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In the International Standard Atmosphere (ISA)…

  • A

    MSL pressure is 1013 hPa and there is a lapse rate of 1.98°C/2000 ft up to 36 090 ft

  • B

    MSL temperature is 15°C and there is a lapse rate of 1.98°C/1000 ft up to 36 090 ft

  • C

    MSL temperature is 15°C and MSL pressure is 1010 hPa

  • D

    MSL pressure is 1013 hPa and MSL temperature is 10°C

The real atmosphere is unpredictable and changes rapidly with time. In order to provide a benchmark for calculations and planning, the International Standard Atmosphere (ISA) provides some fixed values and assumptions. It is important we memorise these as they are used very frequently in aviation:

The temperature at Mean Sea Level (MSL) is 15°C

The environmental temperature lapse rate is 1.98°C per 1000 ft. This means that as we climb, the temperature decreases by about 2 degrees every 1000 ft.

The pressure at sea level is 1013.25 hPa

The density at sea level is 1.225 kg/m3

The tropopause (top of the troposphere) is at 36 090 ft, at a temperature of -56.5°C

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