Lightning image via Shutterstock

It sounds a bit like an old wives' tale, but you actually can use the speeds of light and sound to get a rough estimate of how far away a storm is.

Scientists have come up with various devices and methods for determining the distance of lightning, but you can estimate it right in your head with just a little counting and a little math, using what’s called the "Flash to Bang" method.

Sound travels through air at, well, the speed of sound. Officially, that’s 1,087 feet per second in dry air at 0 degrees Celsius/32 degrees Fahrenheit. Depending on the local temperature and humidity, that mileage will vary. For a quick calculation in your head, though, the experts at the National Severe Storms Laboratory say you can use 1 mile per five seconds as a good approximation in most conditions.

The speed of light is just a wee bit faster than sound, 186,282.397 miles per second. It’s fast enough that you see the lightning almost the instant it flashes. When that happens, start counting until you hear the thunder, which is caused by a sonic shockwave created from air rapidly expanding in the presence of lightning’s extreme heat and pressure. Divide the number of seconds between the flash of lightning and the bang of thunder by five to account for the sound’s slower speed, and you have a rough idea of how many miles away the lightning struck. If it takes 10 seconds for the thunder to roll in after the flash, the lightning struck about 2 miles miles away.

Of course, getting struck by lightning is not something most of us want to experience, no matter how cool the scars, so if your distance is dwindling while you’re out there crunching numbers, seek shelter.