Thus, certain corrections must apply to the instrumentation, as well as the airplane performance, if the actual operating conditions do not, fit the standard atmosphere. In order to account properly for the nonstandard atmosphere, certain related terms must be defined. Pressure Altitude Pressure altitude is the altitude in the standard atmosphere, corresponding to a particular pressure level. The airplane altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude - the altitude in the standard atmosphere corresponding to a particular value of air density.
Density altitude is pressure altitude corrected for nonstandard temperature.
Density altitude, then, is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. Since density varies directly with pressure, and inversely with temperature, a given pressure altitude may exist for a wide range of temperature by allowing the density to vary. However, a known density occurs for any one temperature and pressure altitude. The density of the air, of course, has a pronounced effect on airplane and engine performance. According to the airplane flight handbook, the ground run on takeoff may require a distance of 790 feet under standard temperature conditions.