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Ever wonder why sometimes the airplane you're flying on decides to lurch suddenly and cause your little baggie of peanuts to spill all over the place? Join Hank on SciShow today as he explores the in and outs and the ups and downs of turbulence.

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Hank: If you've ever flown in an airplane, you've probably felt it, whether you're cruising at ten thousand meters or about to land, turbulence can be a frightening experience. But what's surprising to me is that some scientists have been afraid of turbulence too, but for different reasons.
Richard Feynman described turbulence as "the most important unsolved problem of classical physics." In its scientific definition, turbulence is the flow of a fluid, like air or water, that is irregular, chaotic, and unpredictable. Mathematically, it is vexing, which is why we continue to use wind tunnels when testing aerodynamics or airplanes and cars. We simply don't have computers powerful enough to model the complexities of turbulence. (0:49)
But when laypeople talk about turbulence on an airplane, mostly we're talking about the shaking and the grumbling and the dropping and the lifting that's caused by the actual turbulent air around it, and that can be caused by a number of things. (1:00)
Probably the most common is thunderstorms, also known as convective weather patterns. Turbulence associated with storms is the result of water vapor condensing into droplets. When that droplet condenses, it releases a tiny amount of latent heat, and when this happens a lot, it creates sudden masses of warm air that can cause updrafts and downdrafts strong enough to move a plane up or down by hundreds of meters. (1:24)
This does not happen to your plane, though, because your pilot is not flying straight into the heart of a storm, hopefully, because radar and meteorologists exist, and flights can be quickly re-routed. Additionally, like big churning storms, mountains are pretty easy for pilots to spot too, but even so, mountain range turbulence is still hard to predict. (1:44)
When winds blows perpendicular to a mountain, it's forced over the range, creating swirling eddies and oscillation, much like an ocean wave breaking over a sandbar. Not only can these waves of air be powerful, but they can extend hundreds of kilometers from a mountain, surprising pilots who think that they're clear of turbulent air. (2:00)
And then there's clear-air turbulence, which occurs in clear, open conditions, when a large mass of moving air meets another air mass that's moving at a different speed. The most common culprits here are jet streams, the giant currents that form when arctic air meets warmer air from the south. These rivers of air can travel at a hundred kilometers per hour or more; when they meet the slower-moving air around them, they can form lots of large eddies of confused, turbulent air. (2:27)
These rapid changes in wind speed are known as wind shears, and because the boundaries of jet streams are always shifting unpredictably, they can seem to come out of nowhere, even in beautiful blue skies.
A final, kind of unexpected, source of turbulence: airplanes themselves. The tips of airplanes wings cause vortices of wake turbulence, which, while cool to look at, are the reason that pilots have to wait a long time before taking off or landing after the plane in front of them. (2:53)
Thankfully, science has gotten a good enough grasp on the problem of turbulence to help us travel safely and usually comfortably. I know that I, for one, am much more comfortable with things that are just mathematically scary.
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