WRAL WeatherCenter Blog

I've got some age on me, (retired from Wake Co. Schools) and my mama always told us that once we saw a streak of lightening, begin counting (not fast) and when we heard a clap of thunder, that the rain was that many miles away. Have you ever heard this? Try it out if you have not.

Posted July 18, 2007 8:07 a.m. EDT

Related question from Elizabeth - "What is behind the story of counting between when lightning flashes and the thunder sounds. Have you heard anything about this?"


MIKE MOSS SAYS:      Gloria and Elizabeth,    The rule of thumb that you're both referring to states that if you see a lightning flash, and count the number of seconds between the visible flash and the time you hear thunder from that flash, you can estimate the distance to the flash in miles by dividing the number of seconds by five.

For example, if you see a streak of lightning and count ten seconds before the thunder hits, the lightning was about two miles away. The reason for all this is that the light from the flash travels at just under 186,000 miles per second and therefore reaches your location almost instantly, but the sound waves that you hear as thunder travel much more slowly, at a speed of about one mile in five seconds (around 720 mph).

A few things to keep in mind with this rule of thumb: First, it works best when there are relatively few lightning strikes, because when there are a lot of them in a short time span it can be difficult to match up a particular flash with the correct burst of thunder. Second, it dosn't always help much with figuring out the distance to rain, because the lightning can sometimes strike many miles away from the rain shaft. Third, it usually doesn't work past about ten miles away, because thunder is often inaudible from a strike beyond that distance. Finally, it isn't a very helpful safety aid, since some storms are known to produce successive lightning strikes as much as five to ten miles apart, or occasionally to produce strikes that reach ten miles or more from the storm, so you might calculate a strike to be, say, six miles away, and still the very next one could be in your location.

For a few additional details about this subject, see an earlier Ask the Meteorologist post at  http://www.wral.com/weather/blogpost/1435060/