Is humid air lighter than dry air?
— Ernie Blenkle, West Chicago
While it does seem counterintuitive that a given volume of humid air is less dense and therefore weighs less than a similar volume of dry air at the same temperature and air pressure, it is true. The reason that moist air is lighter is that the molecular weight of a water molecule (18) is less than the molecular weight of a nitrogen molecule (28) and also of an oxygen molecule (32), the two gases that make up 99 percent of dry air. When moisture is added to dry air, the lighter water molecules displace heavier nitrogen and oxygen molecules and the air becomes less dense. That’s why a baseball will fly farther in humid air than in dry air — it encounters less resistance as it pushes the lighter moist air aside during its flight.
Moist air is lighter because the molecular weight of a water molecule is less than the molecular weight of a nitrogen molecule or of an oxygen molecule.