Dry air is heavier than humid air. Newton discovered this in 1770 and Avogadro put forward the exact reasons for it. Equal volumes of different gases at the same pressure and temp will contain the same number of molecules. Hence water vapour molecules must displace air molecules in humid air. Water vapour has a molecular weight of 18 while air is approx 29 so the weight for a given volume ( ie density) is less. Don't confuse humid air, which is water vapour and invisible, with cloud or fog which is visible water droplets ( ie liquid water) suspended in air and settles in valleys only because it is colder than the surrounding air.