Editing Altitude (section)

== In aviation==
{{anchor|Aviation}}
{{See also|Sea level#Aviation|Vertical separation (aviation)}}
{{main cat|Altitudes in aviation}}
{{more citations needed section|date=December 2018}}
[[File:Boeing 737 view 1.jpg|alt=A generic Boeing 737-800 cruising at 32,000 feet. Below it are a pack of clouds. Above it is a vivid, ambient blue sky.|thumb|upright=1.5|A [[Boeing 737 Next Generation|Boeing 737-800]] [[cruise (aeronautics)|cruising]] in the [[stratosphere]], where [[airliner]]s typically cruise to avoid [[turbulence]] rampant in the [[troposphere]]. The blue layer is the [[ozone layer]], fading further to the [[mesosphere]]. The [[ozone]] heats the stratosphere, making conditions stable. The stratosphere is also the altitude limit of [[jet aircraft]] and [[weather balloon]]s, as the [[air density]] there is roughly {{fract|1|1000}} of that in the troposphere.<ref>{{cite web |title=The Stratosphere - overview |url=https://scied.ucar.edu/learning-zone/atmosphere/stratosphere-overview#:~:text=Commercial%20jet%20aircraft%20fly%20in,the%20lower,%20more%20humid%20troposphere. |website=scied.ucar.edu |publisher=[[University Corporation for Atmospheric Research]] |access-date=2021-02-06}}</ref>]]

[[File:vertical distances.svg|thumb|upright=1.7|Vertical distance comparison]]

The term ''altitude'' can have several meanings, and is always qualified by explicitly adding a modifier (e.g. "true altitude"), or implicitly through the context of the communication. Parties exchanging altitude information must be clear which definition is being used.

Aviation altitude is measured using either [[mean sea level]] (MSL) or local ground level (above ground level, or AGL) as the reference datum.

[[Pressure altitude]] divided by 100 feet (30&nbsp;m) is the [[flight level]], and is used above the [[transition altitude]] ({{convert|18000|ft|m}} in the US, but may be as low as {{convert|3000|ft|m}} in other jurisdictions). So when the altimeter reads the country-specific flight level on the standard pressure setting the aircraft is said to be at "Flight level XXX/100" (where XXX is the transition altitude). When flying at a flight level, the altimeter is always set to standard pressure (29.92&nbsp;[[Inches of mercury|inHg]] or 1013.25&nbsp;[[Pascal (unit)|hPa]]).

On the flight deck, the definitive instrument for measuring altitude is the pressure [[altimeter]], which is an [[barometer#Aneroid barometers|aneroid barometer]] with a front face indicating distance (feet or metres) instead of [[atmospheric pressure]].

There are several types of altitude in aviation:

* '''Indicated altitude''' is the reading on the altimeter when it is set to the [[QNH|local barometric pressure at mean sea level]]. In UK aviation radiotelephony usage, ''the vertical distance of a level, a point or an object considered as a point, measured from mean sea level''; this is referred to over the radio as '''altitude'''.(see [[QNH]])<ref name="CAP413">{{cite book | date=1 January 1995 | title=Radiotelephony Manual |publisher=UK Civil Aviation Authority| id=CAP413| isbn=978-0-86039-601-7 }}</ref>
* '''Absolute altitude''' is the vertical distance of the aircraft above the terrain over which it is flying.<ref name="AFM_51-40">{{cite book |title=Air Navigation |date=1 December 1989 |publisher=Department of the Air Force |id=AFM 51-40}}</ref>{{rp|ii}} It can be measured using a [[radar altimeter]] (or "absolute altimeter").<ref name="AFM_51-40" /> Also referred to as "radar height" or feet/metres [[Above Ground Level|above ground level]] (AGL).
* '''True altitude''' is the actual elevation above [[mean sea level]].<ref name="AFM_51-40"/>{{rp|ii}} It is indicated altitude corrected for non-standard temperature and pressure.
* '''Height''' is the vertical distance above a reference point, commonly the terrain elevation. In UK aviation radiotelephony usage, ''the vertical distance of a level, a point or an object considered as a point, measured from a specified datum''; this is referred to over the radio as '''height''', where the specified datum is the airfield elevation (see [[Mean sea level pressure|QFE]])<ref name="CAP413" />
* '''Pressure altitude''' is the elevation above a standard datum air-pressure plane (typically, 1013.25 millibars or 29.92" Hg). Pressure altitude is used to indicate "flight level" which is the standard for altitude reporting in the U.S. in Class A airspace (above roughly 18,000 feet). Pressure altitude and indicated altitude are the same when the altimeter setting is 29.92" Hg or 1013.25 millibars.
*'''[[Density altitude]]''' is the altitude corrected for non-ISA [[International Standard Atmosphere]] atmospheric conditions. Aircraft performance depends on density altitude, which is affected by barometric pressure, humidity and temperature. On a very hot day, density altitude at an airport (especially one at a high elevation) may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

These types of altitude can be explained more simply as various ways of measuring the altitude:
*'''Indicated altitude''' – the altitude shown on the altimeter.
*'''Absolute altitude''' – altitude in terms of the distance above the ground directly below
*'''True altitude''' – altitude in terms of elevation above sea level
*'''Height''' – vertical distance above a certain point
*'''Pressure altitude''' – the [[air pressure]] in terms of altitude in the International Standard Atmosphere
*'''Density altitude''' – the density of the air in terms of altitude in the International Standard Atmosphere in the air