Q: Why is air lighter when moisture is added to it? (Dee, Detroit, Michigan)
A: Given the same temperature and pressure, water vapor is less dense than
dry air, because the mass of a water molecule is less than the mass of either
nitrogen or oxygen molecules, which are the dominant components of air. Thus,
adding light water vapor to dry air lightens it like adding helium to air in a
balloon.
Further Reading:
Clouds in a glass of beer by Craig Bohren
Moisture in air by Tim Padfield, the physics of the museum environment
Cirrus clouds and climate by K.N. Liou, Monthly Weather Review., <114, 1167-1198, 1986;
(Answered Oct. 16, 2006; updated Nov. 2, 2007)