Atmospheric Pressure and Gravity

[kakoka]

I am wondering about the causes of atmospheric pressure. All the sights I have found say that air pressure is caused by the weight of air above a point pressing down upon air below it. This seems to contradict Dalton's law of partial pressure because air cannot press on other air and have an effect. Am I missing something?

I was thinking that maybe gravity is more of the reason for the pressure, being that gravity is stronger closer to the core, so the weaker effect of gravity at higher altitudes could cause changes in air density or something. Could somebody help me understand this, please?

[Psycho]

It isn't pushing down on it that is why there is higher pressure, it is mixing in the same area.

 

The reason there is higher pressure is because there is more of it at ground level than high up, the stronger the gravity the more that is pulled closer to the surface and the higher the pressure. The the case of light gases such as hydrogen and helium the gravitational force on earth isn't high enough so they just float off.

[kakoka]

So im on the right track with gravity causing air pressure? I don't know what exactly you mean by "mixing in the same area." So does having less atmosphere above higher altitude locations have no effect on the air pressure?

[Psycho]

So im on the right track with gravity causing air pressure? I don't know what exactly you mean by "mixing in the same area." So does having less atmosphere above higher altitude locations have no effect on the air pressure?

The only reason there is less atmosphere above higher elevations is because they are higher and therefore the air pressure is lower to start with at higher altitudes than at sea level.

 

Primarily in air pressure terms if you go up a 3000m mountain the air pressure is the same as if you were in a plane at 3000m.

Edited October 23, 2011 by Psycho

[kakoka]

thanks, very helpful.

but im still curious; is air denser at lower elevations? does gravity effect the movement of air particles, like a downward sloping movement or something? are sound waves affected by gravity, like sloping along the curve of the earth or something?

[insane_alien]

air is denser at lower altitudes.

 

gavity does affect the trajectory of air particles. this isn't noticable down here because they tend to hit off other particles in a very short distance.

 

soundwaves aren't because they have no mass.

[Psycho]

thanks, very helpful.

but im still curious; is air denser at lower elevations? does gravity effect the movement of air particles, like a downward sloping movement or something? are sound waves affected by gravity, like sloping along the curve of the earth or something?

That is what pressure is, a higher density, in terms of a gases anyway.

 

You seem to be misunderstanding that the air on earth is all one linked system if there is low pressure somewhere the air moves from another place on earth to "fill" the gap, that is the cause of wind, air moving from a high pressure area to a low pressure area. High pressure can be formed by cooling of air causing it to shrink in size and pull more air from above to ground level, so there are variation in sea level air pressure.

[insane_alien]

That is what pressure is, a higher density, in terms of a gases anyway.

 

no, these are often, but by no means always equivalent. temperature plays a huge role.

[Psycho]

no, these are often, but by no means always equivalent. temperature plays a huge role.

I don't disagree, however the only effect temperature has is to increase the number of collisions due to increased movement of molecules which is equivalent to there being more of them in the same area (though not literally the same).

[Guest Zachery1]

Atmospheric pressure is not affected by gravity because gravity is constant value(almost, because is too small difference between gravity force on equator and earth poles, but it is constant value for coordinates what are taken in consideration,for example: your city ) and is affected by other environment conditions who are correlated. In this keys for atm pressure are temperature and air humidity. Because on different temperature air will have different density and can take different quantity of water in it(named air humidity),and that will cause different weight of air, as result of that condition in correlation with constant surface size giving us different value for atmospheric pressure. As we know, pressure is weight/surface, for example: pound/square inch (psi).Usually for calculations if you don't have specific values, you taking 68*F for temperature and 101325 Pa for pressure who are called Normal(Standard) conditions.

[CaptainPanic]

I don't disagree, however the only effect temperature has is to increase the number of collisions due to increased movement of molecules which is equivalent to there being more of them in the same area (though not literally the same).

Temperature can reduce the density of a gas without changing the pressure.

 

But to go back to the OP, yes, the air in higher layers of air indeed pushes down (it is also attracted by gravity). That weight in turn compresses the air below, which is thus denser.

 

Gravity is not much different at the earth's surface or 10 km up where the air is a lot thinner. So, you cannot say that the air is so much thinner up in the atmosphere because the gravity is less. It's not. (Well, actually it is, but only a tiny tiny fraction).

 

To be honest, I don't see why Dalton's law would contradict with how our atmosphere works. In other words: I don't understand what you don't understand

 

[edit] Oh, I see that my answer is a little late. Nearly 6 weeks late. I hope it's still relevant.

Edited December 9, 2011 by CaptainPanic

[Greg Boyles]

I am wondering about the causes of atmospheric pressure. All the sights I have found say that air pressure is caused by the weight of air above a point pressing down upon air below it. This seems to contradict Dalton's law of partial pressure because air cannot press on other air and have an effect. Am I missing something?

I was thinking that maybe gravity is more of the reason for the pressure, being that gravity is stronger closer to the core, so the weaker effect of gravity at higher altitudes could cause changes in air density or something. Could somebody help me understand this, please?

 

 

My understanding is as follows.

 

Due to gravity the closer you are to the surface the more tightly packed the air molecules are and the more collisions there between air molecules. More collisions = more presure.

 

Gravity acts like a container, confining the air molecules to a region. Although you could have equivalent pressure without gravity if you had the air molecules contained within a pressure vessel of some sort, e.g. a space ship.