Quickly estimating the distance of a thunderstorm is relatively simple. After you see the flash of lightning (either the familiar zigzag or a sudden flash of light), count off the seconds until you hear the thunder, then divide that number by five. The result is an approximation of the distance of the thunderstorm in miles.
If you see a thunderstorm in the distance, see a flash of lightning, and count 15 seconds before you hear the thunder, then the thunderstorm you see is three miles away (15 seconds / 5 = 3 miles). If the lightning and thunder seem to strike at the same time, then the thunderstorm is right on top of you (0 seconds / 5 = 0 miles). If you hear the thunder only two seconds after you see the lightning, then the storm is two-fifths of a mile, or a bit less than half a mile, away (2 seconds / 5 = 2/5 mile).
Why this works
This simple and easy-to-remember calculation works because when a bolt is created, both lightning and thunder are produced; but because light and sound travel at different speeds, they each reach you at different times.
Light travels incredibly fast, at 3.0 x 10^8 meters per second, reaching your eyes almost instantaneously. In comparison, sound travels very slowly, at 343 meters per second. At this speed, the time it takes for a person one mile away from a thunderstorm to hear the thunder is five seconds. Essentially, when you are counting the seconds it takes for the thunder to reach you, you are using the speed of sound to measure your distance to the thunderstorm. The same principle applies if you are trying to judge the distance of a gunshot: if you see the gun go off and then don’t hear the gunshot for five seconds, then the gun is a mile away. If the sound from thunder takes less than five seconds to reach you, then the storm is a fraction of a mile away.
It is important to note that this is just a rough estimate and not an exact distance to your thunderstorm. Many different factors, from human error to differences in humidity or temperature can throw off your estimate by half a mile to several miles. If you have a stopwatch, you can eliminate most of the human error involved in counting the seconds, but it will still be just an estimate.
Estimating the distance of a thunderstorm can be useful and fun, and this easy-to-remember calculation makes it simple. So long as you remember that this is only a rough estimate and not hard scientific data, you can enjoy watching the thunderstorms and preparing for their arrival.