How can we determine how far away lightning is?
Because of the vast differences in the speed of light and the speed of sound, the flash of lightning precedes the rumble of thunder. It takes sound waves five seconds to travel one mile, whereas the flash of lightning travels the same distance in less than one one-hundred thousandth of a second. To determine how far away the lightning is, use the flash-to-bang rule. When you see the FLASH, count the seconds to the BANG. Every five seconds equals one mile. For example, if 15 seconds elapse between a lightning bolt and when you hear its thunder, the bolt was three miles away.
Just because a lightning bolt is a few miles away doesn’t mean you are safe. The National Weather Service advocates the 30/30 rule: Take cover if you hear thunder within 30 seconds of the lightning, and then wait at least 30 minutes after the last lightning flash before resuming outside activities.
Here are some safety rules. Try not to be outside when there is lightning. Avoid all metal objects, such as flagpoles, metal fences, golf carts, baseball dugouts, and farm equipment. Avoid being in or near high places or in open fields. In open areas, go to a low place and crouch down on the balls of your feet. Don’t lie down; if lightning strikes nearby, you minimize your risk of burns by crouching instead of lying. Vehicles with the windows rolled up usually provide good shelter from lightning—but avoid contact with any metal.