Lightning is probably the most common element of a thunderstorm. After all, you can't have thunder without lightning. Electricity is all around us, and a thundercloud, like a battery, has positive and negative electrical charges. There are positive and negative electrical charges on items on the ground, like homes, trees and, of course, people.
As the thundercloud moves along, it pulls positive charges toward it from the ground. If you've ever been under a strong storm, and your hair has stood on end, you're feeling the effects of the positive charges being drawn upward toward the cloud. And if this does happen to you, you're in immediate danger of being struck by lightning! Crouch down onto the balls of your feet, and do not lie flat.
In a cloud to ground lightning strike, the cloud sends down an invisible negatively charged leader. When this charge gets close to the ground, it is met by a positive charge and a channel develops. What you see during a lightning strike is the return positive charge back to the cloud.
But what about the thunder? when the electrical charge returns to the cloud, it heats the channel to 30-thousand degrees Fahrenheit. This causes the air around the lightning strike to expand rapidly. Thunder is the sound of the air rapidly compressing back to its original space. You can use the five-second rule to get a rough idea of how far away a lightning strike is. As soon as you see the lightning flash, begin counting. Stop counting when you hear the thunder. Divide this number by five and this is how many miles away the lightning is.
Generally, if you can hear thunder, you're close enough to be struck by lightning. You do not want to be the tallest object or be near the tallest object with lightning nearby. Get inside or get into an automobile. Do not let blue sky or sunshine fool you, and wait at least 30 minutes after the last flash before you leave your shelter.