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The barometric pressure has dropped to 1/2 of the pressure at sea level at..

  • A

    30000 feet.

  • B

    18000 feet.

  • C

    10000 feet.

  • D

    25000 feet.

The Standard Atmosphere

The ICAO Standard Atmosphere is defined as follows:

a) MSL temperature of +15ºC;
b) MSL pressure of 1013.25 hPa;
c) MSL density of 1225 grammes/cubic metre;
d) a lapse rate of 1.98ºC/1 000 ft (6.5º/km) up to 36 090 ft (11 km) thereafter the temperature remains constant at -56ºC up to 65 617 ft (20 km).

The altitudes in the standard stmosphere that pressure will be 1/4, 1/2 and 3/4 of MSL pressure is approximately:

  • 3/4 MSL => 8 000 ft.
  • 1/2 MSL => 18 000 ft.
  • 1/4 MSL => 36 000 ft.

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