Phases of Water
Water molecules can exist in three distinct phases: Solid (Ice), Liquid, & Gas (Vapour). He we are interested in the later, water in the gas phase held in the air.
The amount of moisture in the air can be found from a combination of the air temperature and its relative humidity.
The relative humidity of air is the measure of the degree of saturation of the air. A relative humidity of 100% means the air is completely saturated, any additional moisture will form as condensation. The same applies if the air cools. A relative humidity of 0% means the air is completely dry.