Why does the atmosphere move with the earth?

In summary: This means that when you stir a pot of soup, the soup moves around a lot more than if you just left it alone. This is because the particles are moving so fast that they can't stick to each other.
  • #1
quasar987
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HallsofIvy wrote in another thread:
HallsofIvy said:
What your friend meant by "because of the atmosphere" is that the atmosphere moves with the earth. You could not, as I have heard said, go up in a balloon and then wait while the Earth revolved under you! The balloon moves with the air which moves with the earth.

Without the atmosphere, the answer might be a little different. When you are standing on the surface of the earth, you are revolving with it: your eastward speed is exactly the same as the surface of the Earth (and so is the speed of the ball in your hand. As the ball moves upward, it retains that motion BUT, since the radius of the circle it is describing eastward is now longer, moving the same speed to the east results in a smaller angle: The ball would, in fact, fall slightly westward of you!
(That is a variation on "coriolis" force.)

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?
 
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  • #2
quasar987 said:
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?

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
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
O.k. for the "why is there an atmosphere", but according to which physical laws exactly does the water spin with the glass?
 
  • #5
quasar987 said:
O.k. for the "why is there an atmosphere", but according to which physical laws exactly does the water spin with the glass?

Navier-Stokes equations for fluid flow are built over the so-called Continuum Medium Hypothesis. If the wall velocity is [tex] u_w[/tex], then the fluid closest to the wall has to have [tex] u=u_w[/tex] 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
quasar987 said:
I.e. why doesn't it just fall down on Earth like a decent piece of matter?
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.
 
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  • #7
quasar987 said:
O.k. for the "why is there an atmosphere", but according to which physical laws exactly does the water spin with the glass?
Gokul said it - viscous friction/drag.
 
  • #8
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
quasar987 said:
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.
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
Got it !

.
 
  • #11
russ_watters said:
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.

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
NateTG said:
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.
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
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 into 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
Just out of curiosity, how did you happen to find this thread whose last post was over four years ago?
 
  • #15
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
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. :smile:
 
  • #17
quasar987 said:
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.

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. :smile:)
 
  • #18
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 ;)..
 
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  • #19
Andrea said:
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?
More or less, yes.
 
  • #20
russ_watters said:
More or less, yes.

Hummmm :confused:

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.
 
  • #21
croghan27 said:
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.

In your model, what's slowing down the upper layers? They've got friction from the lower layers constantly dragging them to match speed with the layers beneath them, so after billions of years, why are they still not spinning with the rest of the planet?

This is a simplification, though. Temperature driven effects on the rotating planet's atmosphere (that is, weather) can actually change the rate of rotation of the atmosphere relative to the planet. In the case of Venus, the atmosphere has spun up to the point that the upper layers in the fastest moving regions circle the planet in 4 days, while the planet itself rotates only once every 243 days. This is an extreme case, though, and without the heat input of the sun to drive it, the atmosphere would quickly settle to match the rotation of the planet.
 
  • #22
croghan27 said:
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.
Why? What would cause it to slow down?
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.)
The fluid in contact with the pipe is slowed due to contact with the pipe...which do you think is analagous to the outer edge of the atmosphere and why?
 
  • #23
Gokul43201 said:
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.

Isn't this completely wrong?
A rough calculation for RMS speed of helium at 25ºC gives around 40m/s. Earth's escape velocity is around 11km/s.
 
  • #24
The Earth has a planetary boundary layer, which is the lowest layer of the troposphere where wind is influenced by friction. This is somewhat similar to the friction encountered near the edge of a pipe. The thickness (depth) of the PBL is not constant. At night and in the cool season the PBL tends to be lower in thickness while during the day and in the warm season it tends to have a higher thickness. Usually when you leave the PLA in an airplane, the winds usually increase and turn counterclockwise.
http://www.theweatherprediction.com/basic/pbl/

As for how the winds vary with altitude in the real world, there is no set pattern. Sometimes they increase, sometimes they decrease and several combinations in between.

Meteorologists take data from Radiosondes and make what I called a Skew-T chart. The link below is an example of one. The winds are on the right side with the wind staff pointing in the direction from which the winds are coming from and show wind direction. The Lines perpendicular to the wind staff indicate 10 knots while half lines represent a 5-knot wind. A half line at a 45 ° angle to the wind staff indicates a wind less than 5 knots. Triangles are used for 50 knots.
http://www.theweatherprediction.com/thermo/skewt/
See 291 to 325 for more charts showing wind speeds. These charts are a snap shot for that time. When they make another chart 12 hours later it will be different.
http://www.theweatherprediction.com/habyhints/


The atmosphere is very complicated. You have properties such as temperature, pressure and density interacting with the amount of moisture present.

As for lighter gases like helium and hydrogen escaping this might help:
http://www-istp.gsfc.nasa.gov/istp/polar/coronal.html
 
  • #25
russ_watters said:
Why? What would cause it to slow down? The fluid in contact with the pipe is slowed due to contact with the pipe...which do you think is analagous to the outer edge of the atmosphere and why?

russ ... are there not all manner of extraneous forces operating on the atmosphere: gravity from the sun and moon just being two, they must have some disruptive effect on the spinning action imparted by the earth; solar winds; heat deltas between the Earth's surface and the edges of space; even the effects of various objects from deeper space like meteors or meteorites; certainly the charges that cause the northern/southern lights must have some effect. At the very least these factors must impart or withdraw energy from the upper layers of our atmospheric cocoon. (Atmospheric Hawking radiation? :wink:)

NateTG points out: the rotation of the Earth during the movement of air north and south along the Earth has significant and important effects on weather. which indicates to me that the constant thrust of the Earth is needed to maintain the speed of the atmosphere the farther it is from the centre.

