did you know you can tell how far away lightning is by counting seconds?

a lightning bolt produces light and sound at the same moment. but they travel at wildly different speeds.

light covers a mile in about five millionths of a second — effectively instant over the distances that matter here. sound needs about five seconds for the same mile.

the gap between what you see and what you hear is a measuring stick.

the moment you see the flash, start counting seconds. stop when the thunder arrives.

every five seconds of gap means roughly another mile of distance. three seconds of gap means a little over half a kilometre.

distance (miles) ≈ seconds ÷ 5 distance (km) ≈ seconds ÷ 3

a rule of thumb, not a precision instrument — but a surprisingly good one.

sound travels at about 343 metres per second in air, depending on temperature. one mile is 1,609 metres.

1,609 ÷ 343 ≈ 4.7 seconds per mile — close enough to five that “divide by five” works beautifully as a mental rule.

for kilometres: sound needs about 2.9 seconds per kilometre, so “divide by three” does the job.

time = distance ÷ speed

• why does thunder rumble instead of clap? the sound arrives from different points along the bolt at different times.
• if the gap is shrinking between strikes, the storm is moving towards you — that is when to head inside.
• the same trick works with fireworks, and with watching someone chop wood across a field.