Position yourself in a safe location before attempting any measurements. Choose a location where you can view the lightning and hear the thunder.
Watch the second hand on a wrist watch, or use a stop watch, and start measuring the elapsed time from the moment that you see a flash of lightning. Stop the time measurement when you hear the thunder. If you do not have a stop watch or a second hand on your watch, count the seconds by saying the word "thousand" between each number: "one thousand, two thousand, three thousand."
Divide the number of seconds measured between the time you see the lightning and the time you hear the thunder by 5. The result of this calculation is the approximate distance, in miles, between you and the position of the lightning activity.