What is Air Pressure? Understanding the Weight of the Atmosphere

[Graeme M]

Reading a website and I noticed that the author claimed that air pressure is essentially equal to the weight of the atmosphere. I googled that fact and sure enough, everywhere I go there is that statement. But...

Caveat. I have no maths or science background, so won't cope with too many equations in answering my question...

I assume the concept pressure=weight is a sort of generalisation. Because it does not seem obvious to me that this could be so.

Here is a thought experiment. I have a safe of solid steel 1 metre on all sides with an airtight seal. With the safe open and a barometer inside, the air pressure inside will be whatever the extant air pressure is at my location. If I close my safe, there is no longer a column of air acting upon my barometer. True, the atmosphere may be weighing heavily upon my safe's surface, but I see no way for that to affect my barometer. The parcel of air available to exert some kind of vertical pressure is little more than a few cm high. It won't weigh much...

What do I not understand?

 

[tiny-tim]

Welcome to PF!

Hi Graeme! Welcome to PF!

the author claimed that air pressure is essentially equal to the weight of the atmosphere. I googled that fact and sure enough, everywhere I go there is that statement. But...
…
… If I close my safe, there is no longer a column of air acting upon my barometer. True, the atmosphere may be weighing heavily upon my safe's surface, but I see no way for that to affect my barometer. The parcel of air available to exert some kind of vertical pressure is little more than a few cm high. It won't weigh much...

Yes, you're right to be suspicious …

technically, the air pressure is essentially equal to the weight of the atmosphere plus any external forces (other than weight) …​

we usually take a column of air that extends to "outer space", where the external force is zero,

but in your example the column extends only to the roof of the safe, which is exerting a downward force on the air.

(in practice, of course, the air pressure inside the safe is simply whatever it was when you closed the door, and from good ol' https://www.physicsforums.com/library.php?do=view_item&itemid=373" from the roof is equal and opposite to the force of the air on the roof )

 

[Graeme M]

Thanks tim. Hope you can bear with me on this, just a few more questions to clarify that. I'll have to read up on what the third law says (and the first and second too clearly!).

I would have imagined that weight is the force exerted on a surface by an object that is accelerated by gravity. That is (and I am sorry if I don't know the correct physics terms/concepts to describe what I am saying), any object with mass will, in the presence of a gravitational field, be accelerated by that field, presumably only in a direction downwards perpendicular to the surface of the gravitational object. That object will not 'weigh' anything while being so accelerated. It will only exhibit 'weight' when it is prevented from accelerating. So weight is the effect of an object being prevented from responding to a gravitational field. All objects on Earth's surface will fall to the surface, where weight can be measured.

However, the atmosphere is not falling. As I understand it, the atmosphere is composed of free moving molecules which are in effect unconstrained.

If those molecules are not 'connected' and not falling, then they can exert no force from the effects of gravity on any surface ?

 

[russ_watters]

Are you sure those websites said "air pressure" and not "atmospheric pressure"?

 

[Graeme M]

Ummm... I am unaware of the distinction. I mean the pressure that one experiences at the surface of the Earth and which is measured by a barometer.

I just read tiny_tim's reaction force link. This is exactly my point. The magic is contained in the statement "between two bodies in contact". Which is precisely what I said above. There must be a surface connection for weight to be exhibited, or for a force to be realized. The atmosphere is not composed of a single object or body - it is free moving molecules which do not seem to compose a 'body'. Whilst gravity may affect each molecule, it would seem to do so independently. Solid objects are composed of molecules in some form of bond (is that E/M forces or chemical bonds?) and thus the effect of gravity on each molecule is effectively communicated across all molecules in that object. Not so an atmosphere I'd have thought?

 

[tiny-tim]

Hi Graeme!

However, the atmosphere is not falling. As I understand it, the atmosphere is composed of free moving molecules which are in effect unconstrained.

If those molecules are not 'connected' and not falling, then they can exert no force from the effects of gravity on any surface ?

The molecules are moving freely, but only until they hit something.

Usually, they just hit each other, but when they hit a solid surface they bounce off it, and that change in momentum exerts (indeed, is) a force on the surface.

