# Why does the atmosphere move with the earth?

1. Dec 16, 2004

### quasar987

Just yesterday I was asking myself that very same question. So thanks for giving such a nice an simple explanation.

I was also wondering why the atmosphere revolves with the earth; something you stated but didn't explained. Is it something that goes back to the time of the creation of the earth? Most unlikely imo. So what is the reason?

And why is there an atmosphere in the first place? I.e. why doesn't it just fall down on earth like a decent piece of matter?

2. Dec 16, 2004

### Clausius2

Surely the atmosphere does not revolve like a solid body with the earth. Lower atmosphere layers are sensible to viscous interaction, so a movement of the ground causes the movement of those lower layers with it. Upper layers are not exposed in such a way to the ground interaction.

3. Dec 16, 2004

### Gokul43201

Staff Emeritus
Actually, it does go back to the time of creation of the earth, but even if it didn't, the atmosphere would rotate due to viscous drag. If you have a glass of water, and you spun the glass about its axis, doesn't the water start swirling too ?

Why is there an atmosphere ? This is basically asking why there are gases. The molecules in a gas, do not fall to earth like the molecules of a liquid or solid. This is because the average (thermal) kinetic energy of each gas molecule (~KT) is large enough for it to overcome its gravitational potential. In fact, the reason, we don't have some of the lighter gases like helium and hydrogen in our atmosphere, is that their average thermal velocity would be greater than the escape velocity for earth.

4. Dec 16, 2004

### quasar987

O.k. for the "why is there an atmosphere", but according to which physical laws exactly does the water spin with the glass?

5. Dec 16, 2004

### Clausius2

Navier-Stokes equations for fluid flow are built over the so-called Continuum Medium Hypothesis. If the wall velocity is $$u_w$$, then the fluid closest to the wall has to have $$u=u_w$$ in order to enhance the continuity of the matter.

This condition is widespread used in Fluid Mech. formulation and is usually called "the Non-Slip boundary condition". This is ultimate caused by viscosity forces.

6. Dec 16, 2004

### alpha_wolf

But it is down on earth - otherwise we'd have nothing to breathe down here!

Think about it: if you pour water into a pit in the ground, it starts filling the pit, until at some point it goes over the edge and starts covering the surrounding ground. If you had enough water, you could cover the entire surface of the earth. The same time happens with the atmosphere - it fills up a huge volume and covers the entire surface of the earth. In theory, if you had a really really really tall mountain, a couple hundred kilometers high (the tallest one we have, Mount Everest, is "only" about 9 km high), its tip would peek out of the atmosphere.

Last edited: Dec 16, 2004
7. Dec 16, 2004

### Staff: Mentor

Gokul said it - viscous friction/drag.

8. Dec 16, 2004

### quasar987

I didn't know that that meant (viscouous friction) so I was hoping there was a simple explanation in terms of forces and torques, etc.

9. Dec 16, 2004

### Staff: Mentor

When you move a fluid, the particles moving past each other "stick" together a little bit. Viscocity is the "thickness" or that stickiness of a fluid. Syrup is more viscous than water, for example.

Now when you stop stirring your coffee or tea, the spinning of the liquid will eventually slow down and stop. The reason for this is that the cup is stationary and friction between the cup and the water (regular drag/friction) and between particles of water (viscous friction/drag) slows it down.

Applied to the atmosphere, if the atmosphere were stationary, there'd be 1000mph winds on the surface. These winds would hit things like trees, mountains, etc and slow down. That's drag. So as the air near the ground slows down, viscous friction between air molecules slows down molecules further up and eventually, just like in your cup of tea, the atmosphere slows to a stop relative to earth.

Now, the way the atmosphere really works is that it came out of the ground as hot gasses as the molten earth cooled. So it was already turning with the earth when we got it. And since there is no force to slow it down (no viscous drag, since above it is a vacuum), it always has turned with the earth.

10. Dec 16, 2004

### quasar987

Got it !

.

11. Dec 17, 2004

### NateTG

Actually, there are external forces that act on the atmosphere (and more significantly the ocean) due to the moon and the sun that are responsible for weather patterns. Moreover, the rotation of the Earth during the movement of air north and south along the earth has significant and important effects on weather.

