Earths Atmospheric Pressure - am I misunderstanding the standard definition ?

Please note that this thread contains no major 'conspiracy or revelation'. It's merely my own observation and ramblings regarding what I believe to be an inaccuracy that continues to be dispensed as the 'standard definition' when used to explain earths atmospheric pressure.
If you have little or no interest in science, please feel free to move onto another thread


Like many of you, I was taught in school that at sea level, the average atmospheric pressure that we experience is approximately 14.7 psi. This pressure we're told, is the result of the total weight of the atmosphere contained in a hypothetical 1 square inch column extending from sea level to the top of the atmosphere.

At sea level, for example, the pressure is 14.7 pounds per square inch. This means that a slice of the atmosphere in the shape of a long, thin column, with a one square inch base and as tall as the top of the atmosphere (at least 120 mi or 200 km), would have air within the column weighing 14.7 lb

Atmospheric pressure definition #1

and

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.

Atmospheric pressure definition #2

In the above definitions, and for the remainder of this post, we can take 'weight', 'pressure' and 'force' to be synonymous.


Here's an image to make the concept clearer:

[atsimg]http://files.abovetopsecret.com/images/member/dc30834774b0.jpg[/atsimg]


As mentioned, the above is the standard explanation that we're taught ... that effectively a weight of approximately 14.7 lbs is pressing against every square inch of the earth's surface, and that this pressure is caused by the total weight of all the air above each of those square inch areas, extending upwards in a column from the surface to the top of the atmosphere.


However, now I'm just somewhat disappointed that I simply accepted that explanation for many years without really examining the underlying scientific basis. If I had, it would have become quickly obvious that such an explanation really makes little scientific sense ... at least to me !

Looking at one of the above definitions again

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.

this assumes that each and every molecule of air in that square inch column is effectively transmitting a downward directed force (weight) and its the sum total of each of those downward directed forces that generates the atmospheric pressure.
The fallacy here is that we have an implication that the force (weight) of each air molecule is ultimately felt upon the surface directly below that column of air molecules. In other words, the force (weight) imparted by a molecule of air at a 100 kms altitude, along with the force (weight) imparted by every intermediary air molecule, is felt along with the force (weight) of those molecules of air in direct contact with the surface.
But for this to be so, that means that every molecule of air in that column has to be permanently and physically in contact as a single and cohesive mass (i.e. as a bulk liquid or solid) for all of their individual forces (weight) to be felt at the surface.


To make this a little clearer, lets take a look at that column of air extending from the surface all the way to the top of the atmosphere. In this example, a 2D rendering will be sufficient.

[atsimg]http://files.abovetopsecret.com/images/member/cab3250147d9.jpg[/atsimg]

It's quite obvious that the greatest density of the atmosphere is to be found at the bottom of that column and that atmospheric density decreases with altitude with the minimum density to be found at the top of the column. A decrease in density means that there is a subsequent increase in the average distances between air molecules i.e. the molecules spread further and further apart as the density decreases.


Now lets take a close up look at a small section of that atmospheric column and magnify the air molecules.

[atsimg]http://files.abovetopsecret.com/images/member/0efae85b5750.jpg[/atsimg]
As can be seen, the individual air molecules are not in permanent physical contact with each other and because each air molecule has a measurable amount of kinetic energy, they are also moving in random directions (and with random velocities) with respect to each other. In fact, the only time that the air molecules come into relative and momentary contact is when their respective paths happen to intersect, otherwise the vast majority of the air molecules within that column are independent of each other and not in physical contact.

A direct corollary of the above means that there is no way that air molecules that are not in a direct and continuous chain of contact with each other can contribute to the final force (weight) measured upon a surface.


Let me try to use an example.

If we take every air molecule in that square inch column and "glue" them together, we'll end up with a single object comprised of countless billions of individual molecules. At this point, if we measure the 'weight' of this air-object, we'll obtain a definitive and fixed weight that doesn't change and is the sum of every one of those individual air molecules.

Now lets modify the above. This time we'll take all the air molecules in the top half of the column and 'glue' them together as an object, then take all the air molecules in the lower half of the column and 'glue' them together as a second object. If we keep the 2 air-objects separated and one above the other in the column, the weight we now measure will now be much less as we're only measuring the combined weight of the air molecules within the lower air-object, and which are in contact with our surface. The weight of the upper air-object has no effect as it's physically separate from the lower air-object.

Now lets take this example to the ultimate degree and physically separate every air molecule in that column (i.e. it's normal state). Then the only weight we will be able to measure will be that very small percentage of air molecules actually in physical contact with our surface. The weight of any air molecule not directly in contact with the surface (or with another molecule thats directly in contact with the surface) will therefore not have any consequence and can be ignored.


