When a lightning bolt goes from cloud-to-cloud or cloud-to-ground it suddenly heats the air through which it passes. This sudden increase in temperature causes thunder that occurs at the same time as the lightning. Although it takes virtually no time to see the lightning, depending upon the observer’s distance from the lightning, it can take quite a while to hear the thunder.

The speed of sound at room temperature is about 1,100 feet per second. A mile is 5,280 feet, so it takes about five seconds for sound to travel one mile. So, to determine how far away lightning has struck, when you see the lightning, count the number of seconds it takes before hearing thunder. Divide the number of seconds between the lightning and thunder by five to determine how many miles away the thunder and lightning occurred. For example, if you see a flash of lightning and approximately 10 seconds later you hear the thunder, divide the 10 seconds by the 5 seconds per mile to find that the lightning occurred 2 miles away.