thunder, see why it rumbles, and why hearing thunder means it's time to head indoors.
Transcript
How far away was that lightning? The flash and the thunder are made at the same instant. A bolt heats the air around it to about thirty thousand degrees Celsius, five times hotter than the surface of the Sun, and that air bursts outward with a crack. The light reaches you almost instantly. The sound crawls along at about three hundred forty meters a second. So the gap between them is a stopwatch for distance. Count the seconds from flash to thunder. Now drag the strike closer. Every three seconds is about one kilometer, or five seconds for a mile. A ten-second count puts the strike about three and a half kilometers away. Cold air slows sound a little, but not enough to spoil your count. And the long rumble? A bolt can be kilometers long. Sound from its nearest part arrives first, then from farther and farther along it. Here's the catch: lightning can strike ten miles from its storm. If you can hear thunder at all, head indoors. Try it yourself at Explainer Tools dot com.