Let's say our cubic whatever contains 100 molecules of dry air.

78 molecules Nitrogen                 78 x 14 atomic wt x 2 atoms   = 2184
21 molecules Oxygen                   21 x 16 atomic wt x 2 atoms   =   672
                      
                      Total Atomic Weight  Of Dry Air = 2856

If we add 4% water vapor we can proportionately lose 3 molecules of nitrogen and 1 molecule of oxygen.

75 molecules Nitrogen                 75 x 14 atomic wt x 2 atoms  = 2100
20 molecules Oxygen                   20 x 16 atomic wt x 2 atoms  =   640
4  molecules Hydrogen                   4 x  1  atomic wt x 2 atoms   =      8 
4  molecules Oxygen                       4 x 16  atomic wt x 1 atom    =    64
           
    

Total Atomic Weight With 4% Water Vapor   =      2812
Total Atomic Weight  Of Dry Air                     =      2856

These calculations rely on the temperature and the pressure remaining the same. This, however, is unlikely to occur. There are days which are clear, cloudless, and still. These days can have humidity levels from low to high. It would be expected that if the air was absorbing water vapor and displacing some molecules, the effect would be seen as wind , as the extra molecules try to dissipate.

It appears in fact that the molecules stay where they are, and the air pressure rises. This is what a high pressure area generally is. Warm, clear, still ,fine weather.

Although some moisture rises on a thermal expansion basis, it seems the majority rises because of the atomic weight difference.

All this is fairly simple science. However, what keeps the clouds up after the water vapor condenses into water droplets may not be so simple.

I believe that as water vapor condenses in the air it changes state in two ways. The first is the normal way in which latent heat is released. The other is that the electrical properties or charge also change, and the water becomes weightless, or antigravitational.