That's the Bad Meteorologist's Number One Error. See http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

In sum, it's got nothing to do with the air. Air does not have a "capacity" for holding water vapour. If you have an evacuated flask and put some liquid water in it, a characteristic amount of water vapour will evaporate above it at a given temperature. Add some air, and it won't make any difference to the amount of water vapour above the liquid in the flask. Cool it down, and some of the water vapour it will condense. The presence or absence of air will make no difference to the quantity.