# How Can You Tell How Far Away Lightning Is?

You can tell how far away lightning is by counting the seconds between the lightning flash and the clap of thunder. Lightning and thunder begin almost at the same time. But you see the flash before you hear the noise. This happens because light travels faster than sound.

It takes about five seconds for sound to travel one mile, if you divide the time between the flash and the thunder by five, you get the number of miles away the flash is. When you see a flash of lightning, begin counting, “one thunder, two thunder,” and so on, until you hear thunder. If you count five seconds, the lightning is a mile away. Ten seconds means the storm is two miles away.

Most students are aware that Sound waves propagate through different materials at different speeds. In air at a temperature of 70° F (22.2° C), sound travels at a speed of about 1,129 feet (344 meters) per second, which is roughly equivalent to one-fifth of a mile in one second, or one mile in five seconds (or one kilometer in about three seconds).

Counting the seconds between the strike and the arrival of the thunder and dividing the number of seconds by five gives us a rough estimate of the distance to the strike in miles (dividing by three for a distance in kilometers).