There is water on your car’s windshield in the morning and it did not rain the night before. Where did the water come from? Psychrometry or hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common example is the mixture of water vapor and air, more commonly known as dew point.
Dew point is the temperature at which water vapor begins to condense out of the air. Alternatively, it can be defined as the temperature at which air becomes completely saturated. In dehumidification by cooling and condensation, it is the temperature to which the moist air must be cooled to allow water removal. The lower the absolute amount of moisture in air, the lower the dew point of that air sample will be.