THE INTERNATIONAL STANDARD ATMOSPHERE

PRESSURE SETTINGS AND RELATED MATTERS

Based on the International Standard Atmosphere – for dry air (ICAO 1964)
  1. At mean sea level pressure=1013.25 hPa, temperatur = 15 degC
  2. From MSL to 11 km = decrease in temperature 6.5 degC/km
  3. From 11 km to 20 km = isothermal temperature - 56.5 degC
  4. From 20 km to 32 km = a rise in temperature of about 1 degC/km
hPA (mbar) height (metres) heigth (feet) temperature (deg C)
10 31'055 101'855 -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 definition 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.

QNE

When the ISA mean sea level standard pressure of 1013.2 hPa is set on an aircraft altimeter sub scale, 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.

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.
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.