Sit on the porch during a summer storm and you'll notice something odd. The lightning is over in a flash, but the thunder that follows can roll and grumble for many seconds. If lightning is so quick, why isn't thunder just one sharp bang?
The answer is that a lightning bolt is long, often miles long. Every part of it makes noise at nearly the same instant, but those sounds have to travel different distances to reach your ears. Sound from the closest part arrives first and sound from the farthest parts arrives last, stretching one blast into a long rumble.
What Makes the Boom
Lightning heats the air in its path to astonishing temperatures. Measurements show the channel can briefly reach about 53,500 degrees Fahrenheit, and it averages around 36,300 degrees. That's several times hotter than the surface of the Sun.
Air heated that fast expands violently, pushing outward faster than the speed of sound. That creates a shock wave, much like the one from an explosion or a jet breaking the sound barrier. Up close, thunder can reach 165 to 180 decibels.
People haven't always understood this. Ancient Greek thinkers blamed wind hitting clouds, and the Roman philosopher Lucretius thought it was hail colliding in the clouds. By the mid-1800s, the popular theory was that lightning created a vacuum that collapsed with a bang. Scientists now agree it begins with a shock wave from the superheated air.
A Long Bolt Makes a Long Sound
A lightning bolt isn't a single point. It's a long, crooked channel stretching from the cloud toward the ground, or across the sky. The record holder, seen over Texas and Kansas in 2017, stretched about 515 miles.
When lightning strikes close by, people describe first a clicking or cloth-tearing sound, then a loud crack like a cannon shot, followed by a continuous rumble. The early sounds come from the nearest parts of the bolt, and the rumble comes from parts that are farther away.
Distance changes the sound, too. Nearby thunder tends to be a sharp crack, while distant thunder is usually a long, low rumble. Researchers who study thunder have found that rumbles are quieter and lower-pitched, and can last more than 30 seconds.
Count the Seconds
You always see lightning before you hear thunder, because light travels far faster than sound. Sound moves through air at about 768 miles per hour, which works out to roughly five seconds per mile. Light, by comparison, makes the trip almost instantly.
That gives you a handy trick. Count the seconds between the flash and the thunder, then divide by five. The result is roughly how many miles away the lightning struck. It's a useful way to tell whether a storm is getting closer.
Thunder Can Sound Louder on Some Nights
Sometimes thunder sounds especially powerful, almost as if the storm is right overhead. That can happen during a temperature inversion, when cooler air sits near the ground beneath warmer air. The sound can't spread upward as easily, so it stays concentrated near the surface where you are.
So the next time a storm rolls through, listen closely. That long rumble is really a sound map of the lightning bolt, arriving piece by piece. And if you're counting less than a few seconds between flash and boom, it's time to head indoors.
