lightning causes thunder. lightning happens instantaneously, but thunder (at a distance) sounds like it’s coming, then it claps, then fades away. this don’t make no damn sense given that light and sound move at fixed rates.
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The speed of sound actually does depend on humidity/temperature, but the primary source of the “rolling” is echoes as the thunder bounces off trees, houses, the ground, etc., on its way to you.
Fork lightning often happens in a single strike, but the rolling thunder is usually sheet lighting, which is a lot of smaller discharges within a cloud or between nearby clouds… You can have rolling thunder from fork lightning, when there are multiple discharges, but it’s usually chains of sheet lightning
Quite simply a lightning strike is very large. It isn’t one single source of sound, but causes by a very long extended source.
The strike can be hundreds of yards long.
Let’s take the largest “rolling thunder” scenario:
If a strike is horizontal (cloud to cloud), and long, and pointed in your direction, you’ll hear the thunder from the point closest to you first, then points further and further away.
You’ll get a long roll of thunder as the sound travels to you, each moment having traveled a larger distance.
Now if the strike is short, and pointed perpendicular to you (either right/left or cloud to ground) then most of the sound vibes from one general region. You’ll hear a shorter more intense clap of thunder.
The thunder isn’t created at a single point. When lightning strikes, it moves in a path and thunder is created along that path. What were hearing is the pressure wave created by a bunch of air heating up really fast.
Now yes, all of that thunder is going to travel at approximately the same speed, but it isn’t all going to travel the same distance. Let’s say a lightning bolt goes from could to ground in a vertical line and it’s close enough that we don’t need to worry about the curve of the earth. The sound from where the lightning hits the ground is going to get to you before the from where the lightning came from the cloud because it’s further away. Combine this with all the branches lightning can creat or cloud to cloud lightning that may be moving towards or away from you, and you get a large array of ways thunder can sound to an observer.
[This video](https://youtu.be/5liPK64MAeY) does a pretty good job explaining it. He sets up multiple explosives to go off at different times and different distances to make one loud boom in the front, various booms with appropriate timing from the side, and various booms that are much further apart from behind
And then you also have echos so you hear the same waves again, and then changes in air conditions between you and the lighting, which changes the speed of sound and changes the shape of the wave front adding to the complexity even more
The rolling is the echo of the cracks bouncing off of stuff. Thunder is a loud crack when your very close to it.
A lightening bolt is an extended object which may be multiple miles (or km) long. So the sound from the nearer part will get there before the sound from the farthest away part. So that’s why thunder can be more than just one crack.
The people mentioning sound bouncing are not wrong. But you have to keep in mind that the entire length of the lightning also makes a sound. From the part that’s closest to the ground to the part that connects to the clouds. The whole thing makes a sound from the air being converted to plasma in an instant, all the way up. (Most lightning travels “up”, travel of electrons from ground as the ionosphere desires that charge). Most lightning travels as a “desire” for electrons down, until the most direct path for electrons to travel up is established. Then travels back up as a wave of electrons makes its way from ground into the sky.
Edit: Skip to 5 min in
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