Farm Progress

Weather Wise: Here's why you should respect lightning storms.

Andy Eggert

April 1, 2017

2 Min Read
TAKE COVER! When dark clouds approach that could contain lightning, it’s time to take cover. Stop what you’re doing and head for the barn! Lightning can strike long before you hear thunder.

Is it true that if you see lightning and count the number of seconds before you hear the thunder, you know the storm is the same number of miles away? Whether that’s true or not, it’s fact that you see lightning first. Why does that happen?

Lightning is nothing more than the transfer of charges within a cloud, from a cloud to another cloud or from a cloud to the ground. It’s comparable to a shock you might receive from touching a doorknob in the winter, but on a much, much larger scale. 

Lightning bolts can generate temperatures upward of 50,000 degrees F, which is five times hotter than the sun! It can turn air into plasma similar to what happens in a neon sign, but again on a much larger scale.  Thunder is simply the result of the shockwave from the rapid heating and expansion of the air immediately surrounding the lightning bolt.

Storm math
Using thunder and lightning to calculate storm distance is certainly not a new concept, but the conversion in this question is slightly off. Since light travels so much faster than sound, you can see lightning almost instantly as it happens. 

Thunder only travels at the speed of sound, which is just over 767 miles per hour. Doing some math and conversions, sound, and therefore thunder, travels 1 mile in approximately 4.7 seconds. Therefore, to calculate the distance of the storm, divide the number of seconds between seeing lighting and hearing thunder by 5 and you will have the approximate distance between you and the storm. So if there are 10 seconds between seeing the flash and hearing the thunder, the storm is about 2 miles away.

Thunder is rarely heard from distances of more than 12 miles, so after about a minute, you can stop counting and assume the thunder was too soft or too far away to hear.

Eggert is with the Indiana State Climate Office. He writes from West Lafayette.

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