So hopefully from the above, I've managed to successfully show why the standard definition of atmospheric air pressure is inaccurate from a scientific standpoint.


A few common examples will help to also show this inaccuracy contained in the standard explanation.

If standard atmospheric pressure (1 atm at sea level) is dependent on the combined weight of all those billions of air molecules contained in that 1 square inch column of air extending from the surface to the top of the atmosphere, then how can there still be 1 atm of pressure when measured inside a room ? The ceiling will act as a physical and solid divider within the column of air and negate any possible weight effects from the majority of the air above the ceiling. So the only measurable weight can only come from the air between the ceiling and the surface which according to the standard definition must produce a much reduced pressure value and be way below 1 atm in value.

Another example:

We're told that the pressure on our bodies is equally distributed. In other words, if we hold our hand out parallel to the ground (palm down), that there is 1 atm of pressure acting on the top of our hand and also 1 atm of pressure acting on the palm. But this makes no sense as there should be no 'weight' due to the small amount of air between the ground and the palm (facing the ground).

Another example:
The majority of the interior of the International Space Station (ISS) is kept at normal sea-level atmospheric pressure.

... while the rest of the station remains at the normal sea level atmospheric pressure of 14.7 psi

ISS atmospheric pressure

It's quite obvious that normal atmospheric pressure is NOT dependent on a quantity of air many kilometers deep and can in fact be easily produced within the very limited confines of the ISS module or even just within the few centimeters of space between an astronauts body and his space suit.


So what's an alternative definition for air pressure ?

Quite simply, it can be explained as the direct force being exerted by ONLY those air molecules that come in DIRECT contact with a surface. Any air molecule that does NOT come into direct contact with a surface can be disregarded and have no contribution to the measured pressure.

Using the 'standard column' example, it's not the total and combined weight of every air molecule in the column that determines the measured pressure value but instead ONLY that very small percentage of air molecules at the base of the column and that come into direct contact with the surface below the column. Therefore the greater proportion of air molecules in the column are superfluous and have no effect on the measured pressure value.

(Note that the measured pressure is proportional to the average kinetic energy of only those few air molecules that come into actual surface contact. The higher the average kinetic energy (i.e. temperature), the higher the pressure.)

Originally posted by tauristercus

In the above definitions, and for the remainder of this post, we can take 'weight', 'pressure' and 'force' to be synonymous.

These terms are usually not synonymous in classical discussions of physics. A force is any influence that causes a mass to accelerate. Weight is a measure of the gravitational force multiplied by a mass. Pressure is that amount of force applied to the surface of a mass.

Is there some special sense in which you are taking all these terms to by synonymous?

[edit on 8/25/2010 by Toromos]

Originally posted by Toromos

Originally posted by tauristercus

In the above definitions, and for the remainder of this post, we can take 'weight', 'pressure' and 'force' to be synonymous.

Is there some special sense in which you are taking all these terms to by synonymous?

I agree with the point you're trying to make but purely for the purposes of making my thread topic easier to understand (hopefully !), I tried to consolidate what was being described in the definitions that I supplied.

e.g.
At sea level, for example, the pressure is 14.7 pounds per square inch ... would have air within the column weighing 14.7 lb

and

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface

The pressure at sea level is omnidirectional and every molecule in the column is acting equally on every adjacent molecule regardless of direction. They may not be in actual contact with each other but they're as close as the temperature and pressure at any given altitude dictates. You might have noticed your car seems to run a little better on cold days and that's due to the molecular activity being less at lower temperatures making the air a little denser IE more oxygen per unit volume.

From wikipedia:

When describing a container of gas, the term pressure (or absolute pressure) refers to the average force the gas exerts on the surface area of the container. Within this volume, it is sometimes easier to visualize the gas particles moving in straight lines until they collide with the container (see diagram at top of the article). The force imparted by a gas particle into the container during this collision is the change in momentum of the particle. As a reminder from classical mechanics, momentum, by definition, is the product of mass and velocity.[9] Notice that during a collision only the normal component of velocity changes. A particle traveling parallel to the wall never changes its momentum. So the average force on a surface must be the average change in linear momentum from all of these gas particle collisions. To be more precise, pressure is the sum of all the normal components of force exerted by the particles impacting the walls of the container divided by the surface area of the wall. The image "Pressurized gases" depicts gas pressure and temperature spikes used in the entertainment industry.

In case of atmosphere, the particles are bumping into each other instead of a container, caused by the downward gravitational force.

You do have a point that the downward pressure is greater than the sidewards and upward pressure. But this difference can be neglected because random movement of the particles is magnitudes greater than the directional movement caused by gravity.

The interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image.

