Name: Kelvin-Helmholtz Instability - Sixty Symbols
Uploaded: 2020-03-30T04:20:16.000Z
Duration: 4 min 32 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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I thought I would talk about the very strange cloud formation in this rather beautiful picture

taken on the Norfolk coast a week or so back.

Their technical name amongst cloud people is they call fluctus clouds.

Amongst physicists, they're called Kelvin-Helmholtz clouds and the reason why they called Kelvin-Helmholtz clouds

is they illustrate a phenomenon called the Kelvin-Helmholtz instability -

discovered by Lord Kelvin and Hermann von Helmholtz in the mid 19th century.

[Brady]: I'm going to call them hook clouds. [Professor Merrifield]: Okay, cool.

I mean, you're right, they tilt over. It turns out it's a very common phenomenon.

I mean you see it in clouds from time to time,

but actually it's the same physics that if you have wind blowing over water.

It's why you get ripples appearing in the surface of the water, you see these kind of Kelvin-Helmholtz instabilities

If you have a jet of material coming out of the center of a galaxy and it starts to wiggle around, that's probably this Kelvin-Helmholtz instability as well.

It's basically wherever you have two streams of material traveling at different speeds, this instability will kick in.

Let me draw you a picture. Two streams of stuff, and they're just traveling at different speeds

but just for simplicity we'll start one going that way and one going that way.

It could be one still and the other moving but they just have to be moving relative to each other.

Instability is something where if something starts to grow it will continue to grow.

If you start with a little perturbation it just becomes a bigger and bigger and bigger perturbation.

And it turns out there is this thing called the Kelvin-Helmholtz instability which will kick in in this case.

So let me draw the same thing again, but with just the beginnings of kind of an instability in the middles.

It's tiny, I mean it can be very very small,

It's just a question of you know will it go away again, in which case nothing happens;

or will it keep growing and growing growing, in which case it's interesting.

So what happens in this case is if you look at the air flowing here,

you can see it's the same kind of volume of air is moving all the way along here, but you can see here

its passage gets a little bit constricted. There it gets wider, there it's constricted again.

In the place where it's constricted, in order to get the same amount of air through, it has to go faster.

Okay, so the air flow here becomes faster

to get through and actually here, there's more space for the air flow below it, becomes slower.

The material traveling over the top of this bump here is actually traveling faster than the material below

and there's this effect, this thing called the Bernoulli effect.

It's the same thing that makes aeroplanes wings have lift,

which is where air is traveling faster and it is decreasing the pressure.

That's why with an aerofoil, with the air going over the top of it, the air travels further

and therefore it travels faster and therefore the pressure decreases and

that causes the wing to go up. So we have the same phenomenon here. We have lift.

Like that the air here is traveling faster than the air here which means the pressure here goes down

So the net effect of that is that then this bit gets tugged up because the pressure above it is lower and pressure below it is higher.

And similarly down here is a decrease in pressure here and that means that this pulled that way.

So you can see that what was a little ripple here is now starting to grow, and then the last thing that happens is

as this thing grows across it's going to get kind of swept along by the air flowing this way,

so it likely end up being curled over that way.

That's the phenomenon you see in these clouds. The little things started to grow out there it grew bigger

and then it got caught up in the flow above and got pushed over that way.

And so those little kind of horse head shapes that appear here are basically this Kelvin-Helmholtz instability,

of the instability first growing upwards and then getting pushed along with the airflow above.

[Brady]: Horse head clouds, that's an even better name. [Professor Merirfield]: That's nice, yes.

So there it is, the Kelvin-Helmholtz Instability.

For an extra video showing how this experiment's done, click on the links on the screen or down in the video description.

And we have another Sixty Symbols video about stripey clouds this time,

or to really complete your cloud fix, why not check out this Objectivity video about the so-called Father of Cloud Studies.

There's a guy you'd wanna meet. A cloud resembling a lock of hair or a feather