1. A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, 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 adopted in 1976 and is a slight modification of one adopted in 1952 by the International Civil Aeronautical Organization (ICAO), which, in turn, supplanted the NACA Standard Atmosphere (or U. S. Standard Atmosphere) prepared in 1925. It assumes sea level values as follows: