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[[File:Altitude.svg|thumb|Altitude is a vertical measurement between a reference datum and an object.]]
'''Altitude''' is a distance measurement, usually in the vertical or "up" direction, between a reference [[Datum (geodesy)|datum]] and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometry, geographical survey, sport, or atmospheric pressure). Although the term ''altitude'' is commonly used to mean the [[height above sea level]] of a location, in [[geography]] the term [[elevation]] is often preferred for this usage.

In aviation, altitude is typically measured relative to mean sea level or above ground level to ensure safe navigation and flight operations. In geometry and geographical surveys, altitude helps create accurate topographic maps and understand the terrain's elevation. For high-altitude trekking and sports, knowing and adapting to altitude is vital for performance and safety. Higher altitudes mean reduced oxygen levels, which can lead to altitude sickness if proper acclimatization measures are not taken.

Vertical distance measurements in the "down" direction are commonly referred to as ''[[Depth (coordinate)|depth]]''.

== In aviation==
[[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">LBI</ref>
* '''Absolute altitude''' is the vertical distance of the aircraft above the terrain over which it is flying.<ref name="AFM_51-40">LBI</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

==In atmospheric studies==
===Atmospheric layers===
The [[Earth's atmosphere]] is divided into several altitude regions. These regions start and finish at varying heights depending on season and distance from the poles.  The altitudes stated below are averages:<ref>{{cite web | title=Layers of the Atmosphere |work=JetStream, the National Weather Service Online Weather School | publisher=National Weather Service | url=http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm | access-date=22 December 2005| archive-url= https://web.archive.org/web/20051219190158/http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm| archive-date= 19 December 2005 | url-status= live}}</ref>
* [[Troposphere]]: surface to {{convert|8000|m|mi}} at the poles, {{convert|18000|m|miles}} at the [[Equator]], ending at the Tropopause
* [[Stratosphere]]: Troposphere to {{convert|50|km|mi}}
* [[Mesosphere]]: Stratosphere to {{convert|85|km|mi}}
* [[Thermosphere]]: Mesosphere to {{convert|675|km|mi}}
* [[Exosphere]]: Thermosphere to {{convert|10000|km|mi}}

The [[Kármán line]], at an altitude of {{convert|100|km|mi}} above [[sea level]], by convention defines represents the demarcation between the atmosphere and [[Outer space|space]].<ref>{{cite web|url=http://www.fai.org/icare-records/100km-altitude-boundary-for-astronautics|title=The 100&nbsp;km Boundary for Astronautics|author=Dr. S. Sanz Fernández de Córdoba|publisher=[[Fédération Aéronautique Internationale]]|date=24 June 2004|archive-url=https://web.archive.org/web/20110809093537/http://www.fai.org/astronautics/100km.asp|archive-date=9 August 2011}}</ref> The thermosphere and exosphere (along with the higher parts of the mesosphere) are regions of the atmosphere that are conventionally defined as space.

===High altitude and low pressure===
Regions on the [[Earth]]'s surface (or in its atmosphere) that are high above mean sea level are referred to as '''high altitude'''. High altitude is sometimes defined to begin at {{convert|8000|ft|meters|order=flip}} above sea level.<ref name="websterMed">LBI</ref><ref>{{cite web|url=http://www.ismmed.org/np_altitude_tutorial.htm |title=An Altitude Tutorial |publisher=International Society for Mountain Medicine |access-date=22 June 2011 |archive-url=https://web.archive.org/web/20110719194849/http://www.ismmed.org/np_altitude_tutorial.htm |archive-date=19 July 2011  }}</ref><ref name=MedicalProblems/>

At high altitude, [[atmospheric pressure]] is lower than that at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand.<ref name=nova>{{cite web|url=https://www.pbs.org/wgbh/nova/everest/exposure/pressure.html|title=Atmospheric pressure|work=NOVA Online Everest|publisher=Public Broadcasting Service|access-date=23 January 2009| archive-url= https://web.archive.org/web/20090125053918/http://www.pbs.org/wgbh/nova/everest/exposure/pressure.html| archive-date= 25 January 2009 | url-status= live}}</ref>

