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# What will the temperature be at flight level 50 if the ICAO standard lapse rate is applied?

I need help understanding how to solve problems like this one:

The temperature at FL (flight level) 110 is -5 degrees C. What will the temperature be at FL 50 if the ICAO standard lapse rate is applied?

If anyone can show the work for how to do this, it would be greatly appreciated, since the book I have is totally useless and doesn't give a formula to use. I'm not looking for just an answer, I need to know how to work through the problem. Thanks for any help.

### 2 Answers

- Michel VerheugheLv 710 years agoFavorite Answer
As the owner and pilot of a light aircraft (actually, I went flying today) and a meteorology instructor at my club, I'll answer this one:

The International Standard Atmosphere (ISA) and not the ICAO, set the average adiabatic lapse rate on earth to be 0.65 C per 100 meter (roughly 2 C per 1,000 ft or, 3 F per 1,000 ft).

FL 50 is 5,000 ft above sea level (ASL). The difference between 110 and 50 is, 60, or 6,000 ft. At the rate of 2 C per 1,000 ft, the temperature at FL 50 would be 12 degrees higher or, 7 C.

The ICAO doesn't set any atmospheric standard, it is the ISA. Two other standards you should remember if you intend to be a pilot is: The average pressure at sea level is 1013.25 hPa and the average temperature is 15 C. Those are the values all instruments, and especially aviation instruments are calibrated for.

For this reason, if you e.g. fly at at a temperature below 15 C (which I often do, here in the Norwegian winter!) you fly actually lower than what your altimeter indicates.

What I also do prior to flying is to check the dew point temperature. Its difference with the actual temperature is called the spread. Let's say that the temperature is 15 C and the dew point, 11 C. The spread is then 4 C and I can expect the cloud ceiling to be at roughly 2,000 ft since it will be the altitude where dew point temperature is expected and therefore, full saturation of the air moisture.

Another rule of thumb I use is: Relative humidity = 100 - (spread *5). Say, the spread is 4 C, as above, the relative humidity at ground level will then be 100 - 20 = 80 percent.

- Anonymous5 years ago
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