To enable comparison between aircraft heights over a wide range of meteorological situations the International Civil Aviation Organization (ICAO) has adopted an International Standard Atmosphere (ISA); it is a model atmosphere for which the average variation of temperature with height is specified. From this, the corresponding variations of pressure and density can be deduced.

It specifies the following:

a sea-level pressure of 1013.25 hPa

a sea-level temperature of 15°C

a fixed lapse rate for the temperature of 1.98°C per 1000 ft up to 36,090 feet (the theoretical tropopause height of the ISA), where the temperature is assumed to be 56.5° C

For practical purposes, the ISA can be assumed to have sea level pressure of 1013 hPa at a temperature of 15°C and a lapse rate of 2°C per 1000 feet up to 36 000 feet with a corresponding temperature of 57° C.

The ISA is used for altimetry and as a reference for aircraft performance standards.

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