Below is a great explanation of the effect of water vapor on air density, and why water vapor makes air less dense not more dense. This was not intuitive to me but it certainly makes sense when you read it.
Temperature and pressure make the big difference. What you may not realize is that the pressure they give you on TV, or to pilots for that matter, at any station above sea level is not the actual pressure but an "adjusted" pressure for that altitude. So at Cheyenne the pressure right now is being reported at 30.24". That isn't really the pressure. Pressure decreases with altitude which is called the pressure lapse rate PLR. The PLR is not linear but 1" per 1,000' at lower altitudes works for our purposes. Therefore since CYS is just over 6,000' the pressure in CYS is actually more like 24.24" than the reported 30.24". They are adjusting the number for altitude so 1. we have a common reference and 2. So pilots can set their altimeters.
It is therefore the decreasing pressure with altitude that makes the higher elevation airports less dense and challenging for pilots. Add a hot day and unfortunately you can have serious problems.
The density altitude in Cheyenne the last several days has been 9,000' give or take. Really makes inverted autos challenging! http://wahiduddin.net/calc/density_altitude.htmMoist Air is Less Dense...
As you may have noticed, moist air is less dense than dry air. It may seem reasonable to try to argue against that simple fact based on the observation that water is denser than dry air... which is certainly true, but irrelevant.
Solids, liquids and gasses each have their own unique laws, so it is not possible to equate the behavior of liquid water with the behavior of water vapor.
The ideal gas law says that a certain volume of air at a certain pressure has a certain number of molecules. That's just the way this world works, and that simple fact is expressed as the ideal gas law, which was shown above in equation 1.
Note that this is the gas law... not a liquid law, nor a solid law, but a gas law. Hence comparisons to a liquid are of little help in understanding what is going on in the air, and may simply result in more confusion.
According to the ideal gas law, a cubic meter of air around you, wherever you are right now, has a certain number of molecules in it, and each of those molecules has a certain weight.
Most of the air is made up of nitrogen molecules N2 with a somewhat lesser amount of oxygen O2 molecules, and then other molecules such as water vapor.
Since density is weight divided by volume, we need to consider the weight of each of the molecules in the air. Nitrogen has an atomic weight of 14, so an N2 molecule has a weight of 28. For oxygen, the atomic weight is 16, so an O2 molecule has a weight of 32.
Now along comes a water molecule, H2O. Hydrogen has an atomic weight of 1. So the molecule H20 has a weight of 18. Notice that a water molecule is lighter weight than either a nitrogen molecule or an oxygen molecule.
Therefore, when a given volume of air, which contains only a certain number of molecules, has some water molecules in it (which are very light weight), it will weight less than the same volume of air without any water molecules.