Counting between flash and bang
How Far Away Is the Storm?
Light arrives instantly, sound takes three seconds per kilometre, and the gap is your ruler.
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Step 1 of 5
Both start at the same instant.
One event, not two. They only arrive at different times. 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.
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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.
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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.
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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.
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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.
The short version
The flash and the bang leave together, but light is effectively instant and sound takes three seconds per kilometre — so divide your count by three for kilometres, or five for miles. And the rumble's length is the strike's length.
Try it yourself
Time flash to bang twice, a few minutes apart. If the count is shrinking, the storm is heading toward you.