===Temperature profile===
The temperature profile of the atmosphere is a result of an interaction between [[radiation]] and [[convection]]. Sunlight in the [[visible spectrum]] hits the ground and heats it. The ground then heats the air at the surface. If [[radiation]] were the only way to transfer heat from the ground to space, the [[greenhouse effect]] of gases in the atmosphere would keep the ground at roughly {{convert|333|K|C F}}, and the temperature would decay exponentially with height.<ref name="goodywilson">LBI</ref>

However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of [[convection]]. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an [[adiabatic process]], which has a characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the [[adiabatic lapse rate]], which is approximately 9.8&nbsp;°C per kilometer (or {{convert|5.4|F-change|C-change|abbr=on|disp=sqbr}} per 1000&nbsp;feet) of altitude.<ref name=goodywilson/>

The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent [[heat of vaporization]]. As air rises and cools, it eventually becomes [[Dew point|saturated]] and cannot hold its quantity of water vapor. The water vapor condenses (forming [[cloud]]s), and releases heat, which changes the lapse rate from the [[dry adiabatic lapse rate]] to the [[moist adiabatic lapse rate]] (5.5&nbsp;°C per kilometer or {{convert|3|F-change|C-change|abbr=on|disp=sqbr}} per 1000&nbsp;feet).
As an average, the International Civil Aviation Organization (ICAO) defines an [[international standard atmosphere]] (ISA) with a temperature [[lapse rate]] of 6.49&nbsp;°C per kilometer (3.56&nbsp;°F per 1,000&nbsp;feet). The actual lapse rate can vary by altitude and by location.

Finally, only the [[troposphere]] (up to approximately {{convert|11|km|ft}} of altitude) in the Earth's atmosphere undergoes notable convection; in the [[stratosphere]], there is little vertical convection.

==Effects on organisms==

===Humans===
Medicine recognizes that altitudes above {{convert|1500|m|ft}} start to affect humans,<ref>{{cite web|title=Non-Physician Altitude Tutorial |publisher=International Society for Mountain Medicine |url=http://www.ismmed.org/np_altitude_tutorial.htm |access-date=22 December 2005 |archive-url=https://web.archive.org/web/20051223065508/http://www.ismmed.org/np_altitude_tutorial.htm |archive-date=23 December 2005  }}</ref> and there is no record of humans living at extreme altitudes above {{convert|5500|-|6000|m|ft}} for more than two years.<ref name="highestHabitation">LBI</ref> As the altitude increases, atmospheric pressure decreases, which affects humans by reducing the [[partial pressure]] of [[oxygen]].<ref name="pubmed">LBI</ref> The lack of oxygen above {{convert|8000|ft|m|order=flip}} can cause serious illnesses such as [[altitude sickness]], [[high altitude pulmonary edema]], and [[high altitude cerebral edema]].<ref name="MedicalProblems">LBI</ref> The higher the altitude, the more likely are serious effects.<ref name=MedicalProblems/> The human body can [[Altitude acclimatization|adapt to high altitude]] by breathing faster, having a higher heart rate, and adjusting its blood chemistry.<ref name="BordenHuman">LBI</ref><ref name="Acclimatization">LBI</ref> It can take days or weeks to adapt to high altitude. However, above {{convert|8000|m|ft|-3}}, (in the "[[death zone]]"), altitude acclimatization becomes impossible.<ref name="PBS">{{cite web|url= https://www.pbs.org/wgbh/nova/transcripts/2506everest.html|title= Everest:The Death Zone|work= Nova|publisher= PBS|date= 24 February 1998}}</ref>