(that's https://www.physicsforums.com/library.php?do=view_item&itemid=26" … force = rate of change of momentum)

(and Newton's third law applies as between the surface and the molecules that hit it)

 

[Graeme M]

Yes, but it will only be those molecules striking the surface that can impart a force. The molecules 200 feet further up, or 5 km further up, are not striking that surface. Or are you suggesting some kind of instantaneous cascading collision force through the entire atmospheric column at any instant?

 

[tiny-tim]

Or are you suggesting some kind of instantaneous cascading collision force through the entire atmospheric column at any instant?

Yes , except that it doesn't need to be instantaneous, does it?

 

[Graeme M]

Hmmm... yes, it would be largely wouldn't it?

I guess I am talking from ignorance here, but a solid object is solid at most instants isn't it? I mean if we slice down time to ever smaller slices then our solid object remains solid I assume? I am getting into very fuzzy territory now as I have no idea what actually makes a solid object solid, but it is the state of solidness that allows an object to be accelerated by gravity isn't it?

 

[Graeme M]

But getting back to my question, it seems to me that a solid object is a bound object and it is that binding that allows it to exhibit 'weight'. So I can easily conceptualise such an object bearing on a surface. I can't see how individual free moving molecules can transfer the effect of 'weight' so effectively throughout the whole vertical column.

 

[tiny-tim]

But getting back to my question, it seems to me that a solid object is a bound object and it is that binding that allows it to exhibit 'weight'. So I can easily conceptualise such an object bearing on a surface. I can't see how individual free moving molecules can transfer the effect of 'weight' so effectively throughout the whole vertical column.

You can think of a solid as being made of very tight springs.

A force at one side of a solid is transmitted at the speed of sound towards the other side.

Same thing for a fluid (liquid or gas), except that the transmission is by collision instead of by the electromagnetic forces that make up the "springiness" of a solid.

 

[Graeme M]

Forgive me for taking so much time with what must be very basic to many of you, and thanks for your replies so far, it is certainly helping me develop a decent mental model.

I must admit I've never heard of forces in solids being transmitted at the speed of sound! But there you go, you learn something new all the time.

So, you are saying that the molecules in the column of the atmosphere are all colliding with the molecules below them as they are pulled earthwards by gravity and this force is cascading through the column, and at each 'layer' this force is accumulating to create the pressure at the surface? Do air molecules only move downwards, or are they free to move in other directions? if other directions, would there equally be a force in another direction. For example, if I heated the air, the molecules would be excited and I would imagine them to move in all sorts of directions, including up. That force in a sideways or upward direction would be 'pressure' but it wouldn't be 'weight' would it?

 

[yoshtov]

So, you are saying that the molecules in the column of the atmosphere are all colliding with the molecules below them as they are pulled earthwards by gravity and this force is cascading through the column, and at each 'layer' this force is accumulating to create the pressure at the surface? Do air molecules only move downwards, or are they free to move in other directions?

They can move in other directions, absolutely! In general, you don't need to worry about differences in air pressure with respect to height, except when you start considering near-airtight structures that cover a significant vertical part of the atmosphere. There is almost no difference in air pressure as you move up by, say, 1 meter or 2.

if other directions, would there equally be a force in another direction. For example, if I heated the air, the molecules would be excited and I would imagine them to move in all sorts of directions, including up. That force in a sideways or upward direction would be 'pressure' but it wouldn't be 'weight' would it?

Absolutely! Consider the equation $$PV=nRT$$, which says that Pressure x Volume = number of molecules of gas (in moles) x R (a constant) x Temperature. Notice that nowhere does it make mention of mass of the gas involved. If you heated a volume of gas, the gas would exert more pressure on the walls of the container, but since the pressure would be equal across all of the walls of the container, the forces would cancel, and the mass would remain unchanged.

