The International Standard Atmosphere (ISA) is a model which determines the temperature, the pressure, etc., of an (ideal) atmosphere at a given altitude.
ISA sets a reference which allows to compare the actual atmosphere with a standard atmosphere defined by average values. ISA also plays a fundamental role in the definition of the performance of aircrafts.
The International Standard Atmosphere used by ICAO is defined in the DOC 7488.
ISA main parameters:
♦ Dry air
♦ temperature: 15 oC at sea level
♦ temperature lapse rate: -1.98oC / 1000 ft: from sea level up to FL 360
♦ pressure: 1013.25 hPa at sea level
♦ pressure lapse rate:
◊ sea level: ≈ -3.56 hPa / 100 ft (28 ft / hPa)
◊ FL 100: ≈ -2.70 hPa / 100 ft (37 ft / hPa)
◊ FL 200: ≈ -1.95 hPa / 100 ft (51 ft/ hPa)
◊ FL 300: ≈ -1.37 hPa / 100 ft (73 ft/ hPa)
The mobile application allows you to determine the ISA parameters: air temperature, air pressure, air density and the speed of sound. Have fun!