Dave Ansell and Ginny Smith, The Naked Scientists
Ginny - So, weíve been talking about earthquakes and tsunamis, but there are other kinds of event that are more related to weather rather than directly to what's going on under the ground. And today, weíre going to look at one of them. So, what are we going to do?
Dave - So, what weíre going to do is try and actually build a model of a hurricane which is quite difficult in this small a space, but what drives a hurricane is hot water heating up the air above it. You get hot damp air above the water, and then that moves on. So, itís a way of injecting energy into the system.
Ginny - So, when you heat something, it becomes less dense and it rises. So, that's what happens to the water as it heats up. It starts rising. Now, weíre not going to use water here. We thought that would be a bit boring. Weíre going to use fire. Weíre just going to set fire to some fuel here and you'll see that you get a flame that should go upwards. The air is heating up and itís rising, and itís expanding. Once we actually manage to set fire to it, you should see that. Hopefully, it will catch in a minute. And that's the same as what happens to the water. It gets warm and it starts to rise. Now, that's not a problem. You just got something rising. When it becomes a hurricane is when it starts spinning. Now, the reason that happens is because over the oceans, winds can come in from a really far distance because it's flat, there's nothing to stop them. And the earth is curved, so as these winds come in, they're actually spinning very, very gently because the Earth is spinning. Now, I donít know how many of you have ever watched ballet or ice skating. Anyone? Yeah? Have you ever seen it where the dancers beautifully demonstrated by Dave here, heís an expert dancer. They start to spin and they start out spinning quite slowly and they've usually got either their leg or their arm out. So, they're spinning quite slowly and then all of a sudden, they whip their leg in and they suddenly go much, much faster. Have you seen that before? Who liked the demonstration? So, that's happening because of something called the conservation of angular momentum.
Dave - So, there's a couple of effects going on. One of them is that Ė you can imagine going around a big circle and a small circle. The small circle is a lot smaller so it takes you less time to get around, so you'd spin faster. And also, as you pull in, you're actually doing work. If you ever tried to climb into the middle of a roundabout, itís really hard work and that work is actually going into speeding you up. So, as you move weight in the centre, you spin a lot faster.
Ginny - So, what we need to do here to build our hurricane is to get the air spinning, and that's what this is for. So, weíve got a mesh here that when we start spinning it will actually catch the air and make the air spin.
Dave - So, weíve got basically a cylindrical mesh around the fire and as you can see, as the air starts to be spinning slowly by the mesh, as it gets in the centre, it spins really, really fast and forms this spinning column of air which is actually getting a little bit high and you might want to slow down Ginny before we hit the ceiling. And this is essentially, working on the same principle as a hurricane. You do actually get fire tornadoes in large fires occasionally. If you get the air around a big forest fire spinning slightly, then the fire pulls it into the centre, you do get these much larger versions of this very, very scary spinning fire tornadoes.
Chris - Brilliant! Well done, Dave. Thank you, Dave and Ginny. Any questions about extremes of weather, extremes of hurricanes, or other questions about physics, chemistry for Dave and Ginny
Aron - Hi, itís Aron here from Hardwick. I was just wondering, has there ever been a real fire tornado in the scales that we have normal tornadoes in this world?
Dave - I havenít seen any really huge ones going up really high Ė I think it would be difficult keeping the fuel burning very, very large. There's definitely been sort of 50 metres high I've seen pictures of, but that's not to say that I'm an expert on fire tornadoes globally I'm afraid.
Chris - So Dave, if you had a really big fire in a forest or something, could this cause the same sort of phenomenon? Could you get a sort of swirling effect like a hurricane?
Dave - For that to work, you need the air to be spinning to start with. So, you'd have to have some configuration of hills or something around it and the prevailing wind which starts the air spinning a bit then the fire itself will kind of drag the air in and concentrate that spin and speed it up very fast, and you'll get a fire tornado.
Ewan - I'm Ewan from (St. Nichs) and my question is, if there's a hurricane and you can't get away from it and you get sucked in, what would happen to you in the hurricane?
Dave - A hurricane is basically a really, really powerful storm. So in a hurricane, you can get picked up and thrown around. It can pick up cars and throw it around. When you're in the air, you're probably okay. When you hit the ground again, you're probably in trouble.
Ginny - Or if you hit something else, that's also been picked up and thrown around like a car.
Dave - Yeah, I mean, that's Ė the other things which really can pick people up are tornadoes and there's certainly been examples of parachutists dropping into Ė not necessarily tornadoes, but places with very, very strong updraft and they've end up going up and down, up and down, and turn into a large hailstone which is rather unpleasant.
Chris - So Dave, what's going on in the eye of a hurricane? Why do we have an eye where itís really calm and quiet?
Dave - So essentially, with the hurricane, on the outside, itís spinning slowly. As it moves further and further inwards, for this effect, itís spinning faster and faster, and faster until you get a point in the middle whereby if that carried on going, it will be going at an infinite speed, and that doesn't work, physics breaks. So actually, what happens in the middle is you get air sucked in from the top and you get air coming downwards. At which point, this is dry and itís not damp so you get clear with no clouds on the top. And itís coming down and not spinning, so itís perfectly still.
Victoria - Itís Victoria from Cambridge. I was wondering, what's the difference between and hurricane and a tornado?
Dave - A hurricane is one of these huge storms, hundreds of kilometres across which is essentially working on exactly this principle. A tornado is a much, much smaller effect. How it actually forms is a lot more complicated. to be honest I haven't quite got my head around it. Itís a lot more to do with winds and as much more space and its much more violent.
Chris - I think they begin as a very powerful draft running in one direction which pulls another jet stream of air and puts a torque or a twist on it which causes it to angle down. And so, you've got a very tightly rotating column of air which is then pointed down towards the ground and has this obviously devastating effect because itís a lot of force over very small area on the ground.
Fergul - Hi. Itís Fergul from Cambridge. I just wondered why we always hear about severe tornadoes in the states, but not many reports of very fatal tornadoes from elsewhere in the world where the climate I assume is similar?
Chris - Is it true that actually there are more tornadoes in England than there are in America? We just actually donít see the manifestations because they're quite minor.
Dave - I donít know. Is it true, Chris?
Chris - Has anyone else heard that? I'm sure that that's a fact. James...
James - Yeah, we see there are lots of them in hot desserts I know. I've seen many of them in Iran and in parts of the Middle East. They donít seem to get as big as they do in America. And anyway, they're in the dessert so who cares? They don't do very much. They make it very dusty, but they donít actually cause that much damage. The small versions are called dust devils. You see them as corkscrew things, going across the countryside. If you happen to walk into one, you get covered in dust, but the scale is not very big.
Chris - You can experience that in Cambridge at certain times of the day. One over here...
Mark - Hi, there. This is Mark again from St Neots. So, I'm actually Ewanís dad and he was born up in the highlands of Scotland. When we were living there, when he was very young, we caught the tail end of Katrina which still had quite an amazing devastating effect. There were still upturned boats everywhere, roofs of all kinds of things rose into the sea. Do you know the different effect of hurricanes that are travelling over the sea or land, and how much that will affect how they decrease in their power or anything like that?
Dave - So, hurricanes, they get a lot of power from the hot water. So, they tend to lose power as soon as they go on land because there isn't that hot water driving- water vapour is less dense in normal air so it floats upwards and that drives the circulation really strongly. When they get too far north, they also tend to get Ė because you get lots of high speed winds, high up, you tend to kind of blow the top off them a bit and they also get weaker. So, even though it was a strong storm when it was up here, and the sea underneath it isnít as hot either. So, as you go north, they weaken and when they go over land for a long time, they weaken as well.