Com­ment & Analysis

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He­len Cz­erki on what causes tur­bu­lence on air­planes

“PLANES CAN COPE WITH TUR­BU­LENCE, SO AL­THOUGH MY TEA MIGHT GET SPILT, THERE’S LIT­TLE DAN­GER”

For me, be­ing al­lo­cated a win­dow seat on a plane is like be­ing given a front row ticket to the ma­jes­tic show of the sky. We spend our lives pootling about at the bot­tom of the at­mos­phere, but a flight gives us a chance to ap­pre­ci­ate the sky from within. Still, I’m well aware that my eyes can only de­tect a tiny frac­tion of what’s go­ing on. And there’s no bet­ter re­minder of that than the fa­mil­iar an­nounce­ment: ‘the cap­tain has re­quested that we turn on the fas­ten seat­belt signs’, just be­fore the buf­fet­ing starts. Not many peo­ple like tur­bu­lence, and its seem­ingly un­pre­dictable na­ture just makes it worse. Yet when I’m strapped into my seat and star­ing out across the wing, the vast, blue sky doesn’t seem to be in tur­moil. In fact, ev­ery­thing looks quite still. What causes the tur­bu­lence and why does it oc­cur?

Tur­bu­lence is a re­minder of what’s hold­ing the plane up: vast hordes of air mol­e­cules bounc­ing off the un­der­side of the wings. If the plane passes through a re­gion where a col­umn of air mol­e­cules is flow­ing up or down, the plane gets car­ried along with the flow. But winds travel hor­i­zon­tally – the at­mos­phere is gen­er­ally strat­i­fied, which means that it tends to stay in lay­ers. To bump a plane around, you need to dis­turb those lay­ers, and there are two ways of do­ing it.

The first is con­vec­tion, which is com­mon near clouds. Air can be heated ei­ther by the ground or by the en­ergy given off when wa­ter vapour con­denses. Warmer air is less dense than its sur­round­ings, so tends to rise, break­ing the lay­ered struc­ture. But con­vec­tion is only re­spon­si­ble for about a sixth of the tur­bu­lence ex­pe­ri­enced in planes. The sec­ond mech­a­nism is far grander and far more un­ex­pected.

What I can’t see, when I look out of the win­dow, are the lay­ers of the at­mos­phere mov­ing over each other. Winds travel at dif­fer­ent speeds and pos­si­bly in dif­fer­ent di­rec­tions as you go up­wards. The in­vis­i­ble beasts of the sky are the jet­streams. These fast-flow­ing rivers of air snake across the Earth at ex­actly the al­ti­tude of most planes. Their dis­cov­ery made a huge dif­fer­ence to avi­a­tion – a plane can hop on to this con­veyor belt to speed up a transat­lantic jour­ney by an hour or so. But there’s a down­side for the tur­bu­lence-haters.

Think about when you blow onto a cup of tea: the air pushes up waves on the tea’s sur­face. That’s be­cause a fast­mov­ing fluid (air) is mov­ing over a slower one (wa­ter). The same thing hap­pens up in the air – when a faster fluid (the jetstream) moves over a slower fluid (the still air be­low), you get waves in the bound­ary be­tween them. These waves march slowly across the sky, com­pletely in­vis­i­ble to us. The peaks can be separated by many kilo­me­tres, and the greater the dif­fer­ence in wind speed be­tween ad­ja­cent at­mo­spheric lay­ers, the more likely the waves are to form. Like any waves, if they grow steep enough, they will break, gen­er­at­ing com­plex swirling pat­terns as the lay­ers mix. This is thought to be the cause of most clear-air tur­bu­lence – gi­gan­tic break­ing at­mo­spheric waves. They’re hard to fore­cast, and the plane’s radar can’t see them. But planes are built to cope, so al­though my tea might get spilt, there’s lit­tle dan­ger as­so­ci­ated with them.

If you don’t like tur­bu­lence, this prob­a­bly isn’t much con­so­la­tion. But I love look­ing out of a plane win­dow and imag­in­ing the waves lurk­ing out there, es­pe­cially if I’m in an east­bound plane in the core of a jetstream. Since the patches of tur­bu­lence are rel­a­tively small (about five min­utes of fly­ing time on av­er­age), I know that the bump­ing will pass. And I reckon a bit of tur­bu­lence is worth it, just to ex­pe­ri­ence what the sky is hid­ing from our eyes. Dr He­len Czerski is a physi­cist and BBC science pre­sen­ter. Her book,

The Storm In A Teacup, is out now

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