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What is the approximate vertical interval which is equal to a pressure change of 1 hPa at sea level?
  • A
    64 m (210 ft)
  • B
    32 m (105 ft)
  • C
    15 m (50 ft)
  • D
    8 m (27 ft)
  • 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):
MSL = 1013 hPa
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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