International Standard Atmosphere

The ISA mathematical model divides the atmosphere into layers with an assumed linear distribution of absolute temperature T against geopotential altitude h.[2] The other two values (pressure P and density ρ) are computed by simultaneously solving the equations resulting from: 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).

To accommodate the 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.

The ISA model is based on average conditions at mid latitudes, as determined by the ISO's TC 20/SC 6 technical committee.

Humidity effects are accounted for in vehicle or engine analysis by adding water vapor to the thermodynamic state of the air after obtaining the pressure and density from the standard atmosphere model.

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.

[12][13] NRLMSISE-00 is a newer model of the Earth's atmosphere from ground to space, developed by the US Naval Research Laboratory taking actual satellite drag data into account.

A primary use of this model is to aid predictions of satellite orbital decay due to atmospheric drag.