International Standard Atmosphere

The standard atmosphere standard atmosphere standard atmosphere or is a term used in aviation and called idealized properties of the Earth's atmosphere.

The International Standard Atmosphere (English International Standard Atmosphere ISA) has been defined by the International Civil Aviation Organization (ICAO). It represents an atmosphere in which the sizes of air pressure, air temperature, humidity, and temperature decrease are each 100 m altitude level values ​​that are approximately equal to the ruling on the earth averages. This corresponds to the international standard atmosphere about the mid-latitude of 40 ° north latitude prevailing pressure and temperature conditions (15 ° C and 1013.25 hPa).

The intention is to create a globally consistent reference and not the exact description of the current, local atmosphere. The International Standard Atmosphere corresponding to 32 km altitude the U.S. Standard Atmosphere 1976. Previously, a standard was used, which was known as the International Standard Atmosphere. In Germany there was also the DIN 5450 standard atmosphere, 1975, the standard atmosphere in DIN ISO 2533 has been set.

  • 2.1 U.S. Standard Atmosphere 1976
  • 2.2 Jacchia Reference Atmosphere
  • 2.3 NRLMSISE -00 model

Application

Aviation

The defined properties are also necessary to correct for an engine running on a test stand, the power values ​​to a neutral standard and be able to make a statement as to whether this engine produce enough thrust to accelerate an aircraft on the runway sufficient to.

The use of a standard atmosphere is necessary to calculate performance of aircraft, engines and missiles. They also used to calibrate pressure gauges such as altimeter and speedometer.

For this the following conditions apply to medium sea level:

In addition, the division of the atmosphere is described. The following applies:

  • The temperature gradient from sea level to the tropopause is K/1.000 -6.5 m ( -3.564 ° F/1.000 ft).
  • The tropopause is located at an altitude of 11,000 m ( 36,089 ft).
  • ( 216.65 K, -69.7 ° F) in the stratosphere a constant temperature of -56.5 ° C is given -.

The temperature at a level below the tropopause can be calculated with the following formula:

Metric system:

Imperial system:

The unit of height (meters or feet) shortens with the unit length of the temperature gradient or results in an additional conversion factor.

Terrestrial refraction

Similar Models

U.S. Standard Atmosphere 1976

  • Air pressure p0: 1013.25 hPa

The temperature variation with altitude is defined in the following table, which is linearly interpolated between the levels explicitly defined. The top layer is also the upper limit of this model.

Moreover, inter alia, the following values ​​are defined as constants:

  • Gravitational acceleration g0: 9.80665 m / s ² ( roughly equivalent to the real value at 45 ° latitude at sea level)
  • Earth's radius REarth: 6356766 m ( less than the true mean radius of the Earth to account for the conversion of geopotential heights in the geometric opposite a stationary earth somewhat steeper due to the centrifugal force of the rotating decrease the acceleration of gravity )
  • Molar mass M of air: 28.9644 g / mol ( the standard atmosphere contains no water vapor)
  • Universal gas constant R *: 8.31432 J / (mol · K)

The air density at sea level is calculated therefrom to 1.225 kg / m³.

It is believed that the air meets certain regularities in an ideal manner, in particular the general gas equation. This allows the height-dependent atmospheric pressure calculated. For the bottom layer, one obtains the international barometric formula.

The standard atmosphere is working with geopotential heights after the acceleration due to gravity is assumed to be independent of height. At low altitudes, these are consistent with the usual geometric heights fairly good agreement, but for higher accuracy at higher altitudes, you have to place the geometric height z an imaginary homogeneous gravitational field with corresponding geopotential height h of use. The relationship between the two is defined by the equation

Given.

Jacchia Reference Atmosphere

The Jacchia Reference Atmosphere in particular used in the aerospace describes an atmospheric model that is defined for heights 90-2500 km atmosphere, including temperature, density, pressure, and other values ​​. In contrast to the International Standard Atmosphere also different values ​​in dependence on season and latitude, as well as solar and geomagnetic effects are taken into account in addition, however. In addition, another atmospheric model must be consulted for lower heights.

The Jacchia Reference Atmosphere model was first published in 1970 and 1971 and 1977 updated. It is based on air resistance measurement data from space flights and is mainly used for space travel.

NRLMSISE -00 model

The newer NRLMSISE -00 model of 2000 provides reference values ​​for heights from the surface to outer space.

Density of air Ideal gas law
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