How to factor atmospheric pressure into relative humidity calculations

AI Thread Summary
Atmospheric pressure's relevance to relative humidity (RH) calculations varies with altitude and specific processes. While RH is defined by the ratio of water vapor's partial pressure to the saturation vapor pressure at a given temperature, atmospheric pressure does not directly influence RH itself. However, other methods of measuring moisture in the air, such as mass/volume ratios, do require atmospheric pressure considerations. The discussion highlights the importance of using psychometric charts for a broader understanding of humidity parameters. Ultimately, while pressure affects some measurements, it is not a factor in calculating relative humidity.
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ASHRAE's psychrometric charts specify that they are valid at "sea level" / "atmospheric pressure: 101.325 kPa" or for example at "1500 metres" / "84.556 kPa".

Some people say atmospheric pressure is irrelevant to relative humidity, but then why do ASHRAE'S psychrometric charts include those specifications?
psychrometric - @ sea level big.PNG


psychrometric - @ 1500 m.PNG
 
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It's not irrelevant but how relevant depends on the altitude and process.. Get/try a psychometric calc app for your phone and see what the differences are.
 
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Lnewqban said:
Thanks. The section on "How much moisture can the air "hold"?" was very interesting (to me). I should perhaps have stated that I know how to calculate relative humidity at sea level. I'm trying to factor atmospheric pressure into that calculation. Although it's possible I missed an indirect connection, I don't think the article addressed

How to factor atmospheric pressure into relative humidity calculations?​

 
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100% RH occurs when the partial pressure of water vapor (in the air) equals the vapor pressure of water at that temperature.

If you're talking strictly about RH, atmospheric pressure doesn't matter. There are lots of (other) ways to describe how much water is in the air (mass/mass, mass/volume, etc.) - you need to account for atmospheric pressure with those.
 
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Dullard said:
100% RH occurs when the partial pressure of water vapor (in the air) equals the vapor pressure of water at that temperature.

If you're talking strictly about RH, atmospheric pressure doesn't matter. There are lots of (other) ways to describe how much water is in the air (mass/mass, mass/volume, etc.) - you need to account for atmospheric pressure with those.
Good point -- the OP asks about RH specifically (pressure doesn't matter) and the psych chart in general (pressure does matter to some of the other parameters).
 
Thanks - pressure does matter to some of the other parameters on a psychrometric chart, but not to relative humidity. Cool - that sufficiently answers my question. Thanks.
 
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