What effect does humidity have on the density of air?

Humidity decreases the density of air primarily because water vapor is less dense than the nitrogen and oxygen molecules it displaces in the atmosphere. When air becomes more humid, the proportion of water vapor increases, replacing some of the heavier molecules like nitrogen (N2) and oxygen (O2). Since the molecular weight of water vapor (approximately 18 g/mol) is lower than that of dry air (which has an average molecular weight of about 29 g/mol), the overall density of the air decreases.

This effect is significant for aviation because lower air density can influence aircraft performance, including lift, engine efficiency, and fuel consumption. As humidity increases, the air becomes "lighter," which is particularly relevant during takeoff and landing operations. Understanding this relationship is important for pilots and engineers when assessing conditions for flight.