I think that the model of pressure being equal to the weight of the atmosphere has its benefits, but it must be properly illustrated, or it just leads to more confusion. Here's my take:

Air has mass. In the absence of any sort of gravitational field, the air has mass, but no weight. If you subject the air to a gravitational field, now the air has weight. It is attracted to the object that is producing this field. Assuming that there are a lot of molecules of gas, they will all try to get as close as possible, but will stack up on each other. Every molecule of gas wants to be as close to the source as possible, and so the molecules will "push" and "shove" (physics talk: collide) in an attempt to get closer. As you get closer and closer to the graviational source, the collisions become more aggressive. These collisions are what we call "air pressure." The cumulative effect of many, many collisions can produce a substantial force

 

[EnglishScient]

Air pressure is the result of gravity pulling down on the atmosphere and stopping the air from drifting off into space. So at sea level the air pressure is about 14.7 pounds per square inch, that's the weight of a 1 square inch wide column of air from sea level to the top of the atmosphere.

When you open your safe door the air inside the safe is at a pressure of 14.7 pounds per square inch, when you close the door you are trapping the same volume of air at the same pressure so the pressure remains the same inside.

Even if your safe was sent into space the pressure inside would remain the same as long as the safe didnt leak and the temperature stayed the same.

In the vacuum of space the air inside the safe would be pushing against the inside of the safe walls with a pressure of 14.7 pounds per square inch and since the vacuum outside is not pushing against the outside of the safe, the walls of the safe are stressed and the safe is trying to inflate like a balloon. Of course NASA has to make sure that spacecraft have strong enough skin to withstand this.

 

[russ_watters]

Ummm... I am unaware of the distinction. I mean the pressure that one experiences at the surface of the Earth and which is measured by a barometer.

There is: atmospheric pressure is the pressure of the atmospere. Air pressure is of any volume of air: it can be pressure in a tank.

 

[Paul Alexande]

Graeme M
You have hit the jackpot with this one!
No-one can explain how air pressure works!
Congratulations, in the spirit of pure thinking.

 

[sophiecentaur]

Graeme M
You have hit the jackpot with this one!
No-one can explain how air pressure works!
Congratulations, in the spirit of pure thinking.

"no one can explain"?
That's a bit sweeping, isn't it?
The pressure in a gas is due to the motion of the molecules. If the gas is in contact with the walls of a container then the pressure is due to the molecules bouncing against the container sides. Each molecule hitting the sides has it momentum, normal to the side, reversed. That involves an equal and opposite impulse against the side. The rate of the molecules hitting the side and their average velocity (related to the temperature) will tell you what the actual force is on every unit of area. Pressure acts in all directions equally; up down and the sideways directions. That is what pressure is and what 'causes' it. You don't need a surface to be there; the pressure is there, keeping the molecules in one region from encroaching (on average) into a nearby region of a gas, across a virtual surface between them.
The reason that there is pressure in the atmosphere is that all the molecules above us are pulled towards the Earth by gravity. The downward force on a 1square metre just above us is the same as the upward force and is due to sum of the downward forces (the weights) of all the molecules in a 1msquare column , all the way out into space. The fact that molecules are zapping about in all directions doesn't alter the above because it just averages out. The actual temperature affects the speed of the molecules, hence the pressure and, hence, how far the atmosphere extends out into space. If the whole Earth were much colder then the thickness of the atmosphere (assuming the same number of molecules) would be much less but the pressure would be much the same. ~(Very simplified model because, for a start, all the water would have condensed etc. etc. and gravity gets a bit less as you go up).
That was explained to me about 50years ago and it still is a pretty good explanation of what is happening.

 

[Paul Alexande]

Sophie (or "Centaur")
Thank you for providing a reasonable explanation.

 

[Graeme M]

Thanks again, this is good and I am making progress. But still having trouble with some conceptualisation.

I'll come back to Sophie's explanation in a moment. EnglishScientist states that my theoretical safe 'locks' in air pressure, even if my safe were in space (ie a vacuum) air pressure remains at what it was. However that ruins the earlier explanation which invoked the weight of the column acting on the safe and using Newton's third law to accumulate weight inside the safe. EnglishScientists' explanation therefore must explain air pressure as a force caused purely by molecular collisions with the barometer in the safe, surely?

