Why Do We See Lightning Before We Hear Thunder?
Lightning usually reaches our eyes before thunder reaches our ears because light travels faster than sound. Light travels through air at the rate of 186,282 miles (299,792 kilometers) per second.
So you see the lightning the same instant it flashes, even though it may be miles away. But it takes the sound of thunder nearly five seconds to travel one mile. Sound, however, is much slower.
A sound wave travels through the air as vibrating molecules bump into their neighbors, transferring energy. This process takes time, so a sound wave moves about 1,000 feet in that same second (the speed varies a bit, depending on temperature and the amount of water vapor in the air).
You can tell how far away lightning is by counting the seconds between the flash of light and the clap of thunder. If you count five seconds, the lightning is about a mile (a kilometer and a half) away.