Lightning is a streak of electricity that occurs in every thunderstorm. You see lightning before you hear thunder because light travels faster than sound. Lightning is caused when the negative charge of electricity in the rain clouds meets the positive charge of electricity in the falling raindrops. Most lightning occurs from cloud-to-cloud, but some lightning occurs from cloud-to-ground, where it can start fires, melt metal, or be deadly to people. Although 90% of people survive lightning strikes, they can cause major internal injuries, burns, and hearing loss. 240,000 people are struck by lightning or are injured as a result of lightning every year.


Thunder is caused by the rapid expansion of air and temperature inside and around a lightning strike. Such an expansion of air is known as a sonic shock wave. Did you know you can calculate the distance of lightning from its thunder? Thunder occurs after lightning because the light travels much more quickly than sound. In normal circumstances, lightning is 0.2 miles distant from a given location for every second that passes between the lightning and its thunder. Thus, if you see lightning in the sky and hear thunder five seconds later, the lightning is about one mile from your location.