In the United States, an average of 67 people are killed each year by lightning. Each spark of lightning can reach over five miles in length, soar to temperatures of approximately 50,000 degrees Fahrenheit, and contain 100 million electrical volts. The rapid expansion of heated air causes the thunder. Since light travels faster than sound in the atmosphere, the sound will be heard after the lightning. If you see lightning and hear thunder at the same time, that lightning is in your neighborhood! If you are caught outside squat low to the ground. Place your hands on your knees with your head between them. Make yourself the smallest target possible.
Mary Knapp, Weather Data Library