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View Full Version : If our atmosphere is being held by gravity, why isn't it brought down to the surface?



Ross PK81
2008-Nov-22, 02:27 PM
?

Ken G
2008-Nov-22, 02:44 PM
Imagine a super ball that can just bounce and bounce, and never lose energy and never stop bouncing. Would you say such a ball is being "held down" by gravity? Probably, that would be a natural way to describe it. Now imagine lots and lots of such balls-- that's an atmosphere.

Spaceman Spiff
2008-Nov-22, 02:56 PM
?

Because of the finite thermal gas pressure it exerts, due to the random motions of the molecules, at every layer. In the simplest way to think about it the (average) pressure exerted by the gas near sea level multiplied by the surface area of the earth equals the weight of the atmosphere above. Layers at much higher altitudes are at lower pressure to balance a lower weight of the column above.

A better way to think about it is this. Think of a thin volume layer of atmosphere. This volume has weight. That weight is offset by the difference in pressure P(below) between that below and that of the pressure above P(above) that thin layer. If the balance is not in place, that thin volume layer will accelerate: either upward if

P(below) - P(above) > (layer weight)/(layer volume / layer thickness)

or downward if the pressure difference is less than what's on the right hand side. Equality, LHS = RHS, means a balance - the layer does not accelerate. Then consider summing up this balance over all vertical layers. The phenomenon is known as hydrostatic equilibrium (http://en.wikipedia.org/wiki/Hydrostatic_equilibrium).

It isn't too different from how the difference in pressure between the air inside your tires and that of the outside air (what you measure with a tire pressure gauge), multiplied by the ground contact area of the tire, equals the weight of the portion of your car bearing down on that axle.

cjameshuff
2008-Nov-22, 04:05 PM
Fill a balloon with air. The air is being held inside the balloon by the stretchy rubber, right? Why does the balloon not squeeze it all down into a space where the balloon is not stretched? As air is compressed into a smaller space without change in temperature, its pressure increases, and there is a point where the outward pressure balances the inward pressure of the stretched balloon.

The atmosphere isn't all brought down to the surface because it piles up against the ground, and is compressed to a point where it exerts enough upward force to support the weight of the air above it.

Veeger
2008-Nov-22, 08:34 PM
Just to add to the already good answers, gravity is the weakest of the fundemental forces. (Although you may not think so if you fall out of a tree.) Good thing it is, otherwise we too, would be flat against the surface of the earth.

korjik
2008-Nov-23, 07:02 AM
Imagine a super ball that can just bounce and bounce, and never lose energy and never stop bouncing. Would you say such a ball is being "held down" by gravity? Probably, that would be a natural way to describe it. Now imagine lots and lots of such balls-- that's an atmosphere.

Best description ever! :)

cran
2008-Nov-23, 09:02 AM
If our atmosphere is being held by gravity, why isn't it brought down to the surface?

whilst agreeing with every answer so far, Ross PK81 ...

I would add: it is brought down to the surface!

There are no gaps anywhere between the atmosphere and whatever surface lies beneath it - whether it's land surface, water surface, anthropogenic surface, or the surfaces of furry critters roaming the grasslands ...

mugaliens
2008-Nov-23, 11:27 AM
Ditto what cran said about great answers. Would like to add:

Our atmosphere is made up of gases. Both gases and liquids are fluids. Fluids include liquids, gases, plasmas, and to some extent, plastic solids." - Wikipedia

Under gravity, all fluids will continually deform (flow) so that they pool to the lowest possible level.

The difference between water (liquid) and air (gas) is that liquids can freely form a distinct surface. That free surface is not constrained by a container. Gases, on the other hand, due to their rubber ball, er, kinetic energy, bounce around. Thus, their upper surface is not nearly as distinct, and there are great spaces between their molecules. Thus, the "surface" of our atmosphere is rather thick, several hundred miles, during which our atmosphere diminishes until it's the same density as the random vacuum throughout our solar system.

By the way, all liquids under standard sea-level conditions (14.7 psi), when heated, will become a gas. Not all gases, however, become liquids at this pressure. CO2, for example, condenses directly into CO2 ice, known as "dry ice."