[edit on 25-8-2010 by -PLB-]

None of the above is really explaining how the kinetic energy/momentum of each of the billions of air molecules contained in that 100's of kilometer high 'column' is transferred to the bottom of the column, thereby resulting in a pressure value of 14.7 psi showing on the sensor.
There will be billions of molecules high up in the column that never get the opportunity to transfer their momentum to molecules lower in the column. It seems that a pressure sensor will only be reacting to a limited number of molecules located at the very bottom of the column that physically interact with the sensor itself ... so the further a molecule finds itself from the sensor (the higher it is up the column), the lower the probability that it will be able to transfer energy/momentum to the sensor either directly or indirectly through interactions with molecules lower in the column.

Originally posted by tauristercus
None of the above is really explaining how the kinetic energy/momentum of each of the billions of air molecules contained in that 100's of kilometer high 'column' is transferred to the bottom of the column, thereby resulting in a pressure value of 14.7 psi showing on the sensor.
There will be billions of molecules high up in the column that never get the opportunity to transfer their momentum to molecules lower in the column. It seems that a pressure sensor will only be reacting to a limited number of molecules located at the very bottom of the column that physically interact with the sensor itself ... so the further a molecule finds itself from the sensor (the higher it is up the column), the lower the probability that it will be able to transfer energy/momentum to the sensor either directly or indirectly through interactions with molecules lower in the column.

You know those kinetic ball toys some people have in their offices? I may be mistaken, but it's a bit like that.

reply to post by tauristercus


The explanation is that the particles bump into each other. So a particle in the highest levels of the atmosphere bumps into a particle that is a bit lower and transfers its momentum caused by gravity to the other particle. This particle will bump in yet another particle that is a bit lower, and you get a chain reaction all the way to the ground.

Of course you have to imagine that the particles are a large soup and bump into each other in all directions, so what I write above is overly simplified. The point is that the direction the particles bump into each other is slightly skewed downward because of the gravitational force.

Random collisions spread the weight of individual molecules, as explained so well by Einstein in his 1905 paper on Brownian motion. Although you're not measuring the weight of those particular molecules above a given square inch, you do get an average weight of the atmosphere over a larger area. At least that's what you would get if not for local turbulence and global air currents.

All of you are neglecting the fact that the atmosphere contains more than just air molecules. Everything in the air, including insects, dust and airplanes contributes to the pressure at the ground. If you grounded all air traffic, there would be a very slight reduction in atmospheric pressure at the ground, but no reduction above the air lanes. The wings push down more than up, and turbulence distributes the difference evenly by the time it reaches the ground.

Standard atmospheric pressure is defined in terms of Pascals, not in terms of the weight of the atmosphere. It is intended, however, to approximate the total weight of the atmosphere (and everything in it) divided by the area of the oblate spheroid of Earth at sea level.

reply to post by tauristercus


Air acts as a fluid.

The molecules in the air bump into each other just like the molecules in water. The molecules in water are not necessarily permanently and physically in contact with each other (although they are bumping around more than in a gas).

Would you have a difficult time wondering how water has weight?

[edit on 8/25/2010 by Soylent Green Is People]

Originally posted by Phractal Phil
All of you are neglecting the fact that the atmosphere contains more than just air molecules. Everything in the air, including insects, dust and airplanes contributes to the pressure at the ground. If you grounded all air traffic, there would be a very slight reduction in atmospheric pressure at the ground, but no reduction above the air lanes. The wings push down more than up, and turbulence distributes the difference evenly by the time it reaches the ground.

This is the only argument in the thread that's persuasive that the following definition is incorrect:

Originally posted by tauristercus

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.

Atmospheric pressure definition #2

None of the arguments in the OP are persuasive at all, all the molecules bumping against each other have a cumulative effect that increases pressure as you get closer to the Earth's surface.

I would add that another reason the definitions are perhaps somewhat incomplete are that they do not attempt to account for/explain varying barometric pressure, which has meteorological explanations. An extreme example would be a tornado where the atmospheric pressure is influenced strongly by atmospheric conditions.

But if you want to get picky like Phractal Phil, he's right that the additional pressure created by of all the flying helicopters, planes, insects, birds, bats and bugs would also count in addition to the weight of the air column. If you take all that stuff out of the air and measure pressure in calm weather I think the definitions are fairly accurate.

You are thinking into it too hard.

One atmosphere is the fluid pressure at sea level. The fluid happens to be air but it's still a fluid.

If you were to jump into the ocean and descend to 10 feet below sea level you would be experiencing two atmospheres of pressure (roughly 29.8 psi)

At 20 feet you experience three atmospheres of pressure and so on.

Air and water are both fluid and both exert pressure omnidirectionally.