Ok so let me see if I understand why humid air is less dense than dry air...

Dry air is mostly made up of oxygen and nitrogen. Water is oxygen and hydrogen. Hydrogen is much lighter than nitrogen. Furthermore, the oxygen in the atmosphere is O2 (nitrogen is N2 also, I believe), but there is only one atom of oxygen in a water molecule. Thus, dry air is significantly heavier than humid air as it contains more oxygen, and nitrogen.

It just seems crazy to think that water is lighter than air on a molecular level. But it's also makes sense when you look at water's structure. Two tiny hydrogen atoms and one oxygen atom. That's ten electrons per molecule. That's less than both N2 and O2, before we consider CO2 and the other trace gasses.