Moisture shall be lighter than air to some extend, because clouds are in the sky water must go up there.
I'd bet cold air - either moist or dry - will be always heavier due to condensation (vapor replaced by mist).
Such formulas are misleading. With twice as much wind you should get the same traction from a sail four times smaller, which is obviously not the case. You may be comfortably powered with 8mē in 12 Kts but you will never fly with 2mē in 24 Kts. There are certainly other factors at play (like sail+sailor efficiency/drag ratio). A few years back there was a very good article from Jim Drake about balance of forces, but it is no more online.
Last edited by Farlo; 3rd June 2013 at 09:34 AM.