Is humid air “heavier” than dry air?
Broadcasters of baseball games in mid-summer commonly suggest that the air on a hot, muggy night is “heavy” and that, as a consequence, the ball will not travel as far when hit. But is humid (i.e. moist) air really heavier than dry air?
First of all, “moist air” is air with a high water vapor content. Water vapor, the invisible, gaseous form of water, occurs in highly variable amounts in the atmosphere. Water is composed of a hydrogen atom and two oxygen atoms (H2O) and has a molecular weight of 18 grams per mole. (One mole of a gas, at standard conditions, has a volume of 22.4 liters.)
“Dry” air contains no water vapor, and is mostly a mixture of molecular nitrogen (N2) and molecular oxygen (O2). The molecular weight of dry air is 28.97 grams per mole.
Since mole per mole, dry air is heavier than water vapor, any mixture of dry air with water vapor will weigh less than pure dry air. The more water vapor in the air, the smaller its molecular weight will be.
Therefore the notion that humid air is heavier than dry air on a summer night at the ballpark strikes out when compared to the physical reality. We may feel lethargic on a humid day, and that may weaken a slugger’s swing – but don’t blame the air, at least, not directly!