Why is Air Colder and Less Dense at Higher Altitudes?

[Studiot]

Be aware that this is not your only misconception (or slack terminology whatever you wish).

Thus, hot air at the surface creates space for the more rapid motion of its molecules by pushing cooler air aside, keeping its own density as low as possible to accommodate the greater need for freedom of motion for its high-energy molecules.

The atmosphere is bound to Earth by the Earth's gravity.
The pressure energy due to the weight of the air column is due to gravity.

 

[BadBrain]

Be aware that this is not your only misconception (or slack terminology whatever you wish).



The atmosphere is bound to Earth by the Earth's gravity.
The pressure energy due to the weight of the air column is due to gravity.

That's what I said:

"In terms of physics, the air at that altitude needs to resist far less gravitational potential energy trying to become gravitational kinetic than does the air further down, which has so much more air bearing down upon it"

But gravity is not the sole source of air pressure: heat is another source of air pressure, as is sound (read about the unmanned Apollo 4 mission, which was flown before the installation of the launch complex's sound suppression system).

 

[DaveC426913]

as is sound (read about the unmanned Apollo 4 mission, which was flown before the installation of the launch complex's sound suppression system).

While that is fascinating, surely you don't suggest that heating due to liftoff of rockets (or any other sounds) affects the climate of our atmosphere...

 

[BadBrain]

While that is fascinating, surely you don't suggest that heating due to liftoff of rockets (or any other sounds) affects the climate of our atmosphere...

That's NOT what I'm saying!

By sound pressure I am referring to acoustical overpressure, NOT to heating.

And I'm NOT claiming that this has any impact on a planetary scale.

All I'm claiming is that the building in which Walter Cronkite was seated suffered severe shaking and a ceiling collapse during the liftoff, which phenomena he reported on-air.

See:

 

[DaveC426913]

That's NOT what I'm saying!

By sound pressure I am referring to acoustical overpressure, NOT to heating.

And I'm NOT claiming that this has any impact on a planetary scale.

All I'm claiming is that the building in which Walter Cronkite was seated suffered severe shaking and a ceiling collapse during the liftoff, which phenomena he reported on-air.

See:

Then why do you bring it up in a discussion about atmospheric pressure, heating and cooling? Seems to be a red herring.

 

[BadBrain]

Then why do you bring it up in a discussion about atmospheric pressure, heating and cooling? Seems to be a red herring.

It's not a red herring. I was simply indicating alternative sources of air pressure besides gravity. Hot air is less dense than cold air, yet a hot air cell exerts greater atmospheric pressure than cold air. What causes the difference in pressure? Thermal energy. Likewise, the acoustic overpressure that brought the ceiling down on Walter Cronkite's head was another source of air pressure (albeit highly localized).

By the way, not only was your "rockets heating up the atmosphere" point a red herring, it was also a straw-man argument. You owe me and the PF community an apology.

 

[DaveC426913]

It's not a red herring. I was simply indicating alternative sources of air pressure besides gravity. Hot air is less dense than cold air, yet a hot air cell exerts greater atmospheric pressure than cold air. What causes the difference in pressure? Thermal energy. Likewise, the acoustic overpressure that brought the ceiling down on Walter Cronkite's head was another source of air pressure (albeit highly localized).

By the way, not only was your "rockets heating up the atmosphere" point a red herring, it was also a straw-man argument. You owe me and the PF community an apology.

Will the court read back the proceedings please. We are discussing hot and cold air masses, pressure, and sources of energy heating up the atmo, and you actually list rockets as a source - somehow you consider this a fact relevant to the discussion.

That is not a strawman (let alone mine); it is a red herring, and it is 100% yours. I simply called you out on it.

(You do realize that other people are following this, and can read everything you wrote, including post 32...)

How about we just stick to the topic.

 

[BadBrain]

DaveC426913:

Here, in part relevant to this discussion, is my post # 32, which I am totally aware that everyone can read, and I hope everyone will read.

"But gravity is not the sole source of air pressure: heat is another source of air pressure, as is sound (read about the unmanned Apollo 4 mission, which was flown before the installation of the launch complex's sound suppression system)."

In response to Studiot's post reminding me of gravity's role in gravitational air pressure, I posted this for the purpose of indicating multiple possible sources of air pressure, and I was correct in so stating.

