# Why does thunder from a distance rumble

Physics is Phun
why does thunder from a distance rumble, but thunder really close make a loud crack. shouldn't the distant thunder just be a quieter version of the loud closer thunder/lightning

my hypothesis is that it is due to different temeratures in the air between me and the lightning so when the lightning strikes far away some of the thunder travels faster than other parts of the thunder due to variances in temperature. the farther the lightning the larger the difference in time it takes the thunder to reach my ears.

Thats just my idea, don't know if it's any good or not...

thanks

Staff Emeritus
That sounds like a good theory to me. Electronic signals experience a similar phenomenon, known as "multi-path interference", whereby a signal takes multiple paths. It's a reasonable guess that sound experiences a similar phenomenon, but I don't actually know for sure.

Gold Member
pervect said:
That sounds like a good theory to me. Electronic signals experience a similar phenomenon, known as "multi-path interference", whereby a signal takes multiple paths. It's a reasonable guess that sound experiences a similar phenomenon, but I don't actually know for sure.
It makes sense, but I'm not sure that it would be the sole explanation. I'm wondering if perhaps there are low frequency vibrations conducted through the ground as well as the multi-spectrum ones through the air. That would bring in the addition of secondary effects from bouncing about through different densities of solid matter.

PatPwnt
That's funny I was wondering about that exact same thing earlier today when a thunder-storm had came through.

Timbuqtu
Actually there is something interesting going on here if we idealize the problem. Let's regard the thunderbolt as a infinite long line which exitates the air and causes soundwaves to travel toward us.

In case the sound was emitted from one point we would sound the same from no matter what distance we listen, apart from being quieter. This is because the fundamental solution of the wave equation in 3 dimensions (or the Green's function of the wave operator) lies on the surface of the sound-cone (the equivalent of the light-cone). Or stated differently, a short pulse at the origin will travel outward in all directions with the speed of sound.

Now let's return to the problem of the infinite line exitation. Because of translational symmetry along the line, we can examine the wave propagation as if it were happening in 2 dimensions. Now it is a mathematical fact that the fundamental solution of the wave equation in two dimensions is not confined to the surface of the sound-cone, it is non-zero inside the cone too. This means that you will not only hear a sound pulse just when the wave front reaches your ears, it will be spread out in time while the amplitude decreases until you cannot hear it anymore.

I'm not sure if this is the answer you were looking for, but I think it is an interesting application of fundamental solutions/green's functions in different dimensions and it explains mathematically why you only see an instantaneous lightning flash, while you hear the thunderclap a longer time.

Physics is Phun
it probably is the answer I am looking for, but I am having a hard time comprehending it. (greens function? translartional symmetry? fundamental solution? sound cone?)

Timbuqtu
The point is that the sound of thunder is not emitted from one point, but from a line (the path of the thunderbolt) which results in a different distance dependence of the sound wave. Not all points on this line have the same distance to the observer and therefore the observers observes the sound spread out over a time interval. (loosely speaking)

Donski
Physics is Phun said:
it probably is the answer I am looking for, but I am having a hard time comprehending it. (greens function? translartional symmetry? fundamental solution? sound cone?)

If you're having a hard time comprehending it, then forget the math and the complex equations. In laymens terms, it's the echo off both the atmosphere and the things on the ground. If the conditions are just right, it can echo back and forth and create rolling thunder. When it hits next to you, it can take a few seconds for the initial echo to reach you but it's there. You will first hear the crack and then later you will hear the rolling thunder from it almost appearing to sound like a separate lightning strike. When it hits far away it echoes off the same atmosphere that the original crack came through, so both the crack and the echo seem to be heard starting at the same time. And every variation in between, depending on how far away the strike was.

Now if you're looking to do research into lighting and thunder, then what Timbuqtu said applies. Another simply way of saying it is that it is "multi-path interference" as Pervect said.

Gold Member
I read somewhere that the sound of thunder (long, rumbling vs. short boom) tells you about its direction. I could just never figure it out.

Gold Member
Timbuqtu said:
Actually there is something interesting going on here if we idealize the problem.
I don't know if this is a situation where having no education is better than having a partial one, but I suspect so. I have no idea what any of those terms that you mentioned mean, but I get an insight (I hope) from your post. Since a lightning bolt superheats the air in a linear sequence, like a zipper opening, it makes sense that there would be a 'boom carpet' as in a supersonic aeroplane passing instead of one huge blast of sound. Even ignoring other factors such as reverberation, that would account for the 'rolling' effect. Did I interpret this correctly?