Altimeter setting is the value of the atmospheric pressure used to adjust the sub-scale of a
pressure altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
so that it indicates the height of an aircraft above a known reference surface.
This reference can be the
mean sea level
There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value (magnitude and sign) of a given data set.
For a data set, the '' ari ...
pressure (
QNH); the pressure at the nearby surface airport (QFE); or the pressure level of {{convert, 1013.25, hPa, inHg, abbr=off, sigfig=4 which gives
pressure altitude Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.
The National Oceanic and Atmospheric Administration (NOAA) published the follow ...
and is used to maintain one of the standard
flight level
In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa ...
s.
The setting of a sensitive pressure altimeter is shown in the
Kollsman window.
The altimeter setting QNH is one of the data included in
METAR messages from weather stations in addition to the surface pressure. An alternative setting is QFE:
#QNH is the barometric altimeter setting that causes an altimeter to read airfield elevation above mean sea level when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield.
#QFE is the barometric altimeter setting that causes an altimeter to read zero when at the reference datum of a particular airfield (in practice, the reference datum is either an airfield center or a runway threshold). In ISA temperature conditions the altimeter will read height above the airfield/runway threshold in the vicinity of the airfield.
QNH and QFE are arbitrary
Q codes rather than abbreviations, but the
mnemonic
A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding.
Mnemonics make use of elaborative encoding, retrieval cues, and imag ...
s "nautical height" (for QNH) and "field elevation" (for QFE) are often used by pilots to distinguish between them.
References
Aviation meteorology