[Phys-L] moist air density; was: The Atlantic's Weekly Planet

[John Denker <jsd@av8n.com>]

The claim that "warmer air has more room between its molecules for moisture" is 
just wrong. In particular, the idea that water molecules occupy spaces in a passive chunk 
of air is seriously wrong. It would suggest that moist air is denser, when in fact the 
opposite is true.

Here's how it actually works. Let's compare a parcel of warm dry air with a 
parcel of warm moist air at the same pressure. By Dalton's law of partial 
pressures, if the dry parcel has 1 bar of dry air then the moist parcel has 
something like 950 millibar of dry air plus 50 millibar of water vapor.

Therefore consider a volume that can 1 mole of dry air. The same volume can 
hold 950 millimoles of dry air plus 50 millimoles of water vapor. Since a water 
molecule has less mass than an air molecule, the moist parcel has less density 
i.e. less mass per unit volume.

Pilots have a mnemonic: The air is less dense if it is hot, high, and/or humid.

You cannot have a dry air mass in static equilibrium with a moist air mass. If 
they are in equilibrium at ground level, they won't be aloft. That's because 
the pressure falls more slowly as altitude increases on the less-dense side. 
The way this works in practice is that the pressure difference is balanced, via 
the Coriolis effect, by a tremendous windshear across the frontal boundary.

It is common to talk about cold fronts and warm fronts, but there need not be a 
temperature difference across the front. There might be just a humidity 
difference, in which case it is called a Marfa front.