rudderrudderrat:
Difference between 0% / 100% relative humidity is about 1% in air density.
True. H2O vapor is lighter than the average air molecule which it displaces. (2xH=2; O=16; total = 18.
2xO=32; 2xN=28; the mixture averages about 29)
That will be reflected in the QNH.
Nope. QNH reflects pressure, not density (which is a function of pressure & temperature as well as H2O content.)