Psychrometrics (or psychrometry, from Greek ψυχρόν (psuchron) 'cold' and μέτρον (metron) 'means of measurement'; also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.

With the inventions of the hygrometer and thermometer, the theories of combining the two began to emerge during the sixteenth and seventeenth centuries. In 1818, a German inventor, Ernst Ferdinand August (1795-1870), patented the term “psychrometer”, from the Greek language meaning “cold measure”. The psychrometer is a hygrometric instrument based on the principle that dry air enhances evaporation, unlike wet air, which slows it.

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, ventilation, and air-conditioning and meteorology. In human terms, our thermal comfort is in large part a consequence of not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor.

Many substances are hygroscopic, meaning they attract water, usually in proportion to the relative humidity or above a critical relative humidity. Such substances include cotton, paper, cellulose, other wood products, sugar, calcium oxide (burned lime) and many chemicals and fertilizers. Industries that use these materials are concerned with relative humidity control in production and storage of such materials. Relative humidity is often controlled in manufacturing areas where flammable materials are handled, to avoid fires caused by the static electricity discharges that can occur in very dry air.

In industrial drying applications, such as drying paper, manufacturers usually try to achieve an optimum between low relative humidity, which increases the drying rate, and energy usage, which decreases as exhaust relative humidity increases. In many industrial applications it is important to avoid condensation that would ruin product or cause corrosion.

Molds and fungi can be controlled by keeping relative humidity low. Wood destroying fungi generally do not grow at relative humidities below 75%.

The dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature. In meteorology and psychrometrics the word temperature by itself without a prefix usually means dry-bulb temperature. Technically, the temperature registered by the dry-bulb thermometer of a psychrometer. The name implies that the sensing bulb or element is in fact dry. WMO provides a 23-page chapter on the measurement of temperature.

The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.