Content

• To step
• Method 1 of 2: Calculate the distance of the lightning
• Method 2 of 2: Calculate the distance of the lightning in meters
• Tips
• Warnings

An approaching thunderstorm, and suddenly you see lightning followed by a deafening thunderclap. It sounded close - very close. Calculating the distance from lightning can just give you peace of mind knowing that you are in a safe place, or that you need to find a safe place as soon as possible. So how close were you to the lightning strike?

To step

Method 1 of 2: Calculate the distance of the lightning

1. Look to the sky for a bolt of lightning.
2. Count the number of seconds until you hear thunder. If you have a digital or analog watch, start counting as soon as you see the lightning and stop as soon as you hear the thunder. If you don't have a watch, carefully count the seconds in mind: One thousand, two thousand ...
3. Calculate the distance of the lightning in kilometers. Sound travels a kilometer every three seconds. So, if you want to know how far away you are from lightning, divide the number of seconds by 3. The delay between seeing lightning and hearing thunder occurs because sound travels much slower than light. More about this:
   • Let's say you counted 18 seconds. To calculate the distance of the lightning in kilometers, divide 18 by 3, so 6 kilometers.
   • While the result is not entirely accurate as the weather can vary in temperature and humidity which can affect the speed of sound, this is a good way to estimate how far away you are from lightning.

Method 2 of 2: Calculate the distance of the lightning in meters

1. Calculate the distance of lightning in meters. Sound travels at a speed of about 344 meters per second. To calculate the distance of the lightning in meters, round 344 to 340 and multiply the number of seconds by 340. More on this:
   • Let's say you counted 3 seconds. Multiply that number by 340 to get the distance in meters. 3 x 340 = 1020 meters.

Tips

• When there are scared children, it is helpful to know how far away the lightning is. By telling them this they are less scared and then they will probably ask "How do you do that?"
• Inform people about this method. Many people still believe that the number of seconds equals the number of kilometers that lightning is away.
• Depending on temperature and relative humidity, sound travels through air at slightly different speeds. However, the difference is quite small and will not significantly affect your calculations. For more information, check out the sound speed calculators in the external links below.
• It can also be used to teach students to calculate distance, speed and time.
• If there is a lightning strike 1 km away, you will see the flash approximately 0.00000436 seconds after the lightning strike while you will hear it approximately 4.75 seconds after the actual lightning strike. If you calculate the difference between these two events, a person will hear a lightning strike approximately 4.71999 seconds after the impact occurred. Therefore, 3 seconds per kilometer is a pretty good estimate.
• Of course, errors are possible with this method. If possible, calculate the distance of several thunders and take the average to be more accurate.
• If you have a map and a compass, try to plot 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 strikes can kill.
• If you discover that the lightning is less than a mile from you, find a safe shelter immediately. You can be struck by lightning.
• You don't do the calculations outside. If you are close enough to hear the thunder, you are close enough to be struck by lightning. Lightning can move quickly and has hit people even 10 kilometers from the storm. If possible, seek shelter immediately.
• Due to the way sound travels, and how different objects such as mountains and buildings interact with sound waves, this is not the most reliable way to predict distance from lightning. Don't let your life depend on it. Listen to the local weather reports.
• If you don't see the lightning directly, the sound you hear may be a reverberation from a building or a mountain, making the time between the two events (the flash and the bang) seem further away than it actually is. Take into account the effect of (especially large) objects / obstacles in the vicinity, because sound must bend and reflect around them. Any indirect path will be greater than the distance you are trying to calculate.