Dear Tom,
What is the formula for calculating the distance of a lightning flash when you know the number of seconds between seeing the flash and hearing the crash of its thunder?
Cal Lodgin, Milwaukee

Dear Cal,
We see lightning practically at the same moment that it occurs because the flash travels at the speed of light, approximately 186,000 miles per second. Thunder, on the other hand, travels much more slowly: At 740 mph when the air temperature is 32 degrees F. and 767 mph at 68 degrees, or about one mile in 4.7 seconds. Therefore, the distance between you and the closest portion of a lightning flash can be approximated by dividing the number of seconds that elapse between seeing the flash and hearing the crash of its thunder by 4.7.