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International Standard Atmosphere

International Standard Atmosphere

The International Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It consists of tables of values at various altitudes, plus some formulas by which those values were derived. The International Organization for Standardization (ISO), publishes the ISA as an international standard, ISO 2533:1975. Other standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States Government, publish extensions or subsets of the same atmospheric model under their own standards-making authority.

Description

The ISA model divides the atmosphere into layers with linear temperature distributions. The other values are computed from basic physical constants and relationships. Thus the standard consists of a table of values at various altitudes, plus some formulas by which those values were derived. For example, at sea level the standard gives a pressure of 1013.25 hPa and a temperature of 15 °C, and an initial lapse rate of −6.5 °C/km. The tabulation continues to 11 km where the pressure has fallen to 22.632 kPa and the temperature to −56.5 °C. Between 11 km and 20 km the temperature remains constant.

Layers in the ISA
Standard Atmosphere 1976
Layer Level
Name
Base
Geopotential
Height
h (in km)
Base
Geometric
Height
z (in km)

Lapse
Rate
(in °C/km)

Base
Temperature
T (in °C)
Base
Atmospheric
Pressure
p (in Pa)
0 Troposphere 0.0 0.0 −6.5 align=center
15.0
101,325
1 Tropopause 11.000 11.019 align=center
0.0
−56.5 22,632
2 Stratosphere 20.000 20.063 align=center
1.0
−56.5 5,474.9
3 Stratosphere 32.000 32.162 align=center
2.8
−44.5 868.02
4 Stratopause 47.000 47.350 align=center
0.0
−2.5 110.91
5 Mesosphere 51.000 51.413 −2.8 −2.5 66.939
6 Mesosphere 71.000 71.802 −2.0 −58.5 3.9564
7 Mesopause 84.852 86.000 −86.2 0.3734

In the above table, geopotential height is calculated from a mathematical model in which the acceleration due to gravity is assumed constant. Geometric height results from the (more accurate) assumption that gravity obeys an inverse square law.

Development of the ISA

The ISA model is based on average conditions at mid latitudes, as determined by ISO's TC 20/SC 6 technical committee. It has been revised from time to time since the middle of the 20th century.

Other standard atmospheres

The International Civil Aviation Organization (ICAO) published their "ICAO Standard Atmosphere" as Doc 7488-CD in 1993. It has the same model as the ISA, but extends the altitude coverage to 80 kilometres (262,500 feet).

The U.S. Standard Atmosphere is models that define values for atmospheric temperature, density, pressure and other properties over a wide range of altitudes. The first model, based on an existing international standard, was published in 1958 by the U.S. Committee on Extension to the Standard Atmosphere, and was updated in 1962, 1966, and 1976. The U.S. Standard Atmosphere, ICAO Standard Atmosphere and WMO (World Meteorological Organization) standard atmospheres are the same as the ISO International Standard Atmosphere for altitudes up to 32 km.

NRLMSISE-00 is an empirical, global model of the Earth's atmosphere from ground to space. It models the temperatures and densities of the atmosphere's components. A primary use of this model is to aid predictions of satellite orbital decay due to atmospheric drag.

The standard conditions for temperature and pressure are a model of gas temperature and pressure used in chemistry.

References

  • Davies, Mark (2003). The Standard Handbook for Aeronautical and Astronautical Engineers. New York: McGraw-Hill.

See also

External links

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