Calculate the Distance from Lightning

A thunderstorm is approaching, and suddenly you see lightning followed by a deafening clap of thunder. It sounded close -- really close. Calculating the distance from lightning can just give you peace of mind if you're in a safe location, or it can help you know if you need to find a safe path as soon as possible. So how close were you to the lightning strike? Read on to find out.

Steps

Calculation Help

Doc:Distance from Lightning Calculation Sheet,Distance from Lightning in Miles Calculator,Distance from Lightning in Meters Calculator

Calculating the Distance from Lightning

  1. Watch the sky for a flash of lightning.
  2. Count the number of seconds until you hear thunder. If you have a digital or analog watch, begin timing as soon as you see the lightning and stop as soon as you hear the thunder. If you don’t have a watch, do your best to count the seconds accurately. Say "One one thousand, two one thousand..." in your mind as you count.
  3. Calculate the distance from lightning in miles or kilometers. Sound travels one mile every five seconds and one kilometer every three seconds. Therefore, if you want to find out how far you are from lightning, divide the number of seconds by 5 if you want the answer in miles and divide it by 3 if you want the answer in kilometers. The delay between when you see lightning and when you hear thunder occurs because sound travels much more slowly than light. Here's what you do:
    • Let's say you counted 18 seconds. To find your distance from the lightning in miles, divide 18 by 5 to get 3.6 miles. To find your distance from the lightning in kilometers, divide 18 by 3 to get 6 kilometers.
    • Though you won't be able to get a completely accurate result because the weather may vary in temperature and humidity, which will slightly affect the speed of sound, this is a good way to estimate how far you are from the lightning.
  4. Calculate the distance from lightning in feet or meters. Sound travels at a speed of about 344 meters, or 1,129 feet, per second. To calculate your distance from the lightning in meters, just round 344 down to 340 and multiply the number of seconds by 340. To calculate your distance from the lightning in feet, just round 1,129 up to 1130 and multiply the number of seconds by 1130. Here's how you do it:
    • Let's say you counted 3 seconds. Multiply that number by 340 to get your distance in meters. 3 x 340 = 1020 meters. Multiply that number by 1130 to get your distance in feet. 3 x 1130 = 3,390 feet.

Tips

  • If there are frightened children around, figure out how far away the strike is and tell them. This will help ease their fears and then they will most likely ask "How did you do that?"
  • Tell people about this method. Many people still believe the myth that the number of seconds you count is equal to the number of miles away the lightning is.
  • Sound travels through air at slightly different speeds depending on air temperature and relative humidity. The difference is fairly small, however, and won’t substantially affect your calculations. For more information, see the sound speed calculators in the external links section below.
  • It can also be used to teach students how to calculate distance, speed and time.
  • 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.
  • Naturally, there is extensive room for error with this method. If possible, calculate the distance of several thunderclaps and average them for improved accuracy.
  • If you have a map and compass, try plotting the location of each lightning strike by drawing a line on the map in the direction of the lightning, and a cross at your calculated distance along this line.

Warnings

  • Lightning can be deadly. See the related wikiHow article for more information on staying safe in a thunderstorm.
  • This is not an exercise to perform outside. If you are close enough to hear the thunder, you're close enough to be struck by lightning. Lightning can travel rapidly and has struck people over 10 miles away from the storm. If possible, find shelter immediately.
  • If you find out that the lightning is less than a mile away, make sure you find/have shelter immediately. Lightning might strike you.
  • Due to the way sound travels and how various objects, such as mountains and buildings, interact with sound waves this is not the most reliable way to predict lightning distance. Don't let your life depend on it. Listen to local weather authorities.
  • If you do not see the lightning strike directly, the sound you hear may be a reflection off a building or a mountain, which adds time between the two events (the flash and the bang, thus making the lightning seem farther away than it really is. Consider the effect of nearby (especially large) objects/obstructions, as sound must "bend" around and bounce off of them. Any indirect path must be larger than the distance which you are trying to calculate.

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