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The following temperatures have been observed over a station at 12:00 UTC. Assume the station is at MSL.

Height Temperature
20 000 ft -12 °C
18 000 ft -11 °C
16 000 ft -10 °C
14 000 ft -10 °C
12 000 ft -6 °C
10 000 ft -2 °C
8 000 ft +2 °C
6 000 ft +6 °C
4 000 ft +12 °C
2 000 ft +15 °C
surface +15 °C
  • A
    The layer between 16 000 and 18 000 ft is absolutely unstable.
  • B
    Assuming that the MSL pressure is 1 013.25 hPa the true altitude of an aircraft would actually be higher than the indicated altitude.
  • C
    The temperature at 10 000 ft is in agreement with the temperature in the International Standard Atmosphere.
  • D
    The height of the freezing level over the station is approximately 12 000 ft.

First step: Calculate the ISA temperature for each of the heights given.

20 000 ft 15 – 2 x 20 = -25ºC
18 000 ft 15 – 2 x 18 = -21ºC
16 000 ft 15 – 2 x 16 = -17ºC
14 000 ft 15 – 2 x 14 = -13ºC
12 000 ft 15 – 2 x 12 = -9ºC
10 000 ft 15 – 2 x 10 = -5ºC
8 000 ft 15 – 2 x 8 = -1ºC
6 000 ft 15 – 2 x 6 = 3ºC
4 000 ft 15 – 2 x 4 = 7º C
2 000 ft 15 – 2 x 2 = 11ºC
Surface 15ºC
Second step: We will now compare the above table (ISA temperature) to the actual temperature at those levels. In order to find out if we are dealing with conditions warmer, colder or equal to ISA.
  • Looking at the question text, we conclude that the actual temperature is warmer than ISA. And we know that, disregarding pressure, for temperatures WARMER THAN ISA; TRUE ALTITUDE > INDICATED ALTITUDE
The layer between 16 000 and 18 000 ft is absolutely unstable.” Incorrect – If ELR > DALR (3°C/1000 f) we have absolute instability. In this case, we have ELR < DALR.
The temperature at 10 000 ft is in agreement with the temperature in the International Standard Atmosphere.” Incorrect – The actual temperature at 10 000 ft is -2ºC; ISA temperature at that same level is -5ºC. ISA +3ºC
The height of the freezing level over the station is approximately 12 000ft.incorrect – The freezing level is approximately at 9 000 ft. (8 000 ft = 2ºC, therefore 0ºC will be at 9 000 ft)

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