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Does pressure/vacuum affect water vapor differently than other gases and hence humidity?

For example, imagine I have a vacuum bell at atmospheric (mean sea level) pressure and the air in it is 50% humidity. If I apply a vacuum to the bell and reduce the pressure to, say, 300mm of Hg (Torr), will the humidity of the air inside the bell change or remain the same?

If it changes, that would seem to suggest that for some reason the water molecules are preferentially being sucked out by the vacuum pump. Why would that be?

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  • $\begingroup$ Possible duplicate of Will a cloud form in a depressurized room. $\endgroup$
    – David Hammen
    Jun 24, 2015 at 8:51
  • $\begingroup$ @DavidHammen The question has nothing to do with condensation or droplet formation. $\endgroup$
    – Ambrose Swasey
    Jun 24, 2015 at 10:50
  • $\begingroup$ The two questions are very closely related, and both hinge on what I think is a misconception on your part of what "relative humidity" represents. $\endgroup$
    – David Hammen
    Jun 24, 2015 at 12:14
  • $\begingroup$ @DavidHammen Since I never used the term relative humidity, which you quoted, as though I had used it, maybe it's YOU who have the misconceptions. As should be obvious from my question, by humidity, I mean the proportion of water molecules to all the gaseous molecules in the bell. $\endgroup$
    – Ambrose Swasey
    Jun 24, 2015 at 13:58
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    $\begingroup$ The absolute humidity -- the ratio of water molecules to air molecules -- will remain constant unless you have a very special pump indeed. $\endgroup$
    – user107153
    May 17, 2016 at 6:13

3 Answers 3

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Will the humidity of the air inside the bell change or remain the same?

Assuming there is no change in temperature, the relative humidity will decrease.


If it changes, that would seem to suggest that for some reason the water molecules are preferentially being sucked out by the vacuum pump. Why would that be?

You may have a misunderstanding of what relative humidity means. From your question, it appears you are thinking relative humidity is the ratio of water molecules to other molecules. That is not what relative humidity is. Relative humidity is the ratio of the partial pressure of water vapor present in air to the saturation partial pressure of water vapor at the same temperature, expressed as a percentage. Removing water vapor while keeping the temperature the same reduces relative humidity.

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  • $\begingroup$ The vacuum pump does not just remove water molecules, it removes air molecules also. $\endgroup$
    – Ambrose Swasey
    Jun 23, 2015 at 14:00
  • $\begingroup$ "Assuming there is no change in temperature." I described the experiment. You tell me: will the temperature change? There is no need to assume anything. What you can assume is that I hooked up a vacuum pump turned it on until it read 300mm, then turned it off. $\endgroup$
    – Ambrose Swasey
    Jun 23, 2015 at 14:03
  • $\begingroup$ @TylerDurden - There will be a slight temperature drop due to the Joule-Thomson effect. This will be negligible compared to the large drop in pressure. The assumption is valid. $\endgroup$
    – David Hammen
    Jun 23, 2015 at 14:53
  • $\begingroup$ @DavidHammen The Joule-Thomson effect is NOT always negligible; when weather reports cite 'snow level at 4500 feet', they are saying that precipitation (100% relative humidity) occurs due to cooling by expansion, with no absolute humidity change, for breeze off the ocean rising in elevation as air goes over a mountain. $\endgroup$
    – Whit3rd
    Sep 3, 2022 at 4:21
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Suppose you have a closed box and you put a glass with water in it. If you wait long enough time you will reach thermal (kinetic) equilibrium where the same amount of water molecules will leave (desorb) from the water surface as will adhere (condense) on the water surface. In this case you have 100% relative humidity. Suppose now you put a solid object inside with some adsorbed water on it (all hydrophilic solids are covered by a water layers of nanometer thickness when kept in the normal atmosphere) but no glass with water. In this case, assuming the original air was dry, some water will desorb from the solid surface but the amount of water in the air will be much less then in the first case. The ratio between the two water (partial) pressures define the humidity. Note that in both cases you have full thermodynamic equilibrium so you can have relative humidity < 100% but still thermal equilibrium if there is no "free" water surface in the box. If you pump out gas from the latter system the relative humidity will decrease but not necessarily scale with the air or gas pressure as it depends on how strongly bound the water molecules are to the solid compared to air molecules. That is, the desorption rate in general will be different for different adsorbed molecules. You would reach full thermal equlibrium only if you pump infinitely long time to get rid of all adsorbed gases. In that case the relative humidity would be 0.

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Looking at natural aspects of the world we can see first that a lack of pressure in no way means water won't and some point coalesce amongst themselves given the ability (eg clouds). So does; by creating a lack of pressure do we also decrease the relative amount of water molecules? NO, but pulling out the molecules via vacuum pump will decrease the overall molecules in the space. However this is usurped by the vessel and the outside environment. If the relative humidity outside a permeable vessel remains higher coupled with a vacuums relative temperature being slightly lower than the outside environment water will permeate through. By achieving a perfect vacuum are you removing all molecules from within the free space, and thus all the humidity, unless the bell is holding water within it and is able to release them once the other molecules have left.

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