Thunder is the sound of rapidly heated air expanding and vibrating, and shock waves. Light travels faster than sound, so we see the lightning before we hear the resulting thunder. Using the speeds of light and sound as well as the atmospheric conditions, you could calculate exactly how far away the storm is. Chances are you're not going to have a calculator handy during a storm. However you can approximate the storm's distance in miles by counting the seconds between the lightning and thunder and dividing by five. It takes about five seconds for the sonic boom to travel one mile, so if you see the sky illuminated by a streak of lightning and count 10 seconds until you hear the low rumble of the thunder, the storm is about two miles away. Of course, lightning is dangerous, so it's always a good idea to seek shelter before you stop to count seconds. Otherwise, it might be your final countdown.