# Moist air - does it rise or drop?

• B
• SentinelAeon
In summary, when things are uniform, the relative humidity of air is the same at all levels. However, if the temperature is not uniform, then the air is denser near the temperature of the warmest object.
Baluncore said:
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.

sophiecentaur said:
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.
Code:
°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

Baluncore said:
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.

sophiecentaur said:
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.

Baluncore said:
I am simply avoiding going all the way back to first principles
Is that the PF way?
Baluncore said:
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.

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.

syfry and sophiecentaur
Baluncore said:
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.

DaveC426913 said:
Hee hee.
He Her ?
I guess it would depend on the degree of frigidity in the relationship.

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