From Glossary of Meteorology
Revision as of 17:58, 25 April 2012 by
- 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:
- —Temperature 288.15 K (15°C)
- —Pressure 101 325 Pa (1013.25 mb, 760 mm of Hg, or 29.92 in. of Hg)
- —Density 1225 g m-2 (1.225 g L-1)
- —Mean molar mass — 28.964 g mole-1.
The parametric assumptions and physical constants used in preparing the current standard atmosphere are as follows.
- The ice point at one standard atmosphere pressure is 273.16 K.
- The acceleration of gravity is 980.665 cm s-2.
- The temperature at zero pressure altitude is 15°C or 288.15 K.
- The density at zero pressure altitude is 0.0012250 gm cm-3
- The pressure altitude of the tropopause is 11 km.
- The temperature at the tropopause is -56.5°C.
The ARDC Model Atmosphere, 1959, extended the above standard approximately as follows:
- The lapse rate from 11 to 25 km is 0°C km-1.
- The lapse rate from 25 to 47 km is +3.0°C km-1; temperature at 47 km is +9.5°C.
- The lapse rate from 47 to 53 km is 0°C km-1.
- The lapse rate from 53 to 75 km is -3.9°C km-1; temperature at 75 km is -76.3°C.
- The lapse rate from 75 to 90 km is 0°C km-1.
- The lapse rate from 90 to 126 km is +3.5°C km-1; temperature at 126 km is +49.7°C (molecular-scale temperatures).
- The lapse rate from 126 to 175 km is +10.0°C km-1; temperature at 175 km is 539.7°C (molecular-scale temperatures).
- The lapse rate from 175 to 500 km is +5.8°C km-1; temperature at 500 km is 2424.7°C (molecular-scale temperatures).
The U.S. Extension to the ICAO Standard Atmosphere is essentially a recomputation of the above data from the surface to 300 km.
A standard unit of atmospheric pressure, the 45° atmosphere, defined as that pressure exerted by a 760-mm column of mercury at 45° latitude at sea level at temperature 0°C (acceleration of gravity = 980.616 cm s-2).
One 45° atmosphere = 760 mm Hg (45°) = 29.9213 in Hg (45°) = 1013.200 mb = 101.320 kPa.
With respect to radio propagation, that hypothetical atmosphere in which standard propagation exists, that is, one in which the index of refraction decreases with height at a rate of 12 N- units per 1000 ft.
COESA 1976. U.S. Standard Atmosphere. U.S. Gov. Print. Off., Wash., D.C.
Jursa, A. S., ed. 1985. Handbook of Geophysics and the Space Environment. Air Force Geophysics Laboratory.
Minzner, R. A., K. S. W. Champion, and H. L. Pond 1959. The ARDC Model Atmosphere, 1959. Air Force Surveys in Geophysics No. 115 (AFCRC-TR-59-267), Air Force Cambridge Research Center.