Moist air, (higher humidity), is less dense than than dry air. Although liquid water is heavier than air, water vapor is lighter than air. At a given temperature and pressure a given volume of any gas can contain only a certain number of molecules. If the air is moist some of the oxygen, nitrogen and carbon dioxide molecules are displaced by the lighter water molecules. Since the prop produces thrust and the wing produces lift by moving these molecules the result is less thrust and lift. The engine also has less power on a humid day because some of the oxygen molecules needed by the fuel are replaced by the water molecules. In flying full size aircraft pilots are cautioned to be aware of the 4-H; hot, high, humid and heavy.