Nib asked
What causes the sound of thunder?
Thunder is not clouds colliding. A lightning bolt heats a thin channel of air to about 30,000 degrees Celsius in microseconds; the air bursts outward as a shock wave, and because the bolt is kilometers long and the sound echoes, you hear…
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1not clouds
People often say thunder is clouds crashing, or the sky splitting open. Neither is true. Clouds are made of tiny droplets and ice, and they cannot clap like hands. Thunder is a sound made by the lightning bolt itself, and the way it is made starts with heat.
Thunder is made by the lightning bolt, not by clouds colliding.
a common mix-upThunder is not clouds banging together.
2super heat
A lightning channel is only a couple of centimeters wide, but a huge current races through it, and it heats the air to roughly 30,000 degrees Celsius in a few millionths of a second. The surface of the Sun is about 5,500 degrees Celsius, so the bolt is about five times hotter.
A bolt heats its channel of air to about 30,000 degrees Celsius almost instantly.
About how much hotter than the Sun's surface is a lightning channel?
Show the answer
About five times hotter.
3the burst
Heated that suddenly, the air in the channel has nowhere to go but out. It expands faster than the speed of sound, pushing a shell of compressed air ahead of it. That is a shock wave, the same kind of thing as an explosion's blast, and as it weakens it becomes the sound we call thunder.
Superheated air expands faster than sound and makes a shock wave: that is the thunderclap.
4count the seconds
Light travels so fast that you see the flash almost instantly. Sound travels about 343 meters a second, so every three seconds is roughly one kilometer, or about five seconds per mile. Count the seconds from the flash to the first thunder, divide by three, and you have the distance in kilometers. Nine seconds means about three kilometers.
Divide the seconds between flash and thunder by three to get kilometers.
The thunder comes 6 seconds after the flash. How far away is the strike?
Show the answer
About 2 kilometers, or a little over a mile.
5the rumble
A lightning bolt is not a point: it can stretch for several kilometers, from the cloud to the ground and sideways. The sound is made along the whole length at the same moment, but the part nearest you is closer, so its sound arrives first. Sound from the far parts arrives later and later. What was one sudden event is spread into a long rolling noise.
A bolt is kilometers long, so its sound reaches you spread out over seconds.
a common mix-upThunder is one bang that gets muffled; in fact it is many arrivals of the same blast.
6echoes
Thunder also bounces off hills, buildings and even layers of air with different temperatures, so extra copies of the sound arrive late from other directions. Together with the bolt's long shape, the echoes turn one flash of heat into a rumble that can last ten seconds or more. Thunder is rarely heard from more than about ten miles away.
Echoes and the bolt's long path stretch thunder into a long rumble.
7the whole idea
Thunder is the sound of lightning. The bolt heats a thin channel of air to about 30,000 degrees Celsius; the air bursts outward as a shock wave; the bolt's length and the echoes stretch the sound into a rumble; and because sound is slow, the delay after the flash tells you the distance, about one kilometer for every three seconds.
Thunder is a shock wave from air heated by lightning.
Next storm, count the seconds: you are timing a sound that was born in air hotter than the Sun.