International Standard Atmosphere
The International Standard Atmosphere begins at a base geopotential altitude of zero meters below sea level. At this starting point, the standard temperature is set to 19 degrees Celsius. A mathematical model divides the atmosphere into layers with an assumed linear distribution of absolute temperature against geopotential altitude. The vertical pressure gradient resulting from hydrostatic balance relates the rate of change of pressure with height. This equation uses g as the standard acceleration of gravity and Rspecific as the specific gas constant for dry air. That constant equals 287.0528 joules per kilogram kelvin. The ideal gas law in molar form connects pressure, density, and temperature at each geopotential altitude. Air density must be calculated to solve for pressure before dynamic pressure can be determined for moving vehicles. Dynamic viscosity serves as an empirical function of temperature while kinematic viscosity comes from dividing dynamic viscosity by density. Thus the standard consists of a tabulation of values at various altitudes plus formulas derived from these principles.
The tropospheric tabulation continues up to 11 kilometers where the temperature has fallen to minus 56.5 degrees Celsius. Pressure drops to 22632 pascals and density reaches 0.3639 kilograms per cubic meter between that boundary and 20 kilometers. Temperature remains constant within this stratospheric section until reaching 20 kilometers. A second stratospheric layer extends from 20 kilometers to 32 kilometers with a lapse rate of negative 1.0 degree Celsius per kilometer. Base atmospheric pressure at 32 kilometers measures 868.0 pascals while density falls to 0.0132 kilograms per cubic meter. The stratopause sits at 47 kilometers where temperature rises slightly to minus 2.5 degrees Celsius. Density at this point equals 0.0014 kilograms per cubic meter. The mesosphere begins at 51 kilometers and extends upward to 71 kilometers. Temperature decreases further to minus 58.5 degrees Celsius as altitude increases through this region. The final layer, the mesopause, reaches an altitude of 84,852 meters. At this peak height, temperature plummets to minus 86.204 degrees Celsius.
The International Standard Atmosphere models a hypothetical standard day to allow reproducible engineering reference for calculation and testing. It does not provide a rigorous meteorological model of actual atmospheric conditions like changes in barometric pressure due to wind. Neither does it account for humidity effects since air is assumed dry and clean with constant composition. Humidity effects are accounted for by adding water vapor to the thermodynamic state after obtaining pressure and density from the standard model. Non-standard days are modeled by adding a specified temperature delta to the standard temperature at altitude. Pressure remains taken as the standard day value while density and viscosity recalculate using the ideal gas equation. Hot day, cold day, tropical, and polar temperature profiles have been defined for use as performance references. United States Department of Defense MIL-STD-210C established one such profile before its successor MIL-HDBK-310 took over. Airspeed indicators calibrate on the assumption they operate at sea level where air density equals 1.225 kilograms per cubic meter. Aviation standards and flying rules rely entirely upon these static reference values for engine testing and vehicle performance calculations.
The International Organization for Standardization publishes the ISA as an international standard designated ISO 2533:1975. Other standards organizations publish extensions or subsets under their own authority including the International Civil Aviation Organization. The ICAO published their document titled Doc 7488-CD in 1993 extending altitude coverage to 80 kilometers. This extension reaches 262,500 feet above mean sea level. The U.S. Committee on Extension to the Standard Atmosphere first published a model based on existing international standards in 1958. That initial version received updates in 1962, 1966, and finally 1976. The U.S. Standard Atmosphere matches the ISO International Standard Atmosphere for altitudes up to 32 kilometers. The World Meteorological Organization maintains similar standard atmospheres that align with the ISO definition within this range. These bodies collectively ensure consistent atmospheric data across different national jurisdictions and industries requiring precise altitude references.
NRLMSISE-00 represents a newer model of Earth's atmosphere from ground to space developed by the US Naval Research Laboratory. It takes actual satellite drag data into account to aid predictions of satellite orbital decay due to atmospheric drag. The COSPAR International Reference Atmosphere 2012 recommends NRLMSISE-00 specifically for composition uses alongside ISO 14222 Earth Atmosphere Density standard. JB2008 serves as another newer model covering distances from 120 kilometers to 2000 kilometers. This system was developed by the US Air Force Space Command and Space Environment Technologies. It accounts for realistic solar irradiances and time evolution of geomagnetic storms during operation. JB2008 proves most useful for calculating satellite orbital decay caused by atmospheric drag forces. Both CIRA 2012 and ISO 14222 recommend JB2008 for mass density in drag applications. These modern computational models contrast sharply with the static ISA reference by incorporating dynamic environmental factors like solar activity.
Common questions
What is the standard temperature at sea level in the International Standard Atmosphere?
The standard temperature at a base geopotential altitude of zero meters below sea level is 19 degrees Celsius. This value serves as the starting point for all calculations within the model.
How does the International Standard Atmosphere define the troposphere and stratosphere layers?
The tropospheric tabulation continues up to 11 kilometers where the temperature falls to minus 56.5 degrees Celsius. The stratospheric section extends from that boundary to 20 kilometers with constant temperature before a second layer begins.
When did the U.S. Committee on Extension to the Standard Atmosphere first publish their model?
The U.S. Committee on Extension to the Standard Atmosphere first published a model based on existing international standards in 1958. That initial version received updates in 1962, 1966, and finally 1976.
Which organization publishes the ISO 2533:1975 designation for the International Standard Atmosphere?
The International Organization for Standardization publishes the ISA as an international standard designated ISO 2533:1975. Other organizations like the International Civil Aviation Organization also publish extensions under their own authority.
What specific values apply at the mesopause altitude of 84,852 meters?
At this peak height, temperature plummets to minus 86.204 degrees Celsius while density reaches 0.0014 kilograms per cubic meter at the stratopause below it. The mesosphere begins at 51 kilometers and extends upward to 71 kilometers before reaching this final layer.