Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Humid or dry air heavier ?

  1. Jan 27, 2009 #1
    Humid or dry air "heavier"?

    It would seem that the addition of water vapor in the air would increase the mass of the volume of air thus making it "heavier." However, the professor in class today said humid air is typically heavier because humid air has water vapor as opposed to the more massive nitrogen and oxygen molecules. Does the water vapor somehow displace/replace the nitrogen and oxygen molecules?
     
  2. jcsd
  3. Jan 27, 2009 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Humid or dry air "heavier"?

    As air is pretty close to an ideal gas, the total number of molecules per unit volume is n=P/(RT). Water molecules have an atomic mass of about 18, a lot less than 28 for nitrogen molecules and 32 for oxygen molecules. Humid air will thus be less dense than dry air, assuming both are at the same pressure and temperature.
     
  4. Jan 27, 2009 #3
    Re: Humid or dry air "heavier"?

    Ok. That's what I was thinking. It's just an introductory meteorology class, so he didn't clarify enough for it to entirely make sense for me (physics major).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?