Sophie's explanation suffers the same fate in my view. In fact, he/she invokes a virtual surface but also says that a surface is not needed in the atmosphere. However that leads me to contemplate the nature of weight. Weight it seems to me cannot be expressed without a surface when we are talking about objects, which I assume we are. The atmosphere is not an object is it? Therefore, it is the molecules we must consider, and these are the 'objects' being acted upon. Without a surface to bear upon, how can we have weight? Weight seems to me to be a function of the extent to which an object accelerated by gravity is prevented from reaching the velocity gravity imposes on it. But our molecules are not doing that, on the whole.

 

[Graeme M]

I also have trouble with the notion that 'collisions' impart weight. Any mass that is accelerated which collides with another mass will impart a greater force than if that first object merely rested on the second object wouldn't it? My thinking suggests that a collision is a different event to the 'contact' proposed by Newton in expressing weight. Gas molecule collisions will impart a force, but that force wouldn't be 'weight' would it?

 

[tiny-tim]

… However that ruins the earlier explanation which invoked the weight of the column acting on the safe and using Newton's third law to accumulate weight inside the safe.

No it didn't, the earlier explanation was that we have to add (1) the weight of the column of air inside the safe and (2) the force from the roof of the safe.

Weight it seems to me cannot be expressed without a surface when we are talking about objects, which I assume we are. The atmosphere is not an object is it?

I also have trouble with the notion that 'collisions' impart weight. Any mass that is accelerated which collides with another mass will impart a greater force than if that first object merely rested on the second object wouldn't it? … Gas molecule collisions will impart a force, but that force wouldn't be 'weight' would it?

You can regard the air as single body, in which case it is stationary (so no change of momentum) and it has weight, which acts as a force on the ground beneath it.

Or you can regard it as lots of separate molecules colliding with each other and with the ground. These molecules are not stationary, and it is the change of their momentum (only of the molecules next to the ground, of course) that provides the force on the ground …

this is equal to the weight calculated by the first method.

 

[Graeme M]

My apologies tiny-tim, I must have misunderstood. I thought you suggested that it was the weight of the air column on the roof of the safe that caused an opposite force to be expressed inside the safe, which then acted upon the air in the safe. In which case, if my safe were in a vacuum, there could be no such force. However, you are saying that the roof of my safe exerts a pressure on the air within? Wouldn't that mean it's just lucky that my safe knows exactly how much force to impart to give 14.7 lbs/sq inch?

 

[tiny-tim]

… Wouldn't that mean it's just lucky that my safe knows exactly how much force to impart to give 14.7 lbs/sq inch?

No, it's 14.7 because that's what it was before the safe door was sealed shut.

(assuming the temperature stays the same)

 

[sophiecentaur]

Thanks again, this is good and I am making progress. But still having trouble with some conceptualisation.

I'll come back to Sophie's explanation in a moment. EnglishScientist states that my theoretical safe 'locks' in air pressure, even if my safe were in space (ie a vacuum) air pressure remains at what it was. However that ruins the earlier explanation which invoked the weight of the column acting on the safe and using Newton's third law to accumulate weight inside the safe. EnglishScientists' explanation therefore must explain air pressure as a force caused purely by molecular collisions with the barometer in the safe, surely?

Sophie's explanation suffers the same fate in my view. In fact, she invokes a virtual surface but also says that a surface is not needed in the atmosphere. However that leads me to contemplate the nature of weight. Weight it seems to me cannot be expressed without a surface when we are talking about objects, which I assume we are. The atmosphere is not an object is it? Therefore, it is the molecules we must consider, and these are the 'objects' being acted upon. Without a surface to bear upon, how can we have weight? Weight seems to me to be a function of the extent to which an object accelerated by gravity is prevented from reaching the velocity gravity imposes on it. But our molecules are not doing that, on the whole.

OI! If I were a girlie, would I have an avatar like that? :grumpy: Do you ever read profiles?

But, on to pressure and in more detail. Why should there not be two ways of creating pressure? Inside a container, the pressure is caused by the force on the walls. Open the container in deep space and the molecules will just carry on in motion outwards and dissipate, bring the pressure inside the container to zero quickly. In the atmosphere, the pressure is there because of the velocities of the gas molecules hitting a 'notional' wall at any level.

