International Standard Atmosphere (ISA)

For a variety of reason it is necessary to establish a standard average atmosphere, describing variations in temperature, pressure and density throughout altitude.

There have been several Standard Atmospheres, but the one in general use now is the 'ICAO ISA', dated 1964 which covers from -16,400ft (-5km) to 262,464ft.

The ISA is needed for:-

a) the calibration of aircraft instruments.
b) the design and testing of aircraft.

The ICAO ISA is defined as follows:

a) a MSL temperature of +15°C,
b) a MSL pressure of 1013.25 milibars,
c) a MSL density of 1225 grammes/cubic metre,
d) from -5km, a lapse rate of 1.98°C/1000ft (6.5deg/km) up to 36,090ft (11km),
e) a constant temperature of -56.5°C up to 65,617ft (20km),
f) an increase of temperature of 0.3°C/1000ft (1deg/km), up to 104,987ft (32km).

 The Iinternational Standard Atmosphere

Ref: JAA ATPL BOOK 9 - METEOROLOGY

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