Hate turbulence? Strap yourself in to find out why it happens.
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 fast-moving 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 presenter. Her latest book is Storm In A Teacup. NEXT ISSUE: HOW PRESSURE HELPS SEAL THINGS