12. Dec 17, 2004

### Staff: Mentor

I realize I simplified it a little, but weather effects are pretty small compared to the 1000 mph winds we would have if the atmosphere didn't rotate with the earth.

13. Apr 8, 2009

### Arjan82

The more advanced explenation of this matter would be this:

Air is made out of molecules. If you look at a random surface on molecular scale, the surface is far from smooth. Air molecules bump up to 'mountains of surface molecules' and this is why at the surface the air moves at exactly the same speed as the surface (it has nothing to do with the continuum of the air as far as I know)

Now the question is why the air directly above the surface would notice... This has to do with the fact that air has a temperature greater than zero. Temperature expresses the fact that molecules move in a random direction and bump in to each other and exchange momentum (momentum is velocity times mass). So if a molecule A, which has the same velocity as the surface moves upward away from the surface, it may collide with an other molecule B of air which had a different speed. Now there has been an exchange in momentum between molecule A and B and thus molecule B is slowed down a bit and molecule A has gained some speed (relative to the surface). This means on a bigger scale that the air closely above the surface is slowed down by the air directly at the surface which on its turn has been slowed down by the surface itself. This is to say that the air has a certain viscosity.

Note that this can only happen when there is a velocity gradient, thus somewhere the air needs to move at a certain speed relative to the surface. So also note than when nothing else would influence the velocity of the flow (so no other forcings would be present) the atmosphere would come to a complete rest relative to the surface. Because any velocity difference is smoothed by viscosity.

Ofcourse as anyone who has ever walked outside knows, the air isn't completely at rest, there is wind, and this is because the air is forced by the sun. The sun heats the earth's surface and this causes high and low pressure regions to form and since air moves form high to low pressure, the air starts to move. (By the way, only moving air feels the Coriolis force)

14. Apr 8, 2009

### Staff: Mentor

Just out of curiosity, how did you happen to find this thread whose last post was over four years ago?

15. Apr 8, 2009

### Arjan82

Lol.... never looked at the date. I just googled a bit (wanted to know until which hight the continuum hypothesis holds in the atmosphere) and found this post. Apparently it interested me and I reacted :)

16. Apr 8, 2009

### Staff: Mentor

OK, no harm done. I was just curious. We've been having a discussion about "necroposts" in the General Discussion forum, and I raised the question of how many are made by people who know they're posting to an ancient thread, or by people who don't realize it. You've provided a data point.

17. Jan 4, 2010

### croghan27

I work (used to work) in oil refineries and have been with nuclear technicians who, in trying to find out the velocity of oil in a line, inject radioactive materials into the lines, as three different depths.

One in the centre of the line;
one about a quarter of the way to the centre;
and another near the pipewall.

The evidence of the radioactive materal is then picked up on sensors as it makes its' way, in the oil, on down the line. As it is within the oil - I guess they use a reverse Navier-Stokes equation to find the velocity of the oil. The sensors very definitely pick up the evidence at three different times - showing the speed of the oil is slower near the pipe wall, slowed by 'friction' and interaction with the wall.

I came to this discussion after looking at the geological report about the volvano in the Philipeans .... as with quasar - I did not notice the time tag. (Dr. Who would understand. )

18. Sep 5, 2010

### Andrea

hello, i'm not sure if i understand this correctly. does it mean that the outer layers of atmosphere move at the same speed as the layers close to the earth's surface? thanks for help with this one :-x (and i'm aware of ancientness of this thread, but only because you talked about it earlier - i wouldn't even think about checking the last post for date ;)...)

edit: i mean the speed relative to earth surface.. ehh.. i think the speed i'm talking about is called angular velocity in english ;)..

Last edited: Sep 5, 2010
19. Sep 6, 2010

### Staff: Mentor

More or less, yes.

20. Sep 6, 2010

### croghan27

Hummmm

Then that is counter intuitive to my experience, as I pointed out in my example of the differing speeds of fluid in a line. That area of the fluid that is in direct contact with the inside of the pipe flows slower than the fluid in the centre of the pipe. (As exhibited by the tracking isotopes.)

I would think that, as it is the earth that is spinning, that part of the atmosphere in contact with it would move in close concert - but the circulating speed would decrease the farther away the atmosphere is from the mass of the ground.