B Moist air - does it rise or drop?

[sophiecentaur]

Is this anything like smoking a cigarette and watching smoke rise and roil, until it stabilizes ( in altitude ) somewhere above.
Heat rises the smoke at the beginning and then it spreads out.

Perhaps the guy could have a smoke box set above his chest(at the chest temperature) , release some smoke and see what happens. An instant visual.

Some effects as in post 9 from @Lnewqban might come into play.

That's a familiar phenomenon that you can see with bonfire smoke and, indeed it's similar with cumulus clouds over warm ground. I'm not too sure how it relates to the humidity / temperature distribution, though. It tells you when the extra thermal energy of the smoke runs out as it gains gpe. Would the smoke stop at the interface between dry and damp air?

I think the OP's question is actually a big one and needs to be 'trimmed' if we want an actual answer. Is the air near the ceiling always warmer and / or is it always damper? In a black metal vertical column (equal temperature all the way up), could we measure a humidity difference as we go up?

The walls would maintain a 'zero' temperature gradient so there would be a steady change in the mixture of molecules on the way up, depending on density.

Hot air will generally rise to the ceiling.
Moisture will generally diffuse throughout the room.

remember that most of our experiences involve a heat source somewhere and a heat sink. How much and where are very important factors.
Diffusion will be affected by molecular mass, too.

 

[Lnewqban]

... So we can just say on top of the box we have an element that produces heat and humidity. And the question i was asking myself was, would that actualy stop the airflow since moist and hot air will stick near top of the box.

That element acts like a source that constantly adds thermal energy to the immediately surrounding air.
At first, it lingers close to the surface of that element, reducing the heat transfer (acting as a powerful insulation film or stagnant air layer).

That increases the temperature of the element and the heat transferred into the air layer, part of which will reach enough temperature and "lightness" to move up as a convective stream.

Part of that stream gets diffused into surrounding cooler air, and most reaches cooler horizontal surfaces, over which some cooling and microscopic condensation occurs.

Then, it starts moving horizontally to give room to new ascending mass of hotter air.
If the room is perfectly sealed to exfiltration and infiltration of exterior ambient air, our mass of air will eventually be forced down to occupy the place of the continuous mass of air heated and humidified by our element.

Not exactly the same cases, but please see the visualizations shown in the link below of air being heated by kitchen equipment and induced to move towards and inside kitchen hoods.

https://www.captiveaire.com/sitedocs/videos/hoods/schlieren/svideo.asp?EXH=1&PSP=1

 

[Baluncore]

One problem here is that a thermal plume forms above the sleeper's head, where they breathe out warm saturated air. That rises and then moves outwards, across the ceiling, then circulates down the walls as it cools.

There is a limit to how much water can be dissolved in the air within a room. Once that limit is reached, if respiration continues to heat air, without sufficient external ventilation exchange, condensation must occur somewhere in the room.

Hot air has a higher capacity to dissolve water, so the RH will actually increase as the air sinks, without the addition of any more moisture, to settle on the floor and under the bed, with 100% RH. The cool underside of the mattress provides the preferred surface for condensation.

That is probably a partial explanation for why the bottom of a mattress is wet. It suggests that better ventilation is required, or that air conditioning should dry the air sufficiently to prevent condensation from 100% RH cool air, near the floor.

 

[sophiecentaur]

There is a limit to how much water can be dissolved in the air within a room.

Air is just a mixture of gas molecules and the proportions depend on the partial pressure of water vapour. It will depend on the temperature of any water surface. I think the term "dissolved" may not be appropriate here because it suggests something different about the water compared with the other air molecules. Air is not really a 'sponge'. It's just a matter of luck that water has a vapour pressure about the same as AP in habitable rooms. Just above the surface of boiling water, the proportion of H2O will be nearly 100%. In a freezer, the proportion will be very low.

This whole thread is really dealing with a dynamic situation - warm, wet surfaces and other warm and cold surfaces. Hard to predict what will happen as water is constantly being transported from place to place

 

[Baluncore]

I think the term "dissolved" may not be appropriate here because it suggests something different about the water compared with the other air molecules.

But there is something different about water compared to the mixture of nitrogen, oxygen and argon, we call air. Those gasses do not condense, to precipitate out, at the temperatures and concentrations found in a bedroom.

Water has a temperature dependent "solubility product" in air. Air saturated with water vapour has a RH = 100%.

