Why do we see lightning before we hear thunder?
Light travels about 900,000 times faster than sound in air, so the flash reaches you almost instantly while the thunder takes about 3 seconds per kilometer.
Lightning and thunder happen at the same moment. The lightning bolt heats the air around it to about 30,000°C (54,000°F) in a fraction of a second. The air expands so violently that it creates a shock wave, and that wave is the sound we call thunder.
Light from the flash moves at about 300,000 kilometers per second, so it arrives almost immediately. Sound moves through air at only about 343 meters per second. The farther away the storm is, the longer the gap between the flash and the rumble.
How to estimate a storm's distance
Count the seconds between the lightning and the thunder. Divide by 3 to get the distance in kilometers, or by 5 for miles. A 15-second gap means the strike was about 5 kilometers (3 miles) away. If the gap is 30 seconds or less, the storm is close enough to be dangerous, so it is time to go indoors.
Did You Know?
Thunder can usually be heard up to about 15 to 25 kilometers (10 to 15 miles) away. Lightning seen without any thunder at all, sometimes called heat lightning, is simply a storm too far away for the sound to reach you.