THE INTERNATIONAL STANDARD ATMOSPHERE, PRESSURE SETTINGSAND RELATED MATTERS

Based on the International Standard Atmosphere:for dry air(ICAO 1964):

1. At mean sea level pressure=1013.25 hPa//temp=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) ......... 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 followingrelationships 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 altimeterset 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, theheight indicated is the height above official airfield datum.

• QNH:
  The pressure 'reduced' to mean sea level, assuming ISA temperatureprofile from the station/airfield to MSL. An altimeter set to the airfieldQNH reads the elevation of the airfield when on the ground.

• QFF:
  Barometric pressure 'reduced' to mean sea level, assuming an isothermalatmosphere from the airfield/station to MSL, using current (screen)temperatures. The difference between QFF and QNH can be considerable whenatmospheric conditions are significantly different from ISA: i.e. at 'hot andhigh' airfields.

• QNE:
  When the ISA mean sea level standard pressure of 1013.2 hPa is set on anaircraft altimeter subscale, the height so indicated upon landing at anairfield is known as the QNE reading. More widely, this is also the PRESSUREALTITUDE, which is alternatively defined as the height of any level in theinternational standard atmosphere (ISA-see above), above the levelcorresponding to a pressure of 1013.2 hPa.

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