Lightning to Thunder: Every 5 Seconds Is About a Mile

You see lightning almost instantly, but its thunder takes time to arrive. The U.S. National Weather Service explains how to use that delay. Since it takes the sound of thunder about 5 seconds to travel a mile, you can count the number of seconds between the flash of lightning and the sound of thunder, then divide by 5, and you will get the distance to the lightning in miles. Five seconds means one mile, 15 seconds means three miles, and no gap at all means very close. The thunder itself comes from heat. The lightning discharge heats the air rapidly, to as much as 50,000 degrees Fahrenheit, and makes it expand. That sudden expansion creates the sound wave we hear. The Weather Service adds a warning: thunder means you are within striking distance and should get to a safe place immediately.