Atmospheric pressure
Atmospheric pressure is the weight of the air column above a point per unit area.
Source: WMO Guide on Meteorological Instruments; ICAO Annex 3.
Origin of pressure — gravity
Air pressure within the Earth's atmosphere develops primarily through the effect of the gravitational force of the Earth on the individual components of the air:
- Gravity pulls air molecules toward Earth.
- At any altitude pressure = weight of air above per area.
- At MSL pressure is maximum (whole atmosphere weighs on it), drops with altitude.
Units
| Unit | Value (MSL ISA) | Use |
|---|---|---|
| Hectopascal (hPa) | 1013.25 hPa | Standard in Europe and aviation |
| Millibar (mb) | 1013.25 mb (= hPa) | Historic, identical to hPa |
| Inch of mercury (inHg) | 29.92 inHg | USA, AFM |
| Bar | 1.01325 bar | Scientific |
| Atmosphere (atm) | 1 atm | Physical |
Pressure decrease with altitude
Pressure falls exponentially with altitude (barometric formula). Aviation rule of thumb:
- In the first thousand feet above MSL: ~1 hPa per 27-28 ft (or 8.5 m).
- With altitude the pressure gradient flattens.
| Altitude AMSL | Pressure (ISA) | vs MSL |
|---|---|---|
| MSL (0 ft) | 1013 hPa | 100 % |
| 5000 ft (1524 m) | 843 hPa | 83 % |
| 10 000 ft (3048 m) | 697 hPa | 69 % |
| 18 000 ft (5500 m) | 506 hPa | 50 % |
| 36 000 ft (11 000 m) | 226 hPa | 22 % |
→ At 5500 m AMSL the atmospheric pressure equals half the sea-level pressure.
Barometric height step
The term "barometric height step" refers to the difference in altitude when the air pressure reduces by 1 hPa:
- At sea level: about 8.5 m / 27-28 ft per 1 hPa.
- At 5500 m: about 16 m / 53 ft per 1 hPa.
- At 18 000 m: about 60 m / 197 ft per 1 hPa.
- Note: the height step grows with altitude (because air density falls).
Altimeter subscale — calibrates measurement
Setting of the pressure value in the subscale of the altimeter calibrates the entire measurement system to the chosen reference pressure:
- The pilot sets e.g. QNH 1018 hPa → altimeter shows altitude relative to the 1018-hPa level at MSL.
- At standard 1013.25 hPa → altimeter shows pressure altitude (flight level).
Q-codes — pressure references
| Q-code | Definition |
|---|---|
| QNH | The actual atmospheric pressure at the airfield, reduced to MSL using ISA values — used for VFR cross-country flights below the transition altitude. |
| QFE | The actual aerodrome pressure, altimeter then shows height above the threshold — rare in aviation, more in military. |
| QFF | Actual pressure reduced to MSL — used on (surface) weather charts for worldwide comparison of pressure values. Difference to QNH: QFF uses actual T conditions, QNH uses ISA standard lapse rate. |
| QNE / standard | 1013.25 hPa — used for flight levels above transition altitude. |
Transition altitude (TA) and transition level (TL)
- Transition altitude (TA) is the altitude at and below which the pilot has to set the altimeter to the QNH value reported by ATC:
- In Germany TA = 5000 ft AMSL (varies regionally).
- Transition level (TL) is the specified first usable flight level that is at least 1000 ft above the transition altitude — marks where aircraft in climb switch from QNH-based altitudes to standard-pressure flight levels, or vice versa in descent.
Instruments — barometers
Aviation meteorological services primarily use bellows-type barometers and mercury barometers to measure air pressure:
- Aneroid (bellows) barometer: light pressure-sensitive capsule, mechanical readout — standard in the cockpit (altimeter).
- Mercury barometer: mercury column, very precise — historically in weather stations.
- Electronic: modern weather stations with semiconductor sensors.
Guide to Meteorological Instruments and Methods of Observation* (WMO-No. 8); ICAO Annex 3; ICAO Doc 7488; FAA-H-8083-25B PHAK Ch. 12.*