Amazing Clouds and the Kelvin-Helmholtz Instability - Sixty Symbols

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're called Phluctus clouds, but amongst physicists they're called Kelvin-Helmholtz clouds. And the reason why they're 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. I'm going to call them Hooke clouds. Okay, call them what you like. Kind of like, I don't know, breaking waves, maybe? I mean, you're right, they're killed 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 on 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 centre 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 travelling at different speeds, this instability will kick in. Let me draw you a picture. Two streams of stuff, and they're just travelling 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. An instability is something where if something starts to grow, it will continue to grow. So 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 a tiny, I mean it can be very, very small, and 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 and 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 it's kind of its passage gets a little bit constricted, there it gets wider, there it gets 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 airflow here becomes faster to get through, and actually here there's more space, so the airflow below actually becomes lower, doesn't have to go so fast. The material travelling over the top of this bump here is actually travelling 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 travelling faster, there's a decrease in the pressure, that's why with an airfoil 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, right, that the air here is travelling 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 the pressure below it is higher. And similarly down here, there's a decrease in pressure here, and that means that this bit gets pulled that way. And so you can see that what was a little ripple here is now starting to grow. And then the last thing that happens is that as this thing grows, of course it's going to get kind of swept along by the air flowing this way, so it'll actually end up being curled over that way. That's the phenomenon you see in these clouds, right? The little thing started to grow up 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 horsehead 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. Horsehead clouds, that's an even better name. Isn't that nice? Yes. So there 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 60 symbols video about stripy 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 want to meet. A cloud resembling a lock of hair or a feather. Yeah, okay.