Thanks to all who responded - it seemed like a stupid question to me in the beginning, one that anyone of the fine trained minds here could answer in their off hand - but there is more here than meets the eye.
 
  • #26
If the atmosphere is moving with the Earth how does it keep up speed with the Earth's rotation. Isn't the drag of a sphere only .47? I realize that's going to be increased a little with mountains and trees and so forth, but 1.00 drag seems a little unrealistic does it not? Of course I would suspect gravity to increase that percentage also, but does it take it up exactly equally, seems unrealistic.
 
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  • #27
Welcome to PF.

Since there is no drag, drag coefficient is irrelevant. Please read the thread - it doesn't look like you've read much of any of it.
 
  • #28
I read that from what I saw there is no drag on the outerpart atmospher the part that touches space, but saying there is no drag at the lower atmospheres I didn't read that anywhere, nor do I comprehend how that could be.
 
  • #29
PGFracing said:
I read that from what I saw there is no drag on the outerpart atmospher the part that touches space, but saying there is no drag at the lower atmospheres I didn't read that anywhere, nor do I comprehend how that could be.
The title of the thread is "why does the atmosphere move with the earth?" If the atmosphere moves with the earth, it doesn't move against the earth. If it doesn't move against the earth, there can be no drag.

The reasons why it doesn't move with respect to the Earth are explained in the thread.

Now the way you asked your question also shows a misunderstanding of drag itself. An object need-not have a drag coefficient of 1.0 to be stopped by aerodynamic drag. In fact, there is nothing special about a drag coefficient of 1.0. It's just a number.
 
  • #30
Sakha said:
Isn't this completely wrong?
A rough calculation for RMS speed of helium at 25ºC gives around 40m/s. Earth's escape velocity is around 11km/s.

(yes, I know this post is a couple of months old)

Are you sure you didn't calculate the speed of He at 25 kelvin? Helium should have a typical molecular speed of around a kilometer per second at typical temperatures in the lower atmosphere, with higher temperatures out in the thermosphere and exosphere (and correspondingly higher speeds). In addition, the typical speed doesn't need to exceed escape velocity for the gas to escape - if it's even somewhat close, the high-speed tail of the distribution will exceed the escape velocity, and the helium will gradually escape.
 
  • #31
cjl said:
(yes, I know this post is a couple of months old)

Are you sure you didn't calculate the speed of He at 25 kelvin? Helium should have a typical molecular speed of around a kilometer per second at typical temperatures in the lower atmosphere, with higher temperatures out in the thermosphere and exosphere (and correspondingly higher speeds). In addition, the typical speed doesn't need to exceed escape velocity for the gas to escape - if it's even somewhat close, the high-speed tail of the distribution will exceed the escape velocity, and the helium will gradually escape.

I think he used the 4 for the molar mass of helium, which is 4 gram, but you need to use 0,004 kg. So he's off by the squareroot of 1000.
 
  • #32
Why are there not chickens floating in the atmosphere? Are chickens not a gas?
 
  • #33
Hi , Just to gain some clarity: could someone chalk out for me all the forces acting on a gas molecule in the atmosphere which keeps it where it is and similarly the forces on a plane and moon. I am still unable visualize why plane in the air has to move along with the atmosphere but a satellite (Artificial or Natural) doesnt.
 
  • #34
There are no such forces in either case!
 
  • #35
So, do you guys get trolled often in these forums?

BTW, the atmosphere moves with us because of centripetal force (the same reason practically anything orbits anything else) combined with pressure and gravity, centripetal force pulls molecules inwards, so does gravity, and pressure pushes molecules apart. These forces are balanced. In the overall state of the atmosphere, there is no lateral force (parallel to the ground) only towards the centre of the Earth and directly away from it.
 
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1. Why does the atmosphere move with the earth?

The atmosphere moves with the earth because of its rotation. The earth's rotation creates a force known as the Coriolis force, which causes air to move in a curved path. This, combined with the earth's gravitational pull, keeps the atmosphere in place and moving along with the earth.

2. How does the atmosphere stay attached to the earth?

The atmosphere stays attached to the earth due to gravity. The earth's gravitational pull is strong enough to keep the gases in the atmosphere from escaping into space. This is also why the atmosphere moves along with the earth's rotation.

3. Does the atmosphere move at the same speed as the earth?

No, the atmosphere does not move at the same speed as the earth. The atmosphere is affected by various factors such as wind patterns, temperature differences, and pressure systems, which can cause it to move at different speeds and in different directions.

4. How does the atmosphere affect the earth's rotation?

The atmosphere affects the earth's rotation through the Coriolis force. As the earth rotates, the atmosphere is dragged along with it, causing the Coriolis force to act on the air. This force helps to create wind patterns and influences the rotation of the earth.

5. Can the atmosphere ever stop moving with the earth?

No, the atmosphere cannot stop moving with the earth. As long as the earth continues to rotate, the atmosphere will also continue to move along with it. However, the speed and direction of the atmosphere's movement can change due to external factors such as weather patterns and climate changes.

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