An atmosphere (from Ancient Greek ἀτμός (atmós) 'vapour, steam', and σφαῖρα (sphaîra) 'sphere') is a layer of gas or layers of gases that envelope a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which ...

History. The standard atmosphere was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (g n = 9.806 65 m/s 2). It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Celsius temperature scale, which defined 100 °C as the boiling point of water at this pressure.