Altitude
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'''Altitude''' is the [[elevation]] of a point or object from a known level or [[datum (geodesy)|datum]] (plural: data). Common data are [[mean sea level]], local ground level (Above Ground Level, or AGL), or the surface of the [[WGS-84]] [[geoid]], used by [[GPS]]. In [[aviation]], altitude is measured in [[Foot (unit of length)|feet]]. For non-aviation uses, altitude may be measured in other units such as [[metre]]s or miles.
[[Atmospheric pressure]] decreases as altitude increases. This principle is the basis of operation of the pressure [[altimeter]], which is an [[barometer#Aneroid barometers|aneroid barometer]] calibrated to indicate altitude instead of pressure. It is the fall in pressure that leads to a shortage of oxygen ([[Hypoxia (medical)|hypoxia]]) in humans on ascent to high altitude.
==Altitude in aviation==
[[Image:Vertical distances.png|thumb|300px|Vertical distances.]]
In aviation, the term altitude can have several meanings, and is therefore qualified by either explicitly adding a