Why should a molecule not have 'weight'? Weight is defined by mg (no specification about how small m can be nor how it has to be measured). This is, in fact, the force that is accelerating it downwards, whether it is 'resting on something' or falling.

In the end it comes down to this. If there were not a force keeping the molecules from falling to the ground (i.e. counteracting their weight) then they would all FALL to the ground). The fact that they are all up there must imply that there is an upwards force, keeping them there. That force must be equal to their weight. Or can you suggest another value for it?
If you have a problem with reconciling the idea of weight with the bouncing of molecules, consider this model. A man stands on some slow acting bathroom scales, holding a bat and a ball. The scales register his weight plus the weight of the ball and the bat. He then starts bouncing the ball in the air (keepy-uppy). Although, every time he hits the ball upwards, there is a large impulse, transferred onto the scale pan, most of the time the ball is not actually pressing down. The average force on the (slow acting) scales (the weight measurement) will be exactly the same as when he was just holding the ball. It's the same with the molecules, the ground has to support (indirectly) the weight of every molecule above it in the same way as the scales have to support the weight of the bouncing ball.

 

[Graeme M]

Sorry sophiecentaur, I am in the middle of a normal working day and only assigning a meager portion of what I imagine is my mind to this forum. It is all very useful but I didn't notice your avatar till after I posted. I *did* go back and change my post slightly...

Thanks for your latest post, very informative. I'll think about it and respond, though we may have got me there! :)

 

[sophiecentaur]

Sorry sophiecentaur, I am in the middle of a normal working day and only assigning a meager portion of what I imagine is my mind to this forum. It is all very useful but I didn't notice your avatar till after I posted. I *did* go back and change my post slightly...

Thanks for your latest post, very informative. I'll think about it and respond, though we may have got me there! :)

I wasn't really grumpy.

I may have put the problem in a nutshell. I must say, I never really though it through before.
It's my bedtime now!

 

[tiny-tim]

Don't feel bad, Graeme …

I used to think he was half-girl, half-horse …

turns out he's a boat!​

as a fish, i should have recognised that! ​

 

[Drakkith]

Hrmmm. It looks to me like the atmospheric pressure is due to the density and temperature of the air, not the "weight". Although the density of the air would be directly related to it's weight, as that is the reason that there is more air near the surface compared to 100 miles up.

Consider this: If I lock myself in a sealed container, the air pressure inside doesn't suddenly go away even if my container can withstand the full weight and pressure of the atmosphere. It is simply due to the density of the air inside and the temperature.

Now consider an air compressor. The compressor simply forces air into a container. The density of the air increases as the pressure does since we are shoving more air into a sealed space. A temperature change can also increase or decreases pressure, hence why you don't want a sealed container of pressurized gas thrown into a fire.

How does that look?

 

[Graeme M]

Thanks guys, I am pretty much happy with the explanation given. I will summarise, let me know if I am largely on track.

The atmosphere is composed of molecules. The molecules are in motion due to a variety of forces, eg wind, heat etc. However, by and large each molecule has mass and is being attracted to the Earth's surface. When not being pulled about by other forces, molecules fall downwards. On average, this leads to a greater density of molecules the closer to the Earth's surface we are. Also on average, at any level in the atmosphere the collisions of those molecules bears on any surface in the form of a force referred to as 'atmospheric pressure'. This varies with the density of the atmosphere (ie number of molecules). If all of those molecules immediately lost energy and fell to the ground where they could register as 'weight', the weight would be equivalent to the earlier pressure. Hence for the atmosphere, pressure broadly equates (or is exactly equivalent to?) to weight. I assume this is the same for any gas.

Close?

 

[klimatos]

So at sea level the air pressure is about 14.7 pounds per square inch, that's the weight of a 1 square inch wide column of air from sea level to the top of the atmosphere.

Since the atmosphere increases in circumference as you move away from the Earth's surface, the use of a uniform column is dimensionally incorrect. For a circular surface area, you should use a conic section. For a rectangular surface area, you should use a pyramidal section. The barometric formula is dimensionally incorrect in its use of a column.

