atmosphere (atm) /at′məsfir/ [Gk. atmos, vapor, sphaira, sphere] , 1. the natural body of air that covers the surface of the earth. It is composed of approximately 21% oxygen, 78% nitrogen, and 1% argon and other gases, including small amounts of carbon dioxide, hydrogen, and ozone, as well as traces of helium, krypton, neon, and xenon and varying amounts of water vapor. 2. an envelope of gas, which may or may not duplicate the natural atmosphere in chemical components. 3. a unit of gas pressure that is usually defined as being equivalent to the average pressure of the earth’s atmosphere at sea level, or about 14.7 pounds per square inch or 760 mm Hg. −atmospheric, adj.