Spaceman Spiff
2008-Nov-23, 03:21 PM
Best description ever! :)

yeah, Ken G has a gift of providing illumination with great clarity. :)

Tobin Dax
2008-Nov-23, 03:50 PM
P(below) - P(above) > (layer weight)/(layer volume * layer thickness)

Maybe this is because it's almost time to take my pain meds, but I think there's a typo in your equation. Volume times thickness doesn't equal area, so I think your equation should be:

P(below) - P(above) > (layer weight)/(layer volume / layer thickness)

or

P(below) - P(above) > (layer weight * layer thickness)/(layer volume)

Or am I wrong and just too foggy to see it?

Haggis
2008-Nov-23, 04:04 PM
It's really not all that much different than the ocean, or the solid ground for that matter. Why doesn't the water in the ocean all fall to the bottom, and why doesn't the ground I'm standing on fall to the center of the earth? Pressure from the ground or water beneath holds it up.

trinitree88
2008-Nov-23, 04:19 PM
Make a Torricelli barometer. Walk up a mountain. As you climb up out of the ocean of air....the level drops. As you walk down towards sea level, near the bottom of the ocean of air..(unless you head underground in a cave or mineshaft, or selected sites below sea level like Death Valley, the Quattara Depression, or the Dead Sea...)...the level goes up. So, technically it does settle towards the ground...as Arisatotle argued with Democritus over the existence of atoms...just like sand in a sandstorm, but, not completely.

see:http://images.google.com/imgres?imgurl=http://www.quicksilver-barometers.co.uk/torre%2520exp.jpg&imgrefurl=http://www.quicksilver-barometers.co.uk/barometer%2520people.html&usg=__n7m4YBRMNhOknGIo4MVSNTY-H6A=&h=720&w=364&sz=8&hl=en&start=18&um=1&tbnid=pQ6FqFq11euu-M:&tbnh=140&tbnw=71&prev=/images%3Fq%3DTorricellian%2Bbarometer%26um%3D1%26h l%3Den%26sa%3DN


pete

Spaceman Spiff
2008-Nov-23, 09:20 PM
Maybe this is because it's almost time to take my pain meds, but I think there's a typo in your equation. Volume times thickness doesn't equal area, so I think your equation should be:

P(below) - P(above) > (layer weight)/(layer volume / layer thickness)

or

P(below) - P(above) > (layer weight * layer thickness)/(layer volume)

Or am I wrong and just too foggy to see it?

Yes, many thanks! You are correct. A slip of the keypunch. :doh::doh::doh:
(I've corrected the equation in the post (http://www.bautforum.com/questions-answers/81566-if-our-atmosphere-being-held-gravity-why-isnt-brought-down-surface.html#post1371261).)

mugaliens
2008-Nov-23, 11:46 PM
Make a Torricelli barometer. Walk up a mountain.

Better yet, take the funicular (http://en.wikipedia.org/wiki/Funicular). It's faster, more enjoyable, and it keeps with the Italian flavor imparted by a Torricelli barometer.

thorkil2
2008-Nov-24, 12:08 AM
Seems to me that it is brought down to the surface. The thickness of the atmosphere (having a density you would recognize as atmosphere) taken as a fractional part of earth's radius is so miniscule that it only seems deep because falling through it would hurt when you hit bottom. Impressive on a human scale; next to nothing at all on a planetary scale. Aside from that, Ken G hit it squarely on the head.

Spaceman Spiff
2008-Nov-24, 01:55 AM
It's a matter of perspective, I suppose.

To get a feel for the thickness of a planet's atmosphere, one can measure an atmospheric scale height, which for constant gravitational acceleration g is:

h = kT/mg ~ 8 km for Earth's atmosphere

where k is Boltzmann's constant 1.38e-23 J/K, and kT is related to the mean kinetic energy of gas molecules, m is the molar mass of the molecules in kg (for Earth, about 28.964 kg per kmole for dry air, so you divide that number by Avagodro's number per kmole to get m). This characteristic scale height of the atmosphere represents approximately where the pressure drops by 1/e, where e is 2.7128828...). Its origin can be thought of as setting the gravitational potential energy per particle equal to the average kinetic energy per particle (that portion thereof in the vertical or z direction). Many scale heights go by before the density drops to near ambient values. (T is also not a constant.)

This relationship shows that planets with stronger gravitational accelerations will have thinner atmospheres, all else equal (lower T and larger gas particle masses also result in thinner atmospheres, all else equal). Things are actually more complicated than that, but this gets the main ideas across.

