How to factor atmospheric pressure into relative humidity calculations

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Discussion Overview

The discussion centers on how atmospheric pressure factors into calculations of relative humidity (RH). Participants explore the relevance of atmospheric pressure in the context of psychrometric calculations and the specific conditions under which it may or may not be significant.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants suggest that the relevance of atmospheric pressure in relative humidity calculations depends on altitude and specific processes.
  • One participant notes that while atmospheric pressure does not affect relative humidity directly, it is important for other measures of moisture in the air, such as mass/mass or mass/volume ratios.
  • A participant emphasizes that 100% relative humidity occurs when the partial pressure of water vapor equals the vapor pressure of water at a given temperature, indicating that atmospheric pressure is not a factor in this specific case.
  • Another participant acknowledges that while atmospheric pressure is not relevant for relative humidity, it does matter for other parameters on a psychrometric chart.

Areas of Agreement / Disagreement

Participants generally agree that atmospheric pressure does not affect relative humidity calculations directly, but there is some debate about its relevance to other moisture-related measurements.

Contextual Notes

The discussion does not resolve the broader implications of atmospheric pressure on related calculations, nor does it clarify how to incorporate pressure into relative humidity calculations specifically.

Jonathon
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TL;DR
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:
Welcome, Jonathon!

Please see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html

:)
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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