See standard atmosphere.

The envelope of air surrounding the earth and bound to it more or less permanently by virtue of the earth’s gravitational attraction. The system whose chemical properties, dynamic motions, and physical processes constitute the subject matter of meteorology. Also, a unit of pressure. See standard atmosphere.

Infrared radiation emitted by or being propagated through the atmosphere.

In a system of moist air, the ratio of the mass of water vapor to the total volume of the system. Usually expressed as grams per cubic meter (g/m³).

An instrument whose calibration can be determined by means of simple physical measurements on the instrument. Compare to secondary instrument.

Temperature based on an absolute scale. See thermodynamic temperature.

A temperature scale based on absolute zero. See Kelvin temperature scale, Rankine temperature scale.

A hypothetical temperature characterized by a complete absence of heat and defined as 0K, -273.15°C, or -459.67°F.

The process in which incident radiation is retained by a substance. A further process always results from absorption.

A type of hygrometer which measures the water vapor content of the atmosphere by means of the absorption of vapor by a hygroscopic chemical.