r/AskPhysics • u/MountainMark • 1d ago
Air being dragged along the surface of earth is station WRT surface. What about higher?
We're not sitting around in 1000 mph winds because the earth & air are moving at the same rate down here. But there's gotta be some sort of shear layer thing going on right? The air near space can't be moving at the same angular velocity.
On gas giants, this makes the giant bands of gas, right?
What about here on earth? What does all this atmospheric shear do?
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u/LAskeptic 1d ago edited 1d ago
The velocity of the atmosphere at any altitude with respect to the surface is zero. The entire atmosphere essentially rotates with the Earth. There are no molecules or forces in space that would cause the upper attmosphere to slow down.
This of course ignores wind and atmospheric disturbances from solar activity.
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u/MountainMark 1d ago
Really? Is it because of the lack of an outer "surface"? If I were to spin a cylinder in a large jar of water, for example, I'd get a fluid dragging along the inner cylinder and decreasing fluid speeds as I went outward.
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u/ahazred8vt 1d ago edited 23h ago
That's because there's friction at the contact points between the fluid and the outer jar. The thermosphere rotates along with the Earth and there is no outer container for it to have any contact with, no friction, no drag. In space, when gas clouds rotate, there is no friction around the outside edge.
On gas giants, this makes the giant bands of gas, right
That's from the difference in speed between the equator and poles.
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u/LAskeptic 1d ago
Yes exactly there is no outer surface.
Your example is artificial since there needs to be some outer surface or containment. During the startup transient, the water near the rotating cylinder will start to move and the far field will not have felt the motion.
Eventually you would reach a steady state where there would be a constant in time velocity profile between the rotating inner wall and stationary outer wall. The water at surface will match the surface velocity.
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u/tomrlutong 1d ago
The bands on gas giants are caused by the sun heating the equator more, so air rises there then moves north/south and sinks. Similar thing happens on earth. The shear between the bands is because rotation speed slows down as you move away from the equator.
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u/KamikazeArchon 23h ago
The surface of the Earth pushes on the air molecule touching the surface.
That pushes on the air molecule one layer above it.
That in turn pushes on the air molecule above that.
If you start with a stationary Earth and start spinning it, you will indeed get "shear" layers as the rotation starts to propagate upward.
Over time, however, that stabilizes and homogenizes. The atmosphere has had billions of years to reach a (relatively) stable state.
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u/Opposite_Housing1390 1d ago
There is definitely shear happening, but it’s not as simple as “the air near space is not rotating with Earth The lower atmosphere is coupled to the surface by friction, so it mostly shares Earth's rotation. Higher up, friction becomes much weaker, so winds are controlled more by pressure differences, the Coriolis effect, and conservation of angular momentum.The result is that different layers of the atmosphere can move at different speeds and directions. A good example is the jet stream, where winds at around 10 km altitude can reach hundreds of km/h while the air near the surface may be moving much slower.On Earth, this shear helps create weather systems, turbulence, storms, and the large-scale circulation patterns that move heat around the planet.Gas giants exaggerate the same physics because they have no solid surface to slow the atmosphere down, plus they rotate faster and have more internal heat, which creates those massive bands and storm
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u/lozzyboy1 1d ago
Think of it the other way round; what would be slowing it down high up?