Mixtures of air and water vapor are the most common systems encountered in psychrometry.

Psychrometry measures the rates of condensation from and evaporation into the air moisture at various atmospheric pressures and temperatures.

Psychrometrics or 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 system of interest is the mixture of water vapor and air, because of its application in heating, ventilating, and air-conditioning and meteorology.