thumb|Comparison of a graph of International Standard Atmosphere temperature and pressure and approximate altitudes of various objects and successful stratospheric jumps

The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a common reference for temperature and pressure and 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 mathematical model divides the atmosphere into layers with an assumed linear distribution of absolute temperature T against geopotential altitude h. The other two values (pressure P and density ρ) are computed by simultaneously solving the equations resulting from:

  • the vertical pressure gradient resulting from hydrostatic balance, which relates the rate of change of pressure with geopotential altitude:

:: <math>\frac{dP}{dh} = - \rho g </math>, and

  • the ideal gas law in molar form, which relates pressure, density, and temperature:

:: <math>\ P = \rho R_{\rm specific}T </math>

at each geopotential altitude, where g is the standard acceleration of gravity, and R<sub>specific</sub> is the specific gas constant for dry air (287.0528J⋅kg<sup>−1</sup>⋅K<sup>−1</sup>). The solution is given by the barometric formula.

Air density must be calculated in order to solve for the pressure, and is used in calculating dynamic pressure for moving vehicles. Dynamic viscosity is an empirical function of temperature, and kinematic viscosity is calculated by dividing dynamic viscosity by the density.

Thus the standard consists of a tabulation of values at various altitudes, plus some formulas by which those values were derived. To accommodate the lowest points on Earth, the model starts at a base geopotential altitude of below sea level, with standard temperature set at 19&nbsp;°C. With a temperature lapse rate of −6.5&nbsp;°C (-11.7&nbsp;°F) per km (roughly −2&nbsp;°C (-3.6&nbsp;°F) per 1,000&nbsp;ft), the table interpolates to the standard mean sea level values of temperature, (1 atm) pressure, and a density of . The tropospheric tabulation continues to , where the temperature has fallen to , the pressure to , and the density to . Between 11&nbsp;km and 20&nbsp;km, the temperature remains constant.

{| class="wikitable"

|+ Layers in the ISA Standard Atmosphere 1976

! Layer

! Level<br>name

! Base<br>geopotential<br>altitude above MSL

ICAO Standard Atmosphere

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 kilometers (262,500 feet).

The ICAO Standard Atmosphere, like the ISA, does not contain water vapor.

Some of the values defined by ICAO are:

{| class="wikitable" style="margin: 1em auto 1em auto"

|+ ICAO Standard Atmosphere

! Height km & ft || Temperature °C || Pressure hPa || Lapse rate °C/1000&nbsp;ft

!Lapse rate C/1000&nbsp;m

|-

| 0&nbsp;km MSL || 15.0 || 1013.25 || +1.98 (tropospheric)

| +6.5 (tropospheric)

|-

| 11&nbsp;km 36 000&nbsp;ft || &minus;56.5 || 226.00 || 0.00 (stratospheric)

|0.00 (stratospheric)

|-

| 20&nbsp;km 65 000&nbsp;ft || &minus;56.5 || 54.70 || -0.3 (stratospheric)

| -0.1 (stratospheric)

|-

| 32&nbsp;km 105 000&nbsp;ft || &minus;44.5 || 8.68 ||

|

|}

Aviation standards and flying rules are based on the International Standard Atmosphere. Airspeed indicators are calibrated on the assumption that they are operating at sea level in the International Standard Atmosphere where the air density is 1.225&nbsp;kg/m<sup>3</sup>.

Physical properties of the ICAO Standard Atmosphere are:

{| class="wikitable"

|+Physical properties of the ICAO Standard Atmosphere

!Parameter

!Value

|-

|Density

|1.225&nbsp;kg m<sup>−3</sup>

|-

|Kinematic viscosity

|1.4607 × 10<sup>−5</sup> m<sup>2</sup> s<sup>−1</sup>

|-

|Dynamic viscosity

|1.7894 × 10<sup>−5</sup> kg m<sup>−1</sup>&nbsp;s<sup>−1</sup>

|-

|Molar volume

|2.3645 × 10<sup>−2</sup> m<sup>3</sup> mol<sup>−1</sup>

|-

|Molecular weight

|28.966

|-

|Thermal conductivity

|2.5339 × 10<sup>−2</sup> W m<sup>−1</sup> K<sup>−1</sup>

|-

|Mean free path

|6.6317 × 10<sup>−8</sup> m

|-

|Collision frequency

|6.9204 × 10<sup>9</sup> s<sup>−1</sup>

|-

|Particle speed

|4.5894 × 10<sup>2</sup> m s<sup>−1</sup>

|-

|Number density

|2.5475 × 10<sup>25</sup> m<sup>−3</sup>

|}

Other standard atmospheres

The U.S. Standard Atmosphere is a set of 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, International Standard Atmosphere and WMO (World Meteorological Organization) standard atmospheres are the same as the ISO International Standard Atmosphere for altitudes up to 32&nbsp;km.

NRLMSISE-00 is a newer model of the Earth's atmosphere from ground to space, developed by the US Naval Research Laboratory taking actual satellite drag data into account. A primary use of this model is to aid predictions of satellite orbital decay due to atmospheric drag. The COSPAR International Reference Atmosphere (CIRA) 2012 and the ISO 14222 Earth Atmosphere Density standard both recommend NRLMSISE-00 for composition uses.

JB2008 is a newer model of the Earth's atmosphere from 120&nbsp;km to 2000&nbsp;km, developed by the US Air Force Space Command and Space Environment Technologies taking into account realistic solar irradiances and time evolution of geomagnetic storms. It is most useful for calculating satellite orbital decay due to atmospheric drag. Both CIRA 2012 and ISO 14222 recommend JB2008 for mass density in drag uses.

See also

  • Acronyms and abbreviations in avionics
  • Density of air
  • Jet standard atmosphere

References

  • NASA JPL Reference Notes
  • ICAO, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488-CD, Third Edition, 1993, .
  • Online 1976 Standard Atmosphere calculator with table en graph generator. Digital Dutch
  • Multilingual windows calculator which calculates the atmospheric (standard and not standard!) characteristics according to the "1976 standard atmosphere" and convert between various airspeeds (true / equivalent / calibrated) according to the appropriate atmospheric conditions
  • A Free Android version for complete International Standard Atmosphere model
  • NewByte standard atmosphere calculator and speed converter
  • ICAO atmosphere calculator
  • ICAO Standards
  • Complete ISA calculator (1976 model)
  • JB2008 source code and references
  • ICAO standard atmosphere 1993 calculator