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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.

  • Kind of like breaking waves, maybe?

  • 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

  • in the cloud patterns on Jupiter,

  • on the sun.

  • 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.

  • Doesn't have to go so fast.

  • 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

  • Yeah. Okay

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


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B1 中級

開爾文-赫姆霍茲不穩定性--60個符號。 (Kelvin-Helmholtz Instability - Sixty Symbols)

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    林宜悉 發佈於 2021 年 01 月 14 日