Air that is 100% saturated with water is:

Air that is 100% saturated with water vapor is considered less dense than dry air. This reduction in density occurs because water vapor is lighter than the nitrogen and oxygen molecules that make up the majority of dry air. When water vapor enters the atmosphere and displaces some of these heavier air molecules, the overall density of the air decreases.

At saturation, the amount of water vapor in the air reaches its maximum capacity for a given temperature. This results in a state where the partial pressure of the water vapor is equal to the equilibrium vapor pressure over a flat surface of water. Consequently, the total density of the air mixture is reduced compared to dry air, which lacks this additional lighter component.

The context of humidity plays a role in understanding this phenomenon; humidity is a measure of the amount of water vapor in the air, and when air is fully saturated (at 100% humidity), it indicates that there is a maximum amount of water vapor present, further emphasizing how it influences density.