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For those of us looking to get some scientific info. out of summer's last storms;

How to Calculate the Distance from Lightning
You know how it is: a thunderstorm is approaching, and suddenly you hear an absolutely deafening clap of thunder. Woah, that sounded close! But how close is the lightning, really? It is difficult to determine the distance of a lightning bolt just by looking at it, and the volume of the thunder isn’t a good way to tell either. Read on for a simple, potentially life-saving method you can use to approximate how far away lightning is.

Steps
Watch the sky for a flash of lightning.
Count the number of seconds until you hear thunder. If you have a watch with a second hand or a digital watch that does seconds, begin timing as soon as you see the lightning and stop as soon as you hear the thunder start. If you don’t have a watch, do your best to count the seconds accurately. Say "One one thousand" in your mind for each second.
Divide the number of seconds by five to calculate the distance in miles (or divide by 3 for kilometers). In other words if you counted 15 seconds from when you saw the lightning, the strike was 3 miles (~5 kilometers) from your location. The delay between when you see lightning and when you hear thunder occurs because sound travels much, much slower than light. Sound travels through air at about 1100-1200 feet per second — (330-350 m/s) a little more than one mile per five seconds (one kilometre per three seconds).
Seek shelter immediately if a storm is approaching.