- As the altitude increases the pressure of the air decreases. Below follows the ISA atmospheric pressure levels. Each FL corresponds to a specific pressure surface (pressure value):

FL50 = 850 hPa

FL100 = 700 hPa

FL140 = 600 hPa

FL180 = 500 hPa

FL240 = 400 hPa

FL300 = 300 hPa

FL340 = 250 hPa

FL390 = 200 hPa

FL450 = 150 hPa

- In the ISA atmosphere, the pressure decreases at the following rates:

**MSL 1 hPa = 27 ft (8 m)**

18 000 ft (5 500 m) 1 hPa = 50 ft (15 m)

39 000 ft (12 000 m) 1 hPa = 105 ft (32 m)

52 500 ft (16 000 m) 1 hPa = 210 ft (64 m)

Alternatively, you can use the following formula:

The formula to calculate the change in ft/hPa = 96 x T/P

*- Where "T" stands for temperatura in K, and "P" is pressure.*

In this case, we will have ISA temperature at MSL = 15ºC

K = C + 273; K = 15ºC + 273 = 288 K

Change in ft/hPa = 96 x 288/1013 = 27

*Note that the formula above only gives an approximate result - which is close enough to answer this sort of questions and it is valid for ISA temperatures only. i.e. for a cold air, the pressure will fall more rapidly with altitude - and the distance between the molecules is less than in warm air.*

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