Does Humid Air Rise (neglecting the effects of weather turbulence-for example in a closed environment like a sealed room) An internet search will find many people claiming that Humid Air (water vapor) rises. However, I am skeptical because if lighter molecules rose then, it follows that the air would be stratified by molecular weight.
Facts:
Water vapor has molecular weight = 18
Nitrogen has molecular weight = 28
Oxygen has molecular weight = 32
So, Water Vapor has the least mass.
However, If lighter gases rose
then (using the above data)
Nitrogen would rise
Oxygen would fall
and we would be breathing 99% Oxygen (neglecting the 1% of other gasses)
this is not true
So
I believe that gasses do not behave this way
as illustrated by the fact that we breathe 78% nitrogen and 21% oxygen
Therefore, it seems to me that (in a closed environment) gasses mix randomly, with perhaps a very small gravitational contribution.
 A: You have to look at Boltzmann factors:
$$ e^{-\frac{E}{kT}} = e^{-\frac{mgh}{kT}}$$
for $m=18u$ and $m=28u$. Roughly at 300K and uniform $g$. For 1 amu, the scale height is:
$$ h \approx \frac{kT}{mg} = \frac{4\times 10^{-21}J}{1.6\times10^{-26}J/m}=250,000m$$
Divide that by 18 or 28 your get a scale height on the order of 10-14km, which is the scale height of the atmosphere. Hence, you don't see any stratification locally.
A: you are right- at room temperature, the different constituents of air are well-mixed and tend to stay that way. However, gases like butane and carbon dioxide are dense enough relative to air that they will indeed settle toward the floor of a room. regarding humid air, it does indeed have less density than dry air at the same temperature. Airplane pilots know that humid air produces slightly less lift than dry air for that reason. 
A: There are no sources or sinks of oxygen or nitrogen to speak of, so there are no fluctuations in the ratios of these gasses to speak of. Water vapor, on the other hand, is constantly being added and removed--air can be dried by cooling it to condense out the water, and then reheated, for example. It is quite possible to have humid air interacting with dry air of the same temperature.  In that case, until the two air masses mix, the humid one will tend to rise above the dry one.