There is a significantly lower overall mortality rate for permanent residents at higher altitudes.<ref>LBI</ref> Additionally, there is a dose response relationship between increasing elevation and decreasing obesity prevalence in the United States.<ref name="Voss">LBI</ref> In addition, the recent hypothesis suggests that high altitude could be protective against Alzheimer's disease via action of erythropoietin, a hormone released by kidney in response to hypoxia.<ref>LBI</ref>
However, people living at higher elevations have a statistically significant higher rate of suicide.<ref name="Brenner">LBI</ref> The cause for the increased suicide risk is unknown so far.<ref name=Brenner/>

====Athletes====
For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, [[long jump]], [[triple jump]]) the reduction in atmospheric pressure signifies less atmospheric resistance, which generally results in improved athletic performance.<ref>LBI</ref> For endurance events (races of 5,000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at high altitude. Sports organizations acknowledge the effects of altitude on performance: the International Association of Athletic Federations (IAAF), for example, marks record performances achieved at an altitude greater than {{convert|1000|m|ft}} with the letter "A".<ref>{{cite web |url=http://www.iaaf.net/mm/Document/06/32/50/63250_PDF_English.pdf |archive-url=https://web.archive.org/web/20131022135302/http://www.iaaf.net/mm/Document/06/32/50/63250_PDF_English.pdf |archive-date=22 October 2013 |title=IAAF World Indoor Lists 2012 |date=9 March 2012 |url-status=usurped |publisher=IAAF Statistics Office   }}</ref>

Athletes also can take advantage of altitude acclimatization to increase their performance. The same changes that help the body cope with high altitude increase performance back at sea level.<ref name="pmid16497842">LBI</ref><ref name="pmid17805094">LBI</ref> These changes are the basis of [[altitude training]] which forms an integral part of the training of athletes in a number of endurance sports including track and field, distance running, triathlon, cycling and swimming.

===Other organisms===
Decreased oxygen availability and decreased temperature make life at high altitude challenging. Despite these environmental conditions, many species have been successfully [[High-altitude adaptation|adapted at high altitudes]]. Animals have developed physiological adaptations to enhance oxygen uptake and delivery to tissues which can be used to sustain metabolism. The strategies used by animals to adapt to high altitude depend on their [[Morphology (biology)|morphology]] and [[phylogeny]]. For example, small mammals face the challenge of maintaining body heat in cold temperatures, due to their small volume to surface area ratio. As oxygen is used as a source of metabolic heat production, the hypobaric hypoxia at high altitudes is problematic.

There is also a general trend of smaller body sizes and lower [[species richness]] at high altitudes, likely due to lower oxygen partial pressures. These factors may decrease [[Productivity (ecology)|productivity]] in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity.

However, some species, such as birds, thrive at high altitude.<ref>LBI</ref> Birds thrive because of physiological features that are advantageous for high-altitude flight.

==See also==
* [[Atmosphere of Earth]]
* [[Coffin corner (aerodynamics)]] At higher altitudes, the air density is lower than at sea level. At a certain altitude it is very difficult to keep an airplane in stable flight.
* [[Geocentric altitude]]
* [[Near space]]

[[Category:Altitude]]
[[Category:Aircraft altitude]]
[[Category:Pressure altitude]]
[[Category:True altitude]]
[[Category:Density altitude]]
[[Category:Altitude measurement]]
[[Category:Barometric altimeters]]
[[Category:Radar altimeters]]
[[Category:Flight levels]]
[[Category:Cruising altitude]]
[[Category:Minimum safe altitude]]
[[Category:Altitude restrictions]]
[[Category:Controlled airspace]]
[[Category:High altitude operations]]
[[Category:Low altitude flight]]
[[Category:Vertical navigation]]
[[Category:Standard atmosphere]]
[[Category:Transition altitude]]
[[Category:Flight planning]]
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[[Category:Aviation safety]]
[[Category:Airspace classification]]
[[Category:IFR operations]]
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[[Category:Mountain flying]]
[[Category:Airspeed and altitude correlation]]
[[Category:Altimeter setting procedures]]
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