# What is thunder?

Hi!

I wonder what makes thunder sound the way it does. I don't really understand lightning either but it is more easy to understand why it sounds. But what makes thunder sound? What happens up there in the sky?

Best regards, Edison
PS
I see now that there is a similar thread already, in spite of this I hope for an answer.

ProfuselyQuarky
Gold Member
From the link . . .
Because electricity follows the shortest route, most lightning bolts are close to vertical. The shock waves nearer to the ground reach your ear first, followed by the crashing of the shock waves from higher up. Vertical lightning is often heard in one long rumble. However, if a lightning bolt is forked, the sounds change. The shock waves from the different forks of lightning bounce off each other, the low hanging clouds, and nearby hills to create a series of lower, continuous grumbles of thunder.

Let me be even more ignorant, I'm not sure I agree with you. I think there actually can be thunder without lightning. But of course, I might be wrong. But if I'm right, what makes the sound then?

Best regards, Edison

ProfuselyQuarky
Gold Member
Let me be even more ignorant, I'm not sure I agree with you. I think there actually can be thunder without lightning. But of course, I might be wrong. But if I'm right, what makes the sound then?
Well, if you don't think that lightening causes thunder, there not much I can say about that.

When the air rapidly expands, it leaves a channel—pretty much a teeny hole with no air present. When that air collapses, it makes a sound.

There might have been a time when you heard thunder without seeing lightening. Well, you probably just missed the lightening!

May I ask why you don't think lightening is the cause of thunder?

Cheers,
PQ

I thought that maybe charged particles in the sky somehow hit eachother and if a large amount of charge (i.e high voltage) hit another large amount of charge (ions of some sort, preferably nitrogen ;) ) then maybe there, somehow, can be a sound. I don't know how but isn't this kind of what ligtning means? My idea being within the clouds?

Best regards, Edison

Drakkith
Staff Emeritus
I thought that maybe charged particles in the sky somehow hit eachother and if a large amount of charge (i.e high voltage) hit another large amount of charge (ions of some sort, preferably nitrogen ;) ) then maybe there, somehow, can be a sound. I don't know how but isn't this kind of what ligtning means? My idea being within the clouds?

That's simply not how it works. Sorry. Lightning causes thunder, exactly as PQ explained.

russ_watters and davenn
Yes, thunder is associated with lightning. In fact, that's a way to tell how far away the storm is - you measure the time difference between seeing the lightning and hearing the thunder. The lightning comes instantaneously for all practical purposes but the thunder travels at the speed of sound. So speed of sound x time difference = distance to storm.

rcgldr
Homework Helper
What you hear from lightning depends on the distance. If the strike is close enough, you hear the crack of a shock wave. If the strike is further away, the shock wave turns into a conventional sound wave, which is the thunder you normally hear.

I think there actually can be thunder without lightning.

By definition, thunder is the sound caused by lightning.

I'm an electrician, not a physicist. However, in terms of an electrical discharge, a lightning strike is a current flowing between two areas with differing voltage potentials. So lightning can strike from a cloud to the ground, or to another cloud with a different potential, or even to the open air or a passing plane.

Observation of lightning strikes and experiments with arc flashes show that lightning is basically the same phenomena as the electrical power in your home. If you've ever accidentally unplugged your vacuum while it was running your probably heard a small pop (depending on the point in the AC sine wave, and how quickly the plug came out). This pop is the air being ionized and expanding. I don't know the numbers for ionized air, but in an arc blast the copper from the conducters expands to about 30,000 times its original volume and about 35,000 degrees F, with larger shrapnel traveling at around 900 ft p/sec, or about the muzzle velocity of a .45ACP from a M1911 military sidearm. This creates a very loud boom.

So, it is well-established that when an electrical current ionizes the air, it creates a flash and a super-heated path of ionized air between differing potentials, and the hot ionized air expands rapidly as a pressure wave which is audible to the human ear. Why thunder is stretched out and rumbles is explained by others, due to the distances and echoing effects.

Particles hit each other every moment—both charged and not charged. Why would this create a noise in the sky?

You are too cute :)

Force between charged particles can be described as

$$F=qE$$

where E is the electrical field intensity and q is the charge (of a single particle, obs)

For a free charged particle it's energy is (integrated over the force between two charged particles).

$$W=qU=\int_{-\infty}^{r}\frac{qQ_2}{4\pi \epsilon_0 r^2}dr$$

I think :D

Anyway, it amounts to something like

$$qU=\frac{qQ_2}{4\pi \epsilon_0 r}$$

which means we have a potential of

$$U=\frac{Q_2}{4\pi \epsilon_0 r}$$

but we should say

$$U=\frac{\sum_{k=1}^{N}Q_{2k}}{4\pi \epsilon_0 r}$$

to consider all charges.

This potential and also electrical field intensity can be very high and as we look at how this has been derived we see that the energy was meant to be explained for a single charge (q). For the potential we can then strike q to get only Q2. But Q2 is actually the sum of all charges considered.

Now we have a rather large potential for lots of Q2. Equally, we may view another group of charges as Q3. And if the number of charges are hugely different the potential difference is also huge.

Now, my knowledge stops here, but high voltages got to be able to be created in the clouds already and thunder may therefore happen even within the clouds.

Another way of viewing it is that there may be charge gradients within the clouds that can result in discharges within the clouds when the electric field intensity is greater than the dielectric strength of air, I think :)

Best regards, Edison
PS
I am also not a physisist, obviously :D

davenn
Gold Member
Now, my knowledge stops here, but high voltages got to be able to be created in the clouds already and thunder may therefore happen even within the clouds.

if there is a lightning discharge within the cloud, then yes .... no discharge, no thunder ... period

Another way of viewing it is that there may be charge gradients within the clouds that can result in discharges within the clouds

yes, of course, it happens all the time, typically between lower and upper levels in the cloud

one example .... keep in mind that the base of the cloud is more commonly negatively charged
as per the lower image

Dave

Edison Bias
ZapperZ
Staff Emeritus
@Edison Bias

I don't get it.

You asked for something, but then you didn't like the answer you were given. So instead, you made a guess at what the answer might be. Not only is this a speculation that has no physical evidence, but if you already made up your mind, why do you bother asking in the first place?

Here's the deal: can you find verified evidence of thunder happening WITHOUT lightning? You need to establish that evidence FIRST before offering an explanation for something that may not even happen! That's like arguing about the size of a unicorn's horn!

Zz.

ProfuselyQuarky and davenn
berkeman
Mentor
if there is a lightning discharge within the cloud, then yes .... no discharge, no thunder ... period
And with Dave's fine post, this thread is done. The OP's question has been answered by multiple posters.

ProfuselyQuarky