Humidity is the concentration of water vapor present in the air. Water vapor,
the gaseous state of water, is generally invisible to the human eye. Humidity
depends on the temperature and pressure of the system of interest. The
same amount of water vapor results in higher humidity in cool air than warm air.
A related parameter is the dew point. The amount of water vapor needed to
achieve saturation increases as the temperature increases. As the temperature of
a parcel of air decreases it will eventually reach the saturation point without
adding or losing water mass.