As a thunderstorm moves into an area, lightning illuminates the sky followed by rumbles of thunder. Using this sequence of events and applying some simple math, you can estimate how far away the storm is.
Since lightning travels at approximately the speed of light – 186,000 miles per second – you see it almost instantly. Thunder, on the other hand, travels at the speed of sound – about one mile in five seconds. These different rates of travel allow you to estimate the distance between yourself and the lightning.
To do this, count the seconds between seeing the flash of lightning and hearing the clap of thunder. Divide that number by five and you will know how far away the lightning is. For example, if you count fifteen seconds between seeing lightning and hearing thunder, the lightning is about three miles away.
But, remember, if you can hear thunder, lightning is close enough to strike. So, as NOAA recommends, “When thunder roars, go indoors.”