The barometric formula relating pressure to the weight of the overlying air is invalid in many respects. It assumes a constant temperature, which is not the case. It assumes a constant gravitational constant: this assumption is invalid. It assumes a constant molecular mass--this is incorrect.

The weight-force hypothesis was developed when gas molecules were thought to be able to expand indefinitely to "fill" any space into which the gas was released. This expansion allowed molecule-to-molecule "contact" and hence transfer of weight force. The development of kinetic gas theory forced the abandonment of that concept, but the language stays on.

From a kinetic point of view, atmospheric pressure is no different from air pressure. It is the simple product of the number of molecular impacts per square meter and the mean impulse (in Newtons) transferred per impact.

When the wind blows across a building with open windows, barometers in the different rooms will show different pressures. And these pressures will be different from that registered by a barometer in the basket of a nearby drifting balloon at the same elevation.

The near-air-tight case of a barometer serves to dampen its reactions. Take a barometer out of its case during a windstorm and watch how the needle moves wildly. Do you think that this truly represents the "weight of the overlying air"?

 

[russ_watters]

Take a barometer out of its case during a windstorm and watch how the needle moves wildly. Do you think that this truly represents the "weight of the overlying air"?

This and most of the "incorrect assumptions" you listed above mix a concept with a model:

The barometric formula relating pressure to the weight of the overlying air is invalid in many respects.

While a certain formula may require such a simplifying assumption (I don't think it is correct to call them "incorrect assumptions"...but then I am an engineer...), the concept is unaffected by variations in temperature.

When the wind blows across a building with open windows, barometers in the different rooms will show different pressures. And these pressures will be different from that registered by a barometer in the basket of a nearby drifting balloon at the same elevation.

If the barometer is measuring velocity pressure, then it isn't just measuring atmospheric pressure. That's not a flaw in the description of "atmospheric pressure", it's a flaw in our ability to measure it.

Don't let the complexities of a moving atmosphere distract you from the reality that whether you model it as a pile of solid balls or a bunch of bouncing balls, the pressure is the same as long as you have the same number/weight of balls.

The conic section issue is interesting, though. I've never heard it and it sounds reasonable, but how big of a difference does it really make? We're not talking about a very large distance, since most of the atmosphere is packed close to the earth. Similar issue with g. I guess if I were being pedantic with the conic section one though, I'd say that saying "above" implies it must be conic/pyramidal, though I do admit I've never thought of it.

 

[sophiecentaur]

@klimatos
Your comments are mostly just additions to the basic model. As Russ says, the effect of wind is not relevant to the average situation.

Taking up your 'conic' objection and applying the actual figures. Effectively, 'space' takes over at a height of about 100km, beyond which there is very little mass of atmosphere. As a rule of thumb, Atmospheric `pressure doubles every 5000m of altitude. That means that half of the atmosphere's weight is contributed by the lowest 5km. By the time you are 20km up, the AP will be 1/16 of that at sea level; most of the atmosphere will be below you..
Lets examine this frustum (truncated cone) to which you refer and compare it with a 1m radius cylinder. The Earth's radius is about 6000km so, geometry tells us that the radius of a conic section at 20km will be 6020/6000 of that at the surface. Your 1m cylinder (at sea level) has now stretched to a radius if 1.003m. That's a difference in area of about 0.6% extra. Perhaps not worth making a big shout about.
Furthermore, the factor by which the area (and hence the difference in volume of an elemental slice) will be increased is exactly the same as the factor by which g will decrease, due to the inverse square law. So the effect would be exactly the same as for the simpler cylindrical model. There would be more molecules up there but they would all weigh a bit less each. (I rather like that argument actually - preen preen). Your 'dimensional' objection doesn't really seem relevant.

