The flash-to-bang rule
Light arrives almost instantly, but thunder travels at about 343 m/s. Divide the seconds by 3 for kilometres or by 5 for miles.
If the gap is under 30 seconds the storm is within about 10 km – close enough for a strike. Go indoors and wait 30 minutes after the last thunder.
Frequently asked questions
How far can you hear thunder?
Usually up to 15–25 km; beyond that it is refracted upwards by warmer air near the ground.