I intended no red herring, simply a discussion of air pressure, which is the topic of this thread.

Your response was thus:

"While that is fascinating, surely you don't suggest that heating due to liftoff of rockets (or any other sounds) affects the climate of our atmosphere... "

This restatement of my position, which imputes to me a declaration that rockets or other sources of acoustical pressure waves contribute to climate change by means of thermal energy, which declaration cannot reasonably be inferred from my own statement, constitutes a fraudulent restatement of my position, for the apparent purpose of facilitating your attack upon my position.

This is the very definition of a straw-man argument, which does, in fact, constitute academic fraud.

 

[BadBrain]

DaveC426913:

Should you feel that I'm dealing unfairly with you, just press the report button, and let's let the mentors decide.

 

[Studiot]

Before this gets out of hand, let me point out that the net pressure change due to any wave is precisely zero. Sound wave are no exception.

 

[BadBrain]

Before this gets out of hand, let me point out that the net pressure change due to any wave is precisely zero. Sound wave are no exception.

You're precisely right! (Hey, I've heard of the sine wave function, too!)

Please remind me of that when Hurricane Irene hits me this Sunday! I'm sure I'll be greatly comforted by that fact ay that time!

YEAH, RIGHT!

 

[Studiot]

Winds are not sounds, although they can generate sounds; aeolian sounds do not hurt.

 

[BadBrain]

Winds are not sounds, although they can generate sounds; aeolian sounds do not hurt.

I never claimed that winds necessarily result from acoustic overpressure waves (though you should have been with me when I was living in Greenwich Village and a huge petroleum tank farm in New Jersey blew up!). I have, in fact, referred to phenomena (i.e., the tug-of-war between high-pressure systems and low-pressure systems) which primarily involve thermal energy differentials within the respective air masses.

Winds can blow down trees, and wreck homes, and send storm surges ashore to drown those of us who live at sea level.

You're correct in stating that winds, in themselves, are not sounds: they constitute the transfer of air from high-pressure zones to low-pressure zones in order to restore pressure equilibrium. But, while they're busy restoring isobaric equilibrium, they're also busy destroying my neighborhood and killing my neighbors.

GET IT?

 

[DaveC426913]

...they're also busy destroying my neighborhood and killing my neighbors.

GET IT?

Now that is a straw man.

And it is still off-topic.

 

[klimatos]

In a perhaps forlorn attempt to get back to the original posting question, I should like to offer a clarification to what appears to be a common misconception of many of the posters. That is, when you change the air temperature the pressure changes in response.

This may be true in confined laboratory vessels, but it is not true in the free atmosphere. It is extremely common for atmospheric temperatures to change significantly without any significant change in atmospheric pressures. What happens is that molecular number density (n=P/kT) changes instead.

We must be very careful in applying laboratory gas laws to the free atmosphere. Many such laws only apply under conditions of equilibrium (not found in the free atmosphere) or when one parameter is held constant (not possible in the free atmosphere).

 

[klimatos]

The majority of the heating of the Earth's atmosphere comes directly from the surface, both land and sea.

The solar irradiation (apart form a few isolated frequencies) largely passes through the atmosphere and is absorbed by the ground/ocean.

The ultimate source of the atmosphere's heat is from solar radiation. The proximate source (according the Kiehl & Trenberth's widely cited 1997 study on the atmosphere's heat budget) is 15% from the Sun and 85% from the Earth's surface. This is why the higher you go the colder it gets.

 

[Rensslin]

So if I drop an unopened can of beer and it doesn't explode, I notice the pressure on the can has increased. Avdgrado...(the French guy) states that the contents have not changed. While I put energy into the beer via dropping it therefore energy is not matter; or at least it does not exist independently of itself.

 

[BadBrain]

So if I drop an unopened can of beer and it doesn't explode, I notice the pressure on the can has increased. Avdgrado...(the French guy) states that the contents have not changed. While I put energy into the beer via dropping it therefore energy is not matter; or at least it does not exist independently of itself.

Actually, you're referring to Avogadro, who was Italian, not French.

