Does Humid Air Rise or Fall?

By Staff WriterLast Updated Apr 14, 2020 2:17:06 PM ET

Given the same temperature and pressure, humid air rises compared to dry air. As the humidity increases, so does the tendency of air to rise.


This seems counter-intuitive, because objects seem to get heavier when water is added; but in this case, it is water vapor. The molecular mass of water is 18 grams per mole. Air is mainly nitrogen and oxygen, with molecular masses of 28 grams per mole and 32 grams per mole respectively, since they are diatomic. At constant temperature and pressure, the water molecules replace the heavier oxygen and nitrogen molecules, resulting in air with a lower density that rises.