You have an objection based on the use of (and possibly failure of) the gas laws and their assumptions. We all know that real molecules have finite radius and that the mean free path of an air molecule is not many mm at AP, so they really can be expected to collide and pass on / exchange their momentum. The upper ones are kept up there by bouncing against the lower ones. Interestingly, if you take an atmosphere of ideal molecules, which never hit each other, you would have a situation in which the only thing keeping molecules up there would be that they would all, at some stage, hit the ground and bounce off, with a range of velocities, determined by the thermal motion of the molecules on the solid ground. They would rise through the atmosphere in a parabolic trajectory and they will lose Kinetic Energy and gain Gravitational Potential Energy, eventually falling to Earth again for another bounce. In a simple model, this would imply that the temperature of the atmosphere would steadily reduce as you go higher because the average KE would reduce. But, effectively, each molecule, by striking the ground once every so often, would be just like the ball which my man on the scales keeps hitting upwards. The average force will just be due to its weight. So, with or without the gas laws, the 'weight' argument must still apply.

But, in the end, what is the problem with relating AP to the weight of the atmosphere? Numerically, it's the same, irrespective of the varying constitution of it with height. I think the main problem that was expressed in this thread was the question of how a nebulous thing like a gas can have the same effect as a solid piston. I think that;s something one just has to come to terms with - because it can clearly be shown to be true.

 

[Feeble Wonk]

From one non-physicist to another... perhaps the most basic way of thinking about this is that the atmosphere is simply an accumulation of gas molecules that respond to the forces placed on them. Gravity acts on them because they are massive particles, thus pulling them toward the Earth's surface. The electromagnetic forces applied by the negatively charged electron shells repel the individual molecules away from each other. Air pressure builds as these forces compete against each other. Therefore, as more air molecules are accumulated in a given area of atmosphere, the air pressure rises, and will attempt to reduce the pressure by moving the air mass toward adjacent areas that lesser accumulations of air molecules. Obviously, any additional factors that effect molecular action/force (such as temperature) will have their effect as well. It all is simply the result of the system, as a whole, trying to achieve higher entropy.

 

[sophiecentaur]

From one non-physicist to another... perhaps the most basic way of thinking about this is that the atmosphere is simply an accumulation of gas molecules that respond to the forces placed on them. Gravity acts on them because they are massive particles, thus pulling them toward the Earth's surface. The electromagnetic forces applied by the negatively charged electron shells repel the individual molecules away from each other. Air pressure builds as these forces compete against each other. Therefore, as more air molecules are accumulated in a given area of atmosphere, the air pressure rises, and will attempt to reduce the pressure by moving the air mass toward adjacent areas that lesser accumulations of air molecules. Obviously, any additional factors that effect molecular action/force (such as temperature) will have their effect as well. It all is simply the result of the system, as a whole, trying to achieve higher entropy.

Pretty well summed up - the molecules are kept apart by just the same forces that keep the molecules of a solid apart - it's just that they are more sporadically applied.

 

[klimatos]

This and most of the "incorrect assumptions" you listed above mix a concept with a model: While a certain formula may require such a simplifying assumption (I don't think it is correct to call them "incorrect assumptions"...but then I am an engineer...), the concept is unaffected by variations in temperature.

We may have a semantic problem here. My position is that if they do not match the observed characteristics of the real atmosphere, then they are incorrect.

[/QUOTE] If the barometer is measuring velocity pressure, then it isn't just measuring atmospheric pressure. That's not a flaw in the description of "atmospheric pressure", it's a flaw in our ability to measure it.[/QUOTE]

A barometer is just another form of manometer. It has no magical properties. It cannot separate out the "weight of the atmosphere" from the myriad other factors that influence how often and how hard the air molecules impact on its sensing surface. It measures the pressure of the ambient air. That is all that it does.

[/QUOTE]Don't let the complexities of a moving atmosphere distract you from the reality that whether you model it as a pile of solid balls or a bunch of bouncing balls, the pressure is the same as long as you have the same number/weight of balls.[/QUOTE]

I'm sure you wrote that without thinking it through. Change the "bouncing" speed and the pressure changes. The mass remains the same.

[/QUOTE]The conic section issue is interesting, though. I've never heard it and it sounds reasonable, but how big of a difference does it really make? We're not talking about a very large distance, since most of the atmosphere is packed close to the earth. Similar issue with g. I guess if I were being pedantic with the conic section one though, I'd say that saying "above" implies it must be conic/pyramidal, though I do admit I've never thought of it.[/QUOTE]

It doesn't make a big difference. But it implies sloppy thinking.