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Concept
International Standard Atmosphere
Summary
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.
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:
and
the ideal gas law in molar form, which relates pressure , density, and temperature:
at each geopotential altitude, where g is the standard acceleration of gravity, and Rspecific is the specific gas constant for dry air (287.0528J⋅kg−1⋅K−1). 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 °C. With a temperature lapse rate of −6.
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