Exploring the Mystery of Colder, Less Dense Air at Higher Altitudes

[Rensslin]

If cold air is more dense, why is higher altitude air colder and less dense?

 

[russ_watters]

Two reasons: space is cold and air cools as it expands(it expands as it rises)

 

[Rensslin]

Does its heat sink while the air rises?

 

[Rensslin]

Does the air move up through the heat while the heat stays relatively stationary?
If warm air is thinner than cold air and it expands as it cools, does it somehow re condense?
I'm trying to get my mind around cold dense air versus cold thin air.
So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?

 

[DaveC426913]

So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?

Well, the expansion due to heat will far outstrip the compression due to rise in air pressure, so it will most definitely expand.

 

[Rensslin]

Well, the expansion due to heat will far outstrip the compression due to rise in air pressure, so it will most definitely expand.

So, in this case cold air is not more dense than warm air, and therefore, cold air is not necessarily more dense than warm air, but only in some situations.

 

[Studiot]

Why do you think cold air is by definition 'more dense'?

Consider a pneumatic tyre just after you have pumped it up.

Is the air inside more or less dense than the surroundings?

And is it hotter or colder than the surroundings?

 

[Rensslin]

Why do you think cold air is by definition 'more dense'?

Consider a pneumatic tyre just after you have pumped it up.

Is the air inside more or less dense than the surroundings?

And is it hotter or colder than the surroundings?

I think cold air is more dense than warm air because if I put an empty water bottle in the freezer over night it will crush in on itself. But I don't know if this principal is universal for cold or hot air.
I think your question is a good one. I think I can make a way to get a thermometer inside a can and then pressurize it to see if it changes.

I squirted a blast of air out of a can for cleaning keyboard dust. I noticed that the can got cold. I assume that the can being pressurized must have been endothermic, therefore the release of pressure must be exothermic. Thus, the temperature decrease.

 

[billiards]

Obviously pressure is very important.

At constant pressure air is colder when it is less dense. If you increase the pressure the density of the air will increase (as you are effectively squashing the air into a smaller volume). Consider the case where you increase the pressure on the air, and yet the density remains the same -- in this case something must be fighting against the squeezing tendency due to the increasing pressure, that something is the increase in kinetic energy of the gas particles, that something is an increase in temperature -- the rise in pressure is met with a rise in temperature if density is to be conserved.

So to the OP. What happens to the pressure at higher altitudes? What are the implications for this on the temperature and density of air?

 

[Rensslin]

Aren't pressure and density synonymous? At least when it comes to air?

 

[Evo]

I squirted a blast of air out of a can for cleaning keyboard dust. I noticed that the can got cold. I assume that the can being pressurized must have been endothermic, therefore the release of pressure must be exothermic. Thus, the temperature decrease.

The cold is from the gases (propellants) in the can.

When the can is held upright and activated, gas flows out through the nozzle. The pressure inside the can therefore drops, and is no longer sufficient to keep the contents as a liquid; so some of the liquid boils, until the equilibrium pressure is re-established. The vaporization of a liquid is endothermic; thus, heat is absorbed, and the can becomes cold.

http://en.wikipedia.org/wiki/Gas_duster#Cooling

 

[billiards]

Aren't pressure and density synonymous? At least when it comes to air?

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

 

[russ_watters]

Does the air move up through the heat while the heat stays relatively stationary?

Heat is not a thing, separate from the air. The air has a certain amount of heat, which it carries with it as it rises.

If warm air is thinner than cold air and it expands as it cools, does it somehow re condense?

If there is moisture in it, the moisture can condense. That's how a thunderstorm works.

I'm trying to get my mind around cold dense air versus cold thin air.

The atmosphere has a pressure gradient to it, so it gets less dense as you go up. But at any particular altitude, warmer air is less dense than colder air.

So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself?

It would collapse.

Aren't pressure and density synonymous? At least when it comes to air?

Not quite, no. At any altitude, any non-rigid volumes of air will have the same pressure, but may have different density.

 

[DaveC426913]

So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?

It would collapse.

Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.

Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
Net effect is a collapse.

 

[Studiot]

I think enough examples have now been provided to demonstrate that what happens in a gas is more complicated that one simple statement will allow. Further what happens also depends upon outside circumstances.

The question of does a gas cool or warm when it expands depends upon how the expansion is generated and whether the gas is confined or free.