If you fear the complexity of RH, the psychrometric chart, or the steam tables, you should opt out of the discussion, rather than try to confuse the situation with extreme examples that do not occur in a bedroom.

 

[sophiecentaur]

If you fear the complexity of RH, the psychrometric chart, or the steam tables,

I don't "fear" them but many of the terms that are used can be misinterpreted. They are gradually leaching out (e.g. sensible heat) but they give a doubtful model. The law of Partial Pressures doesn't include "absorb" because each molecule does its own thing. Fact is that there are many examples of practical Science that work fine but actually doesn't involve 'understanding'. Moist Air is one of them; it describes an apparent behaviour but doesn't explain it. The chart and tables you quote are mechanical methods of predicting a result and, these days, can almost certainly be replaced by an App. Very useful, of course but PF usually tries to look a bit deeper and avoid rôte methods. I don't understand why you are rejecting an explanation and terminology that makes more actual sense, in general.

Using the term 'absorb' is, to my mind, putting things the wrong way round. If you evacuated the room and just allowed water vapour from a 'room temperature' source the pressure in the room would be the same as the partial pressure and concentration of water molecules as under normal circumstances. The popular 'sponge' model wouldn't explain that and that's what leaves me unsatisfied.

 

[Baluncore]

If you evacuated the room and just allowed water vapour from a 'room temperature' source the pressure in the room would be the same as the partial pressure and concentration of water molecules as under normal circumstances.

And that limits the maximum absolute humidity possible in the room at that temperature, before condensation begins to occur at 100% RH = saturation.

Absolute humidity equivalent to 100% RH in air at sea level.

°C    g/m3
50    83.0
45    65.4
40    51.1
35    39.6
30    30.4
25    23.0
20    17.3
15    12.8
10    9.4
  5    6.8
  0    4.8
- 5    3.4
-10    2.3
-15    1.6
-20    0.9

 

[sophiecentaur]

And that limits the maximum absolute humidity possible in the room at that temperature, before condensation begins to occur at 100% RH = saturation.

You have given it a name but you are not making the connection with basic Physics mechanisms. The partial pressure at a given temperature is the same with or without other gases present. This is Gay-Lussac's law.

Are you claiming that I am wrong and that there is no connection with my model and your graphs and tables? You see, I am not saying you are wrong or that the tables do not apply; I am just saying that the way the water behaves, when in equilibrium, is just due to the temperature What else can it be due to?

Have you a model that tells us why or how the other gases in the air somehow allow water vapour to exist amongst them - apart from the pressure and temperature? Is the air actually soaking up the water? This is one of those "nature abhors a vacuum" things which was the way the ancients viewed things - without any explanation or mechanism stated.

 

[Baluncore]

Are you claiming that I am wrong and that there is no connection with my model and your graphs and tables?

No.
I am simply avoiding going all the way back to first principles when doing an engineering analysis. The pressure remains at one atmosphere in the bedroom. There is a temperature dependent limit to absolute humidity, saturation, that makes it possible to directly analyse or model the movement and condensation of water in that space.

 

[sophiecentaur]

I am simply avoiding going all the way back to first principles

Is that the PF way?

The pressure remains at one atmosphere in the bedroom.

That's because air has flowed in or out but the partial pressure of water is still the same - set by the temperature- whatever pressure is imposed on the room. I think this is all very relevant to understanding what's happening.

If someone asked why the light bulb glows and I answered "because someone switched the switch" that would not be judged as a suitable PF answer. We would expect a mention of resistance energy and temperature equilibrium. I'm only sticking to the same basic principle.

 

[DrJohn]

As an aside, I asked a well-known AI whose name begins with chat the very same question about dry vs moist air. It got it wrong. I then took it through a series of simple questions on gases with different molecular masses and it got them correct, as it did with mixing two different gases together - it spotted that the volume increased after a bit of repetition. I then asked the same original question and it got it wrong. I then told it that its previous answers where correct and contradicted its moist air answer which was wrong. It apologised (if AI can apologize) and admitted that I was correct in saying that moist air will have a lower density and briefly explained why.

I asked it again a few days later, and it gave the same wrong answer.

Several months later I asked the new updated version the same question and finally it got it correct.

 

[DaveC426913]

nitrogen, oxygen and argon, we call air. Those gasses do not condense, to precipitate out, at the temperatures and concentrations typically found in a bedroom.

Hee hee.

 

[Baluncore]

Hee hee.

He Her ?
I guess it would depend on the degree of frigidity in the relationship.