(1) A hypothetical vertical distribution of atmospheric temperature, pressure, and density which, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc. The air is assumed to obey the perfect gas law and the hydrostatic equation, which, taken together, relate temperature, pressure, and density variations in the vertical. It is further assumed that the air contains no water vapor, and that the acceleration of gravity does not change with height. This last assumption is tantamount to adopting a particular unit of geopotential height in place of a unit of geometric height for representing the measure of vertical displacement, for the two units are numerically equivalent in both the metric and English systems, as defined in connection with the standard atmosphere. The current standard atmosphere is that which was adopted on November 7, 1952, by the International Civil Aeronautical Organization (ICAO), supplanting the NACA Standard Atmosphere (or U.S. Standard Atmosphere) prepared in 1925. The parametric assumptions and physical constants used in preparing the ICAO Standard Atmosphere are as follows: Zero pressure altitude corresponds to that pressure which will support a column of mercury 760 mm high. This pressure is taken to be 1.013250 x 106 dynes/cm2, or 1013.250 mb (and is known as one standard atmosphere or one atmosphere). The gas constant for dry air is 2.8704 x 106 erg/gm/°K. The ice point at one standard atmosphere pressure is 273.16°K. The acceleration of gravity is 980.665 cm/sec2. The temperature at zero pressure altitude is 15°Cor288.16°K. The density at zero pressure altitude is .0012250 gm/cm3. The lapse rate of temperature in the troposphere is 6.5°C/km. The pressure altitude of the tropopause is 11 km. The temperature at the tropopause is -56.5°C. (2) A standard unit of atmospheric pressure, defined as that pressure exerted by a 760 mm column of mercury at standard gravity (980.665 cm/sec2) at temperature 0°C. one standard atmosphere = 760 mm Hg = 29.9213 in Hg = 1013.250 mb