So if we heat a gas in hot air balloon, the gas expands into the confines of the balloon, increasing the pressure and temperature whilst reducing the density.

By constrast if we expand a gas from a cylinder of compressed air into the open air, the gas cools as it expands and its pressure and temperature and density reduces.

Mathematically the controlling physics are known as the Gas Laws and the Laws of Thermodynamics.

go well

 

[russ_watters]

Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.

Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
Net effect is a collapse.

Actually, I read too fast and didn't see the cold mountain part, but thanks for bailing me out!

Regardless, yes, atmospheric pressure drops very quickly as you go up in altitude.

 

[billiards]

Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.

Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
Net effect is a collapse.

I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.

 

[klimatos]

If cold air is more dense, why is higher altitude air colder and less dense?

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.

The second density is number density, that is the number of molecules per cubic meter. The formula follows Avogadro's Law and is n= P/kT. Here n is the number of molecules per cubic meter, P is the pressure in Pascals, k is Boltzman's Constant, and T is the temperature in Kelvins. Obviously, the number density of a parcel of air depends upon both the temperature and the pressure. The number density of high altitude air is less than that of low elevation air at the same temperature because the number density reflects the Maxwell molecular speed distribution for any given temperature. This is a common exercise in discussions of kinetic gas theory and statistical mechanics. That is, showing that the number density reflects the barometric formula.

 

[DaveC426913]

I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.

It will though it is not as simple as that. You tested pressure change only, did you test temp change? Were your crisps* exposed to below freezing temps a thousand metres up?

*whatever crisps are...

 

[Studiot]

I think this is a case for the nspcc.

(National Society for the Prevention of Cruelty to Crisps)

 

[BadBrain]

I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.

I've had something of the same experience, but in reverse to yours.

I once bought some juice boxes (big mistake, as those tiny, child-sized straws are really hard for an adult to work with), and, as I live at sea level, I noticed that the boxes looked a little crushed, and, when I stuck the straw into them, they began to drip the juice out in a steady stream within about a minute, before I'd even sucked on the straw. The only explanation I can think of is that the air pressure differential between where it was packed (which I assume must be somewhere near Top-O'-the-Universe, Colorado, or, possibly, the moon!) and where I live must have been great enough to compress the fluid content of the juice box to a certain maximum, with state of compression the puncture of the box by the straw released, allowing the compressed juice to flow into the less-dense humid sea-level air.

And, if anybody wants to argue with my position on the issue of the siphon effect along the lines of that Australian guy's bizarre gravity theory, please start a new thread under "General Physics". (I'll be looking for it, as I occasionally need a sure-thing victory!)

 

[billiards]

It will though it is not as simple as that. You tested pressure change only, did you test temp change? Were your crisps* exposed to below freezing temps a thousand metres up?

*whatever crisps are...

Are you having a giraffe or what? My favourite are prawn cocktail.

 

[DaveC426913]

Are you having a giraffe or what? My favourite are prawn cocktail.

I'm presuming some sort of potato chip. But I'm also presuming some sort of nuance to it. Like, instead of potato, it's made out of vegamite.

 

[DaveC426913]

when I stuck the straw into them, they began to drip the juice out in a steady stream within about a minute, before I'd even sucked on the straw.

I have noticed they do this too. I think it's endemic, not situational.

 

[Rensslin]

So if you took a liter of cold air and counted all the molecules, it would have more than what you would have in a liter of warm air.
If you then compressed each of them into 500 ml you would double the atmospheric pressure. Right?
If you compressed them again into 250 ml, you would double them again. Right?
Because the cold air has more mass, doesn't the atmospheric pressure exponentiate at a different rate than the warm air? Double is double. Right?

If you blow up a balloon on a hot day and carry it into a walk-in cold box it should rise to the ceiling.
But if you carried it in and then popped it the warm air would mix with the cold air before it could rise. Where would the warmth go?

If you hit seven golf balls at seven o'clock with a seven iron the balls will go a certain distance.
If you wait until eight o'clock and hit eight balls with an eight iron they go about the same distance.
That's because the sun comes up, hits the ground and emits radiant heat. This warms the air and the molecules push against each other and the air expands and becomes thinner. Thus, less resistance on the golf balls.
assuming you didn't get tired.

Sorry I went away. I went to PF jail until I got a new nickname.
I don't like it. I want to change it. I hope I can.
But if they don't let me I'll have another constant reminder of indiscretions past.

