Is humid air really heavier than dry air?

[Shackleford]

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?

 

[D H]

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.

 

[Shackleford]

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.

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).