How does air stay with the ground?

[Tex]

PLEASE forgive me if it sounds like I don't know what I'm talking about! Hear me out-

I was sitting in the waiting room of a private clinic earlier today when I happened to glance outside. It is cold here in Wyoming, and so I saw the condensed water vapor from a nearby chimney floating away. This got me started thinking about reference points: from the right point, it looked like the clinic was flying, and moving at the same speed as the clouds way in the distance. Then this got me thinking: How does the air surrounding Earth keep up with it?

I thought of it this way: If the Earth is rotating around an axis, and the air surrounding it is not directly connected to the Earth, then how can there be "still" air? Think of the situation as a small beach ball in a pool. If you grab that ball and spin it, the water (most of it, at any rate) stays right where it is, and the ball simply spins on top. The water is hardly affected by the ball, except in the case where the ball spins very fast and that little stream of water climbs up the ball.

By this logic, shouldn't the wind have to move at the same speed as the Earth rotates in order to stay "still"? If the air were truly "still", shouldn't we perceive it as wind moving at several thousand kilometers per hour opposite to the Earth's rotation? Or would the Earth's fairly large gravity (combined with the rotational effect) cause the air to stick to the surface much like the small amount of water if the beach ball spins too fast?

OFF TOPIC: Do you think that if the beach ball were spinning fast enough, the entire ball would gather a sheet of water all the way around?

[AletzN1]

we will never know......

seriously i have no clue, but i think it will be something painfully obvious.

[TheGatesofLogic]

your statements are logically sound if your talking about plopping down a planet, setting it to spin, THEN adding placing "stationary" atmosphere around it, but in reality the air does not move because it IS connected to the planet and upward movement of molecules over time give die to a system that maintains very similar speeds to the ground. there is a noticeable effect in this regard and it is commonly known as the Coriolis Effect which spins wind and cloud patterns away from the poles

[PakledHostage]

It is a complex combination of viscosity effects, temperature/pressure variations throughout the atmosphere and coriolis effect. And gravity.

In a nutshell: Gravity holds the air down. Viscosity keeps it rotating with the earth's surface. Temperature and pressure variations throughout the atmosphere cause the air to move continuously. As TheGatesofLogic mentions, Coriolis effect causes weather systems to rotate about their low/high pressure centres as the air in them moves about on the surface of the earth.

[Camacha]

There is a couple of things to this. First of all, air is generally moving - and a lot faster than we perceive. It is just that the lowest layer is moving quite slowly due to a lot of things being in the way. Those things are just the stuff you see around you, like trees, houses, hills, et cetera. If you get just a little big higher than that, wind usually picks up quite dramatically. It is something that pilots actually need to take into account when coming in for a landing, to prevent sudden stalling at a low altitude.

Something that is not entirely accurate with your water analogy it that air is not only a lot less dense than water, but it is also actually only a small layer compared to the rest of the planet. As a very rough comparison, the atmosphere is in reality about as thick as the skin of an apple compared to the earh being the whole apple. Even though it appears huge to us, it is really a small layer that sticks to the planet. When looking closely, a ball spinning in water might actually have a layer that is equally thick on it, as it will be wet all across the surface and also be pulling some water along due to friction and surface tension/viscosity.

Finally, earth roughly formed alongside its atmosphere, to it is only logical the the atmosphere is not static, but also moving roughly along. It is kind of the same thing as with the planets in the solar system rotating in the same direction with the asteroids and debris, although there are more factors involved than just the planet formation governing the wind patterns that we see.

[wbcundiff]

I don't think the beach ball analogy works because the relative masses are so different between your analogy and reality. mass-of-air:mass-of-earth is not similar to mass-of-water:mass-of-inflatable-toy.

[Tex]

Something that is not entirely accurate with your water analogy it that air is not only a lot less dense than water, but it is also actually only a small layer compared to the rest of the planet. As a very rough comparison, the atmosphere is in reality about as thick as the skin of an apple compared to the earh being the whole apple. Even though it appears huge to us, it is really a small layer that sticks to the planet. When looking closely, a ball spinning in water might actually have a layer that is equally thick on it, as it will be wet all across the surface and also be pulling some water along due to friction and surface tension/viscosity.

I don't think the beach ball analogy works because the relative masses are so different between your analogy and reality. mass-of-air:mass-of-earth is not similar to mass-of-water:mass-of-inflatable-toy.

I know of the differences of air and water, I was only using this to demonstrate my point. I apologize...?

Edited January 3, 2014 by Maximus97

[Superfluous J]

Gravity and Friction, man. What can't they do?

[grawl]

I think the beach ball analogy is not that bad.

Drown it entirely, then take it out. A small layer of water will stay on the ball (the ball is "wet").=> Here is our atmosphere.

Start to spin it, slowly. => Here is the rotation of our planet.

Result => The wet layer stays still on the ball. Maybe the moves of the water molecules look a lot like actual high altitude air currents

Of course, if you spin it too fast, or too long, the model will fail, as the water is much more impacted by "local" gravity - your bathroom is not 0g - than the real atmosphere. A bit of imagination is needed as well to enjoy this experiment

[Camacha]

I know of the differences of air and water

I was mostly referring to the thickness of the layer

[acerola]

As 5thHorseman said, gravity and friction makes the air keep up with the Earth. Gravity pulls the air, so that it doesn't float away. Friction/drag forces make the air spin around, keeping up with the Earth.

Imagine a glass full of water, perfectly still on a table. You start to rotate the glass. At first, the water won't move due to inertia. Then a small layer near the glass will start to spin, keeping up with the glass. This outermost layer will spin another layer further inside. And so on. After a while, all the water will be rotating.