what is the rule for figuring the distance of a lightning flash when you hear the thunder?

Dear Tom,

You have probably answered this already, but what is the rule for figuring the distance of a lightning flash when you hear the thunder?

— Roger Kappelmann

Dear Roger,

Count the number of seconds between the flash of lightning and the crack of thunder and divide that number by five. The result is the distance in miles between you and the closest portion of the lightning bolt. The spark of a lightning bolt heats the air it passes through to 54,000 degrees, causing the air to expand explosively. This initiates a shock wave that we hear as thunder. Light from the flash, traveling at 186,000 miles per second, reaches us practically instantaneously, but sound waves, moving at 720-760 mph, travel about one mile in five seconds. Lightning kills about 50 people per year in this country (though fewer in recent years), 79 percent males and 21 percent females.