International Standard Atmosphere [ISA]

The International Civil Aviation Organisation's [ICAO] International Standard Atmosphere [ISA] provides a fixed standard atmospheric model that is used for many purposes, among which are the uniform assessment of aircraft performance and the calibration of some aircraft instruments. The model is akin to the average condition in mid-latitudes, but contains the following assumptions:

dry air is assumed throughout the atmosphere

-the mean sea-level [msl] pressure = 1013.225 hPa

-the msl temperature = 15 °C [288 K]

atmospheric density = 1.225 Kg/m3

also:

-the tropopause is at 36 090 feet [11 km] and the pressure at the tropopause = 226.3 hPa

-the temperature lapse rate to 36 090 feet = 6.5 °C per km, or nearly 2 °C per 1000 feet

-the temperature between 36 090 and 65 600 feet [20 km] remains constant at –56.5 °C (isothermal layer).