Science Puzzle
How Far Away Is the Storm?
During a thunderstorm you see a flash of lightning and start counting. You hear the thunder 9 seconds later. Sound travels through air at about 340 metres per second.
Roughly how far away did the lightning strike?
The Answer
About 3 kilometres away.
The lightning and the thunder happen at the same moment, but they reach you at very different speeds. Light travels at about 300,000 kilometres per second, so the flash arrives a hundred-thousandth of a second after the strike: effectively instantly. Sound plods along at about 340 metres per second, so it covers roughly a kilometre every 3 seconds. Nine seconds of sound is 9 × 340 = 3,060 metres, a little over 3 kilometres.
That gives a handy rule: count the seconds between flash and bang, then divide by three for kilometres (or by five for miles). Count again at the next flash. If the gap is shrinking, the storm is heading your way.
Thunder itself is the sound of air being heated by the lightning to around 30,000 °C in a split second, hotter than the surface of the Sun. The air expands so violently that it makes a shock wave, which we hear as a crack or a long rumble. If you can hear thunder at all, you're close enough to be struck, so it's time to get indoors.
The principle: Light travels almost a million times faster than sound, so the delay between seeing and hearing a distant event is a measure of its distance: about 3 seconds for every kilometre.