When you hear a storm approaching, with thunder rumbling and lightning flashing, did you ever wonder how far away the bad weather is?  Or how close a really big lightening strike was?  There’s an easy way to figure it out.  Just watch for a particularly bright flash, and count to yourself the number of seconds (one Mississippi, two Mississippi, etc.) before you hear the matching thunder clap.  Divide the number by five, and that’s how many miles away the lightening bolt was.

How’s it work?  Well the light (traveling at 186,000 miles per second) reaches your eye just about instantly, but the sound from the lightening bolt takes quite a bit longer to arrive.  It travels at about 1,100 feet per second at sea level, or roughly a mile in 5 seconds.

Of course, if the delay you count is less than a second or two — you better make sure you’re in a safe spot under cover!

Bonus science trivia: Lightning is a really complex phenomena.  A single lightning bolt is actually a number of different electrical discharges traveling down and up between the ground and a cloud (or between clouds) on a millisecond time scale.  The thunder clap you hear is caused by the superheated air along the lightning path expanding in a shock wave.  Particularly long bolts will have a rumbling thunder sound because the shock waves from the farthest parts of the bolt take slightly longer times to reach your ear.