Dear Tom,

What is heavier, dry air or wet air?

—Rolle Nacston, Riverside

Dear Rolle,

Dry air is more dense and weighs more than an equal volume of humid air at the same air pressure and temperature. This might seem contrary to common sense, but absolutely dry air consists of 78.09 percent nitrogen (that exists in the atmosphere as N2), 20.95 percent oxygen, 0.93 percent argon and several trace gases that do not significantly affect the density of air. This gives the molecular weight of an “average” air molecule (that is, the proportionally averaged weight of the gases that constitute air) as 28.95. However, a water molecule, H2O, has a weight of 18.02, less than the weight of an “air molecule.” When water vapor is added to the mix of atmospheric gases, the mix becomes lighter.