 

[BadBrain]

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.

The second density is number density, that is the number of molecules per cubic meter. The formula follows Avogadro's Law and is n= P/kT. Here n is the number of molecules per cubic meter, P is the pressure in Pascals, k is Boltzman's Constant, and T is the temperature in Kelvins. Obviously, the number density of a parcel of air depends upon both the temperature and the pressure. The number density of high altitude air is less than that of low elevation air at the same temperature because the number density reflects the Maxwell molecular speed distribution for any given temperature. This is a common exercise in discussions of kinetic gas theory and statistical mechanics. That is, showing that the number density reflects the barometric formula.

Your answer is PERFECT!

But it's a little technical, and can be more easily explained thus:

Near the surface of the Earth (let's say at sea level, which is where I live), the air is squished by the weight of the air above it. We physicists call this squishing "compression". The higher one climbs into the atmosphere, the less air there is above you to compress, or squish, the air around you. With less compression of the air at your higher altitude (say, the top of a 6,000 ft mountain), the easier it is for the molecules with make up the air to spread out lazily (in line with the Second Law of Thermodynamics, better known to laymen as the Law of Entropy). Thus, the air up there, because it is thin, enjoys low density because there's much less air above it to press down upon it. 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. This allows the higher-altitude air to easily vent any heat, to which we physicists refer as "Thermal Energy", by just moving a little faster for a little while, until the energy, usually derived from solar radiation, is dissipated via the increased motion of the air molecule.

Now, further down, any piece of air that receives a lot of Thermal Energy from the sun has to expend this energy by increasing the movement of its molecules to an extent that forces these molecules further apart from one another, to the point that they must often infringe upon the portion of the Earth-touching atmosphere which has colder, less energetic air. 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. Another way of saying this is that there are two types of air pressure: air pressure due to the weight of the air above the air mass you're standing in, which might be called "Gravitational Air Pressure", or "High-Density Air Pressure", and air pressure due to the average kinetic energy of the molecules (which is the real meaning of the word "temperature") which make up the air mass you're in, which might be called "Thermal Air Pressure", or "Low Density Air Pressure".

Now, as hot air always rises (because it just HAS to squeeze itself out the confines of the higher-density cooler air), it eventually reaches an altitude at which its thermal energy can be more easily vented, as there are fewer molecules around to interfere with this process. This is something like the way a refrigerator or an air conditioner (these two devices actually being, in principle, the same device) works, by taking hot air with air molecules at at a high level of average kinetic energy and, by compressing this air by means of the exertion of mechanical energy (which means squeezing the hot air by squishing it with a piston driven into a cylinder, with the piston being powered by an electric motor) until the average kinetic energy of the air's molecules is severely reduced, with the result that, when the compression is released, the resulting air is much colder than it was when it entered the cylinder. Where does the heat go? Look at your refrigerator or your air conditioner: it certainly has a radiator in the back, which vents the heat into the atmosphere. And, where does the heat of a rising hot air mass covering a large portion of the surface of the Earth go at it rises to a higher altitude? How about outer space? The possibility exists that outer space might just be larger, and, thus, better able to absorb thermal energy, than your apartment!

 

[Studiot]

Another way of saying this is that there are two types of air pressure: air pressure due to the weight of the air above the air mass you're standing in, which might be called "Gravitational Air Pressure", or "High-Density Air Pressure", and air pressure due to the average kinetic energy of the molecules

What a pity you have mixed up this reasonable statement with a bunch of misconceptions.

A really, really poor way to attempt to heat a gas is to shine a light on it (solar radiation).

 

[BadBrain]

What a pity you have mixed up this reasonable statement with a bunch of misconceptions.

A really, really poor way to attempt to heat a gas is to shine a light on it (solar radiation).

Heating a gas by means of light may be inefficient, but are you denying the role of solar radiation in Earth's climate?

Is heat itself not light on the infrared end of the Electromagnetic spectrum?

How do you account for the seasons if not for the oscillating inclination of the Earth relative to the sun?

Do you posit another particle besides the photon as the carrier of the Electromagnetic force?

 

[Studiot]

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.

 

[BadBrain]

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.

OK, so most of it comes INDIRECTLY from the sun, but the sun is still the ultimate source of most of the heat in the Earth's atmosphere.

Your statement does not make false my claim as originally stated:

"... usually derived from solar radiation."

 

[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.