Anyways, by dropping the can of beer onto the floor, you've converted part of the gravitational potential energy contained within the can into kinetic energy, which kinetic energy was converted upon impact with the floor into mechanical energy sufficient to bring some off the beer's CO2 out of solution and convert it to bubbles of gas, which resultant increase in the pressure within the can (due to the increase in the partial pressure of CO2 in gaseous, rather than dissolved, form) reflects an increase in the energy within the contents of the can, without altering its overall chemical constitution (as solution versus gas represents a physical change, rather than a chemical one). The physical change will eventually resolve itself by virtue of the CO2 going back into solution, with the energy from being dropped being released into the ambiance probably via thermal radiation through the can (which should be gas-tight). Some of the kinetic energy might be converted into mechanical energy which dents the can, and/or into thermal energy as the result of friction between the can and the floor.

In any case, Avogadro was primarily interested in partial pressures resulting from material constitution, and a temporary change in pressure due to input of energy from outside the system appears to be beyond his field of interest.

By the way, one of my most prized possessions just so happens to be a sealed soda can with NO SODA INSIDE! (Am I a nerd or what?) It hasn't collapsed yet, but it dents very easily.

Cheers!

 

[BadBrain]

I believe I owe a fuller explanation of my last post, which doesn't quite satisfy me.

While I claim the can to be gas-tight, the canning process obviously requires the expulsion of whatever air remains within the can after sealing through the seal, which obviously cannot resist the pressure of the air while immersed in boiling water (indeed, it's not supposed to be able to resist that pressure if the canning process is to work correctly). But, intuitively, the pressure increase within the can resulting from the energy input from immersion in boiling water should be far greater than that resulting from being dropped a few feet onto a floor which may well be flexible (OK, so that's another possible partial outlet of the kinetic energy from the fall: conversion into mechanical energy resulting in the movement of the floorboards). Therefor, the can is gas-tight to an extent sufficient for our purposes.

The explanation for the fact that my soda-free soda can hasn't collapsed? Probably the fact that exposure to the heat source during the canning process is limited in time to the extent needed to force the air out of the can and to kill any pathogens that may exist within the can. Therefor, as this process would have been developed under the assumption that the can had, indeed, contained soda, it simply would have been insufficient to drive most of the air from the can. Still, the fact of how easily it dents clearly shows that the air pressure within the can is less than the ambient air pressure.

Hope I helped.

 

[Russell5150]

No. There is an interplay between pressure, density, and temperature.

So could there be a scenario where there would be high pressure, high density, high temperature and very little air?

Or the reverse; could there be much air (in a small space) with low press,temp, and density?

Or break down the first question separately;
1. Could you have high density with little air?
2. Could you have high pressure with little air?
And probably the most interesting question; Could you have high temprature and little or no air?
-Russell

 

[Russell5150]

In atmospheric physics, there are two kinds of density.

One is mass density, that is, kilograms per cubic meter. The mass density of a volume of air depends upon its composition, its temperature, and its pressure. Colder humid air is less dense than slightly warmer dry air at the same pressure, because the mass of a vapor molecule is less than the mass of a dry air molecule. However, air at high elevations is less dense than air at lower elevations because the number density is less.

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

 

[klimatos]

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

I said cooler humid air is less dense than "slightly" warmer dry air. Humid air is also less dense than dry air at the same temperature. This is because the average water molecule is much lighter than the average dry air molecule: 2.99E-26 kilograms versus 4.81E-26 kilograms.

 

[klimatos]

Now you wait a minute here.
If cold wet air is heavier, thicker, and can easily displace warm dry air, How can you say that it is less dense?

I said nothing about "thicker" (whatever that means) or being able to "easily displace warm dry air". It is usually the denser air mass that displaces the less dense--other things being equal.

If by "thicker" you mean having more molecules per unit volume, you are mistaken. At the same temperature and pressure moist air does not have more molecules per unit volume than dry air. It has exactly the same number. That's Avogadro's Law.

 

[Russell5150]

I said nothing about "thicker" (whatever that means) or being able to "easily displace warm dry air". It is usually the denser air mass that displaces the less dense--other things being equal.

If by "thicker" you mean having more molecules per unit volume, you are mistaken. At the same temperature and pressure moist air does not have more molecules per unit volume than dry air. It has exactly the same number. That's Avogadro's Law.

My geophysics instructor explained that a moving cold air mass will displace and lift warmer air because the warm air is lighter and less dense than cold air. I guess I mean thicker in the sense that it has more molecules (air) in the same volume.
If you want to test this put your empty soda can into the freezer and watch it crush in on itself.