Counting after a lightning flash is more than just a game

When lightning strikes, the flash and the thunder are created at almost the same moment. Lightning heats the air in an instant, so the air expands sharply and then contracts as it cools, and this movement becomes a sound wave that we hear as thunder. The US National Weather Service explains that the air along the lightning's path can get as hot as about 27,000 degrees Celsius, five times hotter than the surface of the sun.

But they arrive separately. Light travels 300,000 km per second, so even lightning several kilometers away is seen practically right away. Sound travels around 340 m per second, so the farther away the lightning, the longer the thunder lags behind.

Start counting seconds at the flash, then divide the number of seconds until you hear thunder by 3 to get roughly how many kilometers away it is. Six seconds is about 2 km, and 15 seconds is about 5 km. The US National Weather Service puts it in miles as "5 seconds per mile," which is the same calculation.

It is not an exact value. The speed of sound changes a little with temperature: about 331 m per second at 0 degrees and about 343 m per second at 20 degrees. The seconds people count are uneven too, so expect a difference of a few hundred meters.

Lightning hits the ground at a single point, but in the sky it runs a long path several kilometers long. So the sound from the part closest to you arrives first and the sound from farther parts arrives later, which makes thunder rumble on and on.

If you hear a sharp crack, the lightning passed close by. If it only rumbles low, it struck at least several kilometers away. The loud boom that shakes the ground comes when the main lightning channel reaches the ground.

Being able to count the distance does not mean you are safe. Thunder can only be heard up to about 16 km from the lightning, and the US National Weather Service says that if you can hear thunder, you are already within striking distance. It means the next flash could strike near you.

So count the seconds only from a safe place, such as inside a building or a car. If you hear thunder while you are outside, getting to shelter comes before counting. If the seconds you can count keep getting shorter, it is a sign that the storm is coming closer.