What is a jetstream?

What is causing the current heatwave?
10 July 2018



Why do we get heat waves? And what's this "jet stream" we hear so much about?


Chris Smith proposed this hot topic to Weather Quest’s Jim Bacon...

Jim - Really good question. Like all good questions there are short answers and probably fairly long answers. I'll try and make this one a fairly short answer. The atmosphere consists of a lot of heating that goes on around the equator, and not very much heating around the poles, and it's a very imbalanced system. And the jet stream is, in a sense, a response to the imbalance when you get warm air very close to cold air. You could say, you’d think that it would just degrade gradually from the equator to the poles and gradually get colder. But in actual fact, in the real world, what happens is you get zones where it gets to be a very intense demarcation between two different air masses: say, one coming up from the south from the tropics, and one coming down from the north and the poles. And to make the atmospheric forces balance the motion, the jet stream is a strong wind that forms to do just that. So you get around the whole northern hemisphere. I dont know, it's probably easier to go into electronics here, and picture an oscilloscope trace with a sine wave on it, and if you had that and joined it up all around the northern hemisphere you'd have a complete set of undulating ridges and troughs going around.

Chris - Why does it wiggle like that?

Jim - Well it wiggles because of the instability in the flow, and there are two or three types of wave motions. So one of them is what's called a Rossby wave, which is a very long wavelength caused by disturbance over big mountain ranges. And it's a planetary scale thing and you might fit about five of these around the whole northern hemisphere and then superimposed on that you'll get a slight disturbance, a small perturbation, we call them. Which is the sort of thing that represents lows that travel across the Atlantic and give us our weather. And those features can transport warm air northwards much quicker and bring cold air southwards. And they have the ability in this flow stream to amplify and they grow into much bigger systems. So eventually what happens, our nice sine wave, given the right conditions and instabilities, can distort enough to become very high amplitude waves on the oscilloscope. And the general rule is if it's a very shallow thing, they ripple across really quickly. So our weather is very changeable totally unlike now. It changes one day you've got a low coming in with a lot of wind. The next day you've got a fine spell of weather and then the next ripple comes through and brings another low. But what happens when it gets bigger is that the pattern slows down, and the pattern slows down and can at some stages come almost to a halt and tend to want to move westward. So instead of our systems coming from the west to the east, they can want to go from the east to the west, and that's what we in meteorology call a blocked weather pattern. And it's those blocked weather patterns that are associated with particularly long lasting spells of weather where you get the same type of weather for several weeks instead of changing every other day. And there's one special case of blocked pattern where this nice sine wave forms a very contorted pattern which looks like the Greek letter Omega. And that virtually can stay locked to a longitude on the globe for weeks and weeks at a time. Some of you will, doubtless in this country and I'm sure there are equivalences all around the world, where the pattern has become locked in our country. We had, in the UK, we had a big drought in 1976 and that was one of these blocking patterns. We had a barbecue summer, famously, when we didn't actually have a summer when we got stuck in the loop of this blocked pattern.

Chris - And we just got continuously bad weather?

Jim - Continuously bad weather! Because the whole pattern stops moving across the Atlantic and it's stationary. So the interesting thing is you've got to be stationary underneath the block, the region; the Omega. And if you’re not, you’re having the bad weather and that was the barbecue summer. So the idea of it being a good summer was that the block would stop with us under the ridge. But in fact, it was only a few degrees of longitude out, 10 to 20 degrees of longitude out, and that meant that we were under the bad weather. So we had a summer like that. This year, and in ‘76, we are under the top of the Omega.


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