During a summer storm, you might see a bright flash of lightning streak across the dark sky. A few seconds later, a deep rumble of thunder shakes the ground. It seems like the flash and the sound are two separate events, but they actually happen at the exact same moment! So why do we always see lightning before we hear thunder?
The secret comes down to how light and sound travel through the air. Both light and sound move in waves, but they travel at vastly different speeds. Light travels at an incredible speed of about 186,000 miles per second. That is so fast that light can circle the entire Earth seven times in just one second! Because light moves so quickly, the flash of lightning reaches your eyes almost instantly the second it occurs in the cloud.
Sound waves, on the other hand, are much slower. Sound travels through air at about 1,100 feet per second, which is around 750 miles per hour. While that might sound fast compared to a car driving down a highway, it is a tiny fraction of the speed of light. Because sound takes much longer to travel across distances, the loud sound wave created by the lightning strike takes several seconds to reach your ears.
To figure out how far away a storm is, you can use a simple counting trick. The next time you see a flash of lightning, start counting seconds until you hear the thunder rumble. Every five seconds you count means the storm is about one mile away. If you count ten seconds between the flash and the rumble, the lightning struck about two miles from where you are standing.
Understanding the difference between the speed of light and the speed of sound helps explain why storms give us a double show: first a flash to light up the sky, and then a rumble to fill the air!
