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Air can hold water in proportion to partial pressure of water vapor. This is affected by temperature. But is the water capacity of air also affected by ambient pressure?

I did find some answers that water holding capacity of air decreases with increasing pressure. Quora link, UCAR link

But I was unable to find any equations that would tell me exactly how much it changes.

In this case we talk only about change in pressure. So temperature and volume would stay the same.

I will be glad for any sources that can answer me this relationship.

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  • $\begingroup$ What do you mean by "water capacity?" You yourself said that the partial pressure of water vapor remains unchanged when dry air is added to the container. But since you've added air molecules, that means that the fraction of gas molecules that are $\text{H}_2\text{O}$ is reduced. So, which of those numbers do you consider to be the "water capacity?" $\endgroup$ Commented Oct 29, 2023 at 19:30
  • $\begingroup$ I mean absolute capacity. So at 1bar, 20C, 1m3 of air can hold maximum of 17,2g of water vapor. Anything more will condense into liquid water. That means water capacity of this air is 17,2g. Or it can be expressed in mols if we want to know molecule numbers. $\endgroup$
    – Foxtrot
    Commented Oct 29, 2023 at 19:37
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    $\begingroup$ The "water capacity" of air is independent of the ambient pressure and only depends on the saturation vapor pressure of water. That saturation vapor pressure depends only on temperature and can be found with the Antoine equation. $\endgroup$ Commented Oct 29, 2023 at 23:57
  • $\begingroup$ Thanks, so even at 300bar, 20C, 1m3 the maximum amount of water vapor stays the same at 17,3g (0,96mol) $\endgroup$
    – Foxtrot
    Commented Oct 30, 2023 at 10:40
  • $\begingroup$ It is space capacity, not air capacity. Air is a bystander. // In fact, liquid saturated vapor pressure very slightly raises with the total pressure, as the chemical potential of the liquid raises with pressure. This effect is strong near the liquid critical point. E.g. at ambient T, nitrous oxide is at 100 atm liquid, but its mixture 1:1 (v/v) with oxygen at 200 atm is gaseous. $\endgroup$
    – Poutnik
    Commented Nov 9, 2023 at 14:53

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From most of what I've read on the topic I'd infer that the dew-point is only dependent on two things:

  • temperature
  • partial water vapor pressure

Adding dry air containing absolutely no water vapor at the same temperature into the container would affect neither of the two, and therefore not change the dew-point = water carrying capacity of the air-mixture.


One thing that makes me unsure of this conclusion is the answer by user @Chemomechanics to this question: Why atmospheric pressure does not influence vapor pressure of water but only temperature does? where he states that there is a small change because pressure adds energy to the system, though I fail to follow his math sadly.

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