Why the gap tells you the distance
Lightning makes its light and its sound at the same moment. The light reaches you at once. The sound takes its time. The longer you wait for the thunder, the farther away the strike was.
The two speeds
Light travels at about 300,000 km each second. Across a few miles of sky, that is no delay at all. You see the flash when it happens.
Sound travels through air at about 343 m each second. It needs about 4.7 seconds to cover a mile, and about 2.9 seconds to cover a kilometre.
So the gap between flash and bang is the sound's travel time. Multiply it by the speed of sound and you have the distance.
The rule of thumb
Count the seconds from the flash to the thunder. Divide by 5 for miles, or by 3 for kilometres.
People count "one Mississippi, two Mississippi" because each one takes about a second to say. Five Mississippi is about one mile.
| Flash to bang | Miles | Kilometres |
|---|---|---|
| 1 s | 0.21 | 0.34 |
| 3 s | 0.64 | 1.03 |
| 5 s | 1.07 | 1.72 |
| 10 s | 2.13 | 3.43 |
| 15 s | 3.20 | 5.15 |
| 30 s | 6.39 | 10.29 |
| 47 s | 10.02 | 16.12 |
Why far thunder rumbles
A lightning channel can be miles long. Sound from its near end reaches you before sound from its far end, so one strike arrives as a long roll.
Air also soaks up high pitches faster than low ones. A close strike keeps its sharp crack. A far one loses the crack on the way and arrives as a low rumble.
Past about 10 miles, the thunder usually fades out or bends away before it reaches you. You can still see the flash.
What the game does
Each round picks a distance between 0.1 and 10 miles, shows the flash, and then waits the true travel time before the thunder starts. The delay is exact. The reveal shows you the distance and the delay it used.
Two things are simpler than a real storm. The speed of sound is fixed at 343 m/s, the value for air at 20 °C. Real air changes it by a few percent. And the thunder is made by the page, not recorded.
In a real storm
If you can hear thunder, the lightning is close enough to strike you. Go indoors, and stay there until 30 minutes after the last thunder. The count is for curiosity, not for deciding that you are safe.
Real storms do real damage. You can give to storm relief.