You know how it is - a thunderstorm is approaching, and suddenly you hear an absolutely deafening clap of thunder. Whoa, that sounded close! 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.
4. Seek shelter immediately if a storm is approaching.
* If a lot of lightning is occurring close together, it may be difficult to approximate how far away a given flash is, since it will be hard to tell which clap of thunder "belongs" to which flash of lightning. In this case, you’ll need to keep trying the steps above until you can finally be certain that a given bang resulted from a given flash. Of course, if safety is your primary goal, start counting from the last flash you see. This way it won't matter which flash was closest (or furthest), you will know how close the nearest flash was to you irrespective of which flash it was or how bright the flash was, assuming of course that no other flashes interrupt your count.
* If you are in an area with a lot of background noise (if you are on a busy city street, for example, or working around heavy machinery) you may not be able to determine when thunder starts, if you can hear it at all.
* Sound travels through air at slightly different speeds depending on air temperature and relative humidity (density). 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.
* 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.
* 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.
* This is not an exercise to perform outside. If there’s lightning around, get to shelter immediately.
* Lightning can be deadly. These instructions can help you avoid danger, but only if you take action when lightning is near. See the related wikiHow article for more information on staying safe in a thunderstorm.
* Storms can travel very fast, and lightning frequently strikes up to 10 miles from a storm (occasionally up to 50 miles). Thus, even if you determine that the lightning is quite far away, you should seek shelter, as the next strike could be much closer.
* Due to the way sound travels and how various objects, such as mountains and buildings, interact with soundwaves 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.