Description
The ISA model divides the atmosphere into layers with linear temperature distributions. The other values are computed from basic physical constants and relationships. Thus the standard consists of a table of values at various altitudes, plus some formulas by which those values were derived. For example, at sea level the standard gives a pressure of 1013.25 hPa (1 atm) and a temperature of 15 Celsius, and an initial lapse rate of −6.5 °C/km (roughly −2 °C/1,000 ft). The tabulation continues to 11 km where the pressure has fallen to 226.32 hPa and the temperature to −56.5 °C. Between 11 km and 20 km the temperature remains constant.
| Layer | Level Name |
Base Geopotential Height h (in km) |
Base Geometric Height z (in km) |
Lapse Rate (in °C/km) |
Base Temperature T (in °C) |
Base Atmospheric Pressure p (in Pa) |
|---|---|---|---|---|---|---|
| 0 | Troposphere | 0.0 | 0.0 | −6.5 | +15.0 | 101325 |
| 1 | Tropopause | 11.000 | 11.019 | +0.0 | −56.5 | 22632 |
| 2 | Stratosphere | 20.000 | 20.063 | +1.0 | −56.5 | 5474.9 |
| 3 | Stratosphere | 32.000 | 32.162 | +2.8 | −44.5 | 868.02 |
| 4 | Stratopause | 47.000 | 47.350 | +0.0 | −2.5 | 110.91 |
| 5 | Mesosphere | 51.000 | 51.413 | −2.8 | −2.5 | 66.939 |
| 6 | Mesosphere | 71.000 | 71.802 | −2.0 | −58.5 | 3.9564 |
| 7 | Mesopause | 84.852 | 86.000 | — | −86.2 | 0.3734 |
In the above table, geopotential height is calculated from a mathematical model in which the acceleration due to gravity is assumed constant. Geometric height results from the assumption that gravity obeys an inverse square law.
The ISA model is based on average conditions at mid latitudes, as determined by ISO's TC 20/SC 6 technical committee. It has been revised from time to time since the middle of the 20th century.
Read more about this topic: International Standard Atmosphere
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