One of those complications: Earth's atmosphere is illuminated by UV/X-rays from the Sun, and these photons are absorbed in the upper layers(e.g., Ozone absorbs UV in the stratosphere), raising the temperatures of these layers, thereby puffing the upper atmosphere beyond that in the absence of UV/X-ray illumination.

Tobin Dax
2008-Nov-24, 03:48 AM
Yes, many thanks! You are correct. A slip of the keypunch. :doh::doh::doh:
(I've corrected the equation in the post (http://www.bautforum.com/questions-answers/81566-if-our-atmosphere-being-held-gravity-why-isnt-brought-down-surface.html#post1371261).)
I covered pressure in lecture two weeks ago, so it's on my mind. :)
Heck, I was lying in a bed in ICU two days ago, looking at my I.V., and thinking about pressure and fluid mechanics because that's what I had just been teaching. :whistle:

Vallkynn
2008-Nov-24, 01:26 PM
?

Well, it is, but being a gas, is compressed until a limit defined by the intensity of gravity force, originating the actual atmospheric pressure values. Larger planets will have higher atmospheric pressure values due to higher gravity forces.

Vallkynn
2008-Nov-24, 01:32 PM
This relationship shows that planets with stronger gravitational accelerations will have thinner atmospheres, all else equal (lower T and larger gas masses also result in thinner atmospheres, all else equal). Things are actually more complicated that that, but this gets the main ideas across.


Actually the atmosphere will be thinner only if the reference is the same amount of gas that forms the atmosphere. You always get an equilibrium between gravity and pressure. It's not so simple to say if it will be thinner or thicker.

Spaceman Spiff
2008-Nov-24, 02:30 PM
Actually the atmosphere will be thinner only if the reference is the same amount of gas that forms the atmosphere. You always get an equilibrium between gravity and pressure. It's not so simple to say if it will be thinner or thicker.

Ok, I agree.

To be more clear, I should have said "lower T and larger gas particle masses also result in thinner atmospheres, all else equal". (In fact, I've now rewritten it as such.) The scale height argument I gave also presumes a constant g, which is useful in the limit that M(r)/r^2 does not change appreciably as you move through the atmosphere.

Spaceman Spiff
2008-Nov-24, 02:37 PM
I covered pressure in lecture two weeks ago, so it's on my mind. :)
Heck, I was lying in a bed in ICU two days ago, looking at my I.V., and thinking about pressure and fluid mechanics because that's what I had just been teaching. :whistle:

I wish you all the best in a speedy recovery! :)

Vallkynn
2008-Nov-24, 03:36 PM
Ok, I agree.

To be more clear, I should have said "lower T and larger gas particle masses also result in thinner atmospheres, all else equal". (In fact, I've now rewritten it as such.) The scale height argument I gave also presumes a constant g, which is useful in the limit that M(r)/r^2 does not change appreciably as you move through the atmosphere.

I would like to add that it is still not completely understood the influence of earth magnetic field in the atmosphere. It was found that solar wind takes away big chunks of mars atmosphere, mainly because mars magnetic field is not homogeneous but comes up in small "domes". However Titan, doesn't have magnetic field and has an atmosphere 1,5 denser than earth's, despite being a moon and a big fraction of it's orbital path is outside Saturn magnetic field.
Maybe there's also some balance between volcanic activity by releasing new gases into the atmosphere.

mugaliens
2008-Nov-24, 05:21 PM
I would like to add that it is still not completely understood the influence of earth magnetic field in the atmosphere. It was found that solar wind takes away big chunks of mars atmosphere, mainly because mars magnetic field is not homogeneous but comes up in small "domes". However Titan, doesn't have magnetic field and has an atmosphere 1,5 denser than earth's, despite being a moon and a big fraction of it's orbital path is outside Saturn magnetic field.
Maybe there's also some balance between volcanic activity by releasing new gases into the atmosphere.

Titan has less gravity than our Moon, yet an atmospheric pressure 1.5 times that of Earth - that would easily support human-powered flight.

While Titan doesn't have a magnetic field, its ionosphere has two layers of charged particles, one at 63 km, and a second at 1,200 km. This creates a radio resonating chamber, a source of natural ELF waves.

Lastly, Titan is so much further from the Sun that it experiences but a tiny fraction of the solar wind that our Moon does.