How Far Away Is the Storm?

Step 1 of 5

Both start at the same instant.

The flash and the bang are the same event: a channel of air heated to tens of thousands of degrees, exploding outward. They leave together.

Step 2 of 5

Light is effectively instant.

Light crosses ten kilometres in about thirty millionths of a second. For anything you can see, the flash reaches your eye with no delay at all.

Step 3 of 5

Sound crawls by comparison.

Thunder travels as a pressure wave through air at about 340 metres per second — roughly one kilometre every three seconds.

Step 4 of 5

So the gap measures the distance.

Start counting at the flash and stop at the bang. Divide the seconds by three for kilometres, or by five for miles.

Step 5 of 5

A rumble tells you about the shape too.

A sharp crack means one nearby channel. A long rumble means sound arriving from different parts of a strike kilometres long, at different times.

Beyond about fifteen kilometres you usually see the flash but never hear the thunder — the sound bends upward and passes over you.