How to Calculate the Distance from Lightning


You know how it is - a thunderstorm is approaching, and suddenly you hear an absolutely deafening clap of thunder. Whoa, that sounded close! Even at times it becomes frightening! But how close is the lightning, really? It is difficult to determine the distance of a lightning bolt just by looking at it, and the volume of the thunder isn’t a good way to tell either. Read on for a simple, potentially life-saving method you can use to approximate how far away lightning is.


  1. Watch the sky for a flash of lightning.
  2. Count the number of seconds until you hear thunder. If you have a watch with a second hand or a digital watch that has seconds, begin timing as soon as you see the lightning and stop as soon as you hear the thunder start. If you don’t have a watch, do your best to count the seconds accurately. Say "One one thousand, two one thousand etc." in your mind for each second.
  3. Divide the number of seconds by 5 to calculate the distance in miles (or divide by 3 for kilometers). In other words if you counted 18 seconds from when you saw the lightning, the strike was 3.6 miles (6 kilometers) from your location. The delay between when you see lightning and when you hear thunder occurs because sound travels much, much more slowly than light. Sound travels through air at about 1100-1200 feet (330-350 meters) per second (depending on altitude, relative humidity, pressure, etc.), which is a little more than one mile per five seconds (one kilometer per three seconds). In comparison, the speed of light is 983,571,058 feet (299,792,458 meters) per second.

For example, if lightning strikes a point 1 mile away, you will see the strike approximately .00000536 seconds after the strike while you will hear it approximately 4.72 seconds after the actual strike. If you calculate the difference between these two experiences, a person will hear a strike approximately 4.71999 seconds after the strike actually occurred. Therefore, 5 seconds per mile is a fairly robust approximation.




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