TheInternational Standard Atmosphere (ISA) is astatic atmospheric model of how thepressure,temperature,density, andviscosity of theEarth's atmosphere change over a wide range ofaltitudes orelevations. 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. TheInternational Organization for Standardization (ISO) publishes the ISA as aninternational standard, ISO 2533:1975.[1] Otherstandards organizations, such as theInternational Civil Aviation Organization (ICAO) and theUnited States Government, publish extensions or subsets of the same atmospheric model under their own standards-making authority.
The ISAmathematical model divides the atmosphere into layers with an assumed linear distribution ofabsolute temperatureT againstgeopotential altitudeh.[2] The other two values (pressureP and densityρ) are computed bysimultaneously solving the equations resulting from:
at each geopotential altitude, whereg is the standardacceleration of gravity, andRspecific is thespecific gas constant for dry air (287.0528J⋅kg−1⋅K−1). The solution is given by thebarometric formula.
Air density must be calculated in order to solve for the pressure, and is used in calculatingdynamic pressure for moving vehicles.Dynamic viscosity is an empirical function of temperature, andkinematic 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 accommodatethe lowest points on Earth, the model starts at a base geopotential altitude of 610 meters (2,000 ft)below sea level, with standard temperature set at 19 °C. With a temperaturelapse rate of −6.5 °C (-11.7 °F) per km (roughly −2 °C (-3.6 °F) per 1,000 ft), the table interpolates to thestandard mean sea level values of 15 °C (59 °F) temperature, 101,325 pascals (14.6959 psi) (1atm) pressure, and a density of 1.2250 kilograms per cubic meter (0.07647 lb/cu ft). Thetropospheric tabulation continues to 11,000 meters (36,089 ft), where the temperature has fallen to −56.5 °C (−69.7 °F), the pressure to 22,632 pascals (3.2825 psi), and the density to 0.3639 kilograms per cubic meter (0.02272 lb/cu ft). Between 11 km and 20 km, the temperature remains constant.[3][4]
| Layer | Level name | Base geopotential altitude aboveMSL[5] h (m) | Base geometric altitude aboveMSL[5] z (m) | Lapse rate ( °C/km)[a] | Base temperature T (°C[K]) | Base atmospheric pressure p (Pa) | Base atmospheric density ρ (kg/m3) |
|---|---|---|---|---|---|---|---|
| 0 | Troposphere | 0 | 0 | +6.5 | +15.0 (288.15) | 101,325 | 1.225 |
| 1 | Tropopause | 11,000 | 11,019 | 0.0 | −56.5 (216.65) | 22632 | 0.3639 |
| 2 | Stratosphere | 20,000 | 20,063 | -1.0 | −56.5 (216.65) | 5474.9 | 0.0880 |
| 3 | Stratosphere | 32,000 | 32,162 | -2.8 | −44.5 (228.65) | 868.02 | 0.0132 |
| 4 | Stratopause | 47,000 | 47,350 | 0.0 | −2.5 (270.65) | 110.91 | 0.0014 |
| 5 | Mesosphere | 51,000 | 51,412 | +2.8 | −2.5 (270.65) | 66.939 | 0.0009 |
| 6 | Mesosphere | 71,000 | 71,802 | +2.0 | −58.5 (214.65) | 3.9564 | 0.0001 |
| 7 | Mesopause | 84,852 | 86,000 | — | -86.204 (186.946) | 0 | 0 |
In the above table,geopotential altitude is calculated from a mathematical model that adjusts the altitude to include the variation of gravity with height, whilegeometric altitude is the standard direct vertical distance abovemean sea level (MSL).[2]
Note that the Lapse Rates cited in the table are given as °C per kilometer of geopotential altitude, not geometric altitude.
The ISA model is based on average conditions at mid latitudes, as determined by the ISO's TC 20/SC 6 technical committee. It has been revised from time to time since the middle of the 20th century.
The ISA models a hypotheticalstandard day to allow a reproducible engineering reference for calculation and testing of engine and vehicle performance at various altitudes. Itdoes not provide a rigorous meteorological model of actual atmospheric conditions (for example,changes in barometric pressure due to wind conditions). Neither does it account forhumidity effects; air is assumed to be dry and clean and of constant composition. Humidity effects are accounted for in vehicle or engine analysis by adding water vapor to thethermodynamic state of the air after obtaining the pressure and density from the standard atmosphere model.
Non-standard (hot or cold) days are modeled by adding a specified temperature delta to the standard temperature at altitude, but pressure is taken as the standard day value. Density and viscosity are recalculated at the resultant temperature and pressure using the ideal gas equation of state. Hot day, Cold day, Tropical, and Polar temperature profiles with altitude have been defined for use as performance references, such asUnited States Department of Defense MIL-STD-210C, and its successor MIL-HDBK-310.[6]
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).[7]
The ICAO Standard Atmosphere, like the ISA, does not contain water vapor.
Some of the values defined by ICAO are:
| Height km & ft | Temperature °C | Pressure hPa | Lapse rate °C/1000 ft | Lapse rate C/1000 m |
|---|---|---|---|---|
| 0 km MSL | 15.0 | 1013.25 | +1.98 (tropospheric) | +6.5 (tropospheric) |
| 11 km 36 000 ft | −56.5 | 226.00 | 0.00 (stratospheric) | 0.00 (stratospheric) |
| 20 km 65 000 ft | −56.5 | 54.70 | -0.3 (stratospheric) | -0.1 (stratospheric) |
| 32 km 105 000 ft | −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 kg/m3.
Physical properties of the ICAO Standard Atmosphere are:[8]
| Parameter | Value |
|---|---|
| Density | 1.225 kg m−3 |
| Kinematic viscosity | 1.4607 × 10−5 m2 s−1 |
| Dynamic viscosity | 1.7894 × 10−5 kg m−1 s−1 |
| Molar volume | 2.3645 × 10−2 m3 mol−1 |
| Molecular weight | 28.966 |
| Thermal conductivity | 2.5339 × 10−2 W m−1 K−1 |
| Mean free path | 6.6317 × 10−8 m |
| Collision frequency | 6.9204 × 109 s−1 |
| Particle speed | 4.5894 × 102 m s−1 |
| Number density | 2.5475 × 1025 m−3 |
TheU.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,[9] and was updated in 1962,[5] 1966,[10] and 1976.[11] 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 km.[12][13]
NRLMSISE-00 is a newer model of the Earth's atmosphere from ground to space, developed by theUS Naval Research Laboratory taking actual satellite drag data into account. A primary use of this model is to aid predictions ofsatellite orbital decay due toatmospheric drag. TheCOSPAR 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 km to 2000 km, developed by theUS Air Force Space Command andSpace Environment Technologies taking into account realisticsolar irradiances and time evolution of geomagnetic storms.[14] It is most useful for calculating satellite orbital decay due toatmospheric drag. Both CIRA 2012 and ISO 14222 recommend JB2008 for mass density in drag uses.[citation needed]
...the ISO (International Organisation for Standardisation) Standard Atmosphere, 1972. This model is identical to the present Standard Atmospheres of ICAO (International Civil Aviation Organization) and WMO (World Meteorological Organization) up to a height of 32 km
