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According to ISA (ICAO Standard Atmosphere), at what rate does the temperature change with increasing height within the troposphere?

  • A

    Decreases by 2°C/100 m

  • B

    Decreases by 2°C/1000 ft

  • C

    Increases by 2°C/100 m

  • D

    Increases by 2°C/1000 ft

Atmospheric pressure, density, temperature, and lapse rate vary significantly and are constantly changing across different parts of the globe. To provide a consistent reference, the International Civil Aviation Organization (ICAO) has established the International Standard Atmosphere (ISA). This model represents average atmospheric conditions from just below mean sea level up to 32 km. At mean sea level, ISA specifies the following conditions:

  • Temperature = +15°C
  • Pressure = 1013.25 hectopascal (hPa)
  • Density = 1.225 kg/m3

From the surface upwards:

  • The temperature decreases by 0.65°C/100 meters (1.98°C/1000 feet) up to an altitude of 11 km (36 090 feet), where it reaches -56°C.
  • The temperature remains constant at -56.5°C from 11 km (36 090 feet) to 20 km (65 617 feet), or flight level 650 (FL650).

Note: When doing mathematical calculations involving the temperature lapse rate, we often round-up the lapse rate from its precise value of 1.98°C/1000 ft to an easier to handle 2°C/1000 ft.

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