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International Standard Atmosphere (ISA) and Pressure Settings

Based on the International Standard Atmosphere for dry air (ICAO 1964), which is defined as under:-

1. At mean sea level (msl), the pressure = 1013.25 hPa and temperature = 15.0 degC
2. From msl to 11 km, a decrease in temperature (or lapse rate) of 6.5 degC/km
3. From 11 km to 20 km, the temperature is held to be isothermal (not changing) at a value of - 56.5 degC
4. From 20 km to 32 km, an increase in temperature of about 1 degC/km

hPa (mbar) ......... height (metres) ......... height (feet) ......... temperature (degC)
»...... 10 ................... 31 055 ................ 101 885 ................. -45.4
»...... 20 ................... 26 481 .................. 86 881 ................. -50.0
»...... 30 ................... 23 849 .................. 78 244 ................. -52.7
»...... 40 ................... 22 000 .................. 72 177 ................. -54.5
»...... 50 ................... 20 576 .................. 67 507 ................. -55.9
»...... 70 ................... 18 442 .................. 60 504 ................. -56.5
».... 100 ................... 16 180 .................. 53 083 ................. -56.5
».... 150 ................... 13 608 .................. 44 647 ................. -56.5
».... 200 ................... 11 784 .................. 38 662 ................. -56.5
»... (226/ISA TROP.. 11 000 .................. 36 091 ................. -56.5)
».... 250 ................... 10 363 .................. 33 999 ................. -52.3
».... 300 ..................... 9 164 .................. 30 065 ................. -44.5
».... 400 ..................... 7 185 .................. 23 574 ................. -31.7
».... 500 ..................... 5 574 .................. 18 289 ................. -21.2
».... 600 ..................... 4 206 .................. 13 801 ................. -12.3
».... 700 ..................... 3 012 .................... 9 882 ................... -4.6
».... 800 ..................... 1 949 .................... 6 394 ..................... 2.3
».... 850 ..................... 1 457 .................... 4 781 ..................... 5.5
».... 900 ........................ 988 .................... 3 243 ..................... 8.6
».... 950 ........................ 540 .................... 1 773 ................... 11.5
».. 1000 ........................ 111 ....................... 364 ................... 14.3
». (1013.25/ISA MSL ....... 0 ........................... 0 ................... 15.0)
».. 1050 ...................... - 302 .................... - 989 ................... 17.0

For practical aviation purposes, i.e. Flight Weather charts, the following relationships are used:

PRESSURE LEVEL ---------- FLIGHT LEVEL

100 ............................................ F530
150 ............................................ F450
200 ............................................ F390
250 ............................................ F340
300 ............................................ F300
400 ............................................ F240
500 ............................................ F180
600 ............................................ F140
700 ............................................ F100
850 ............................................ F050

(below F050, heights usually expressed in altitude, above mean sea level, rather than pressure altitude - SEE DEFINITIONS BELOW.)

PRESSURE SETTING DEFINITIONS

  • QFE:
    The pressure corrected to the official airfield elevation. An altimeter set to the particular airfield QFE reads zero when an aircraft is on the ground (strictly the height of the altimeter above the ground). In the circuit, the height indicated is the height above official airfield datum.
  • QNH:
    The pressure 'reduced' to mean sea level, assuming ISA temperature profile from the station/airfield to MSL. An altimeter set to the airfield QNH reads the elevation of the airfield when on the ground.
  • QFF:
    Barometric pressure 'reduced' to mean sea level, assuming an isothermal atmosphere from the airfield/station to MSL, using current (screen) temperatures. The difference between QFF and QNH can be considerable when atmospheric conditions are significantly different from ISA: i.e. at 'hot and high' airfields.
  • QNE:
    When the ISA mean sea level standard pressure of 1013.2 hPa is set on an aircraft altimeter subscale, the height so indicated upon landing at an airfield is known as the QNE reading. More widely, this is also the PRESSURE ALTITUDE, which is alternatively defined as the height of any level in the international standard atmosphere (ISA-see above), above the level corresponding to a pressure of 1013.2 hPa.



These definitions are intended to give a quick reference to the various pressure settings in use. Users should refer to standard textbooks (e.g. Handbook of Aviation Meteorology, The Met.Office/HMSO) for more details, and to current air safety/air traffic control regulations for in-flight use.