also called  Standard Atmosphere,  unit of pressure, nearly equal to the mean atmospheric pressure naturally existing at sea level on the surface of the Earth or to the pressure exerted by a vertical column of mercury (as in a barometer) 760 mm (29.9213 inches) high, used in meteorology. It is defined as 101,325 pascals, or newtons of force per square metre (approximately 14.7 pounds per square inch). See also millibar.