WRAL WeatherCenter Blog

Which is it? The storm is moving away after you count from the time it lightings to the time it thunders? How does that work?

Posted March 31, 2008

MIKE MOSS SAYS:        Patsy,    There are a couple of things that come to mind if I'm interpreting your question correctly. First, when it comes to counting seconds as regards thunder and lightning, there is a rule of thumb that you can use to estimate the distance to the closest part of a lightning strike. To do so, you count the number of seconds between seeing the lightning flash and the arrival of thunder. Dividing that number by 5 will yield an estimate of the distance in miles to the lightning strike. For example, if you see the flash and it takes 10 seconds to hear the thunder, the lightning is about two miles away. Of course, if there are a lot of strikes in a short time, it can be difficult to associate the right flash with a particular clap or rumble of thunder. Generally speaking, thunder usually can't be heard for lightning strikes more than about ten miles away, and in rare cases lightning can strike as far as ten miles from the storm cell. So, storm safety experts recommend taking shelter in a safe location if you can hear thunder.

The other thunderstorm safety tip that you might have in mind is the recommendation to wait thirty minutes after hearing the last clap of thunder before resuming outdoor activities in the wake of a storm. Typically, if that much time has passed with no thunder, the storm has either moved out of range for striking your area or has dissipated. Of course, there is no guarantee that a new storm couldn't form nearby or rapidly move into the area at that point, but in most cases the "thirty minute rule" is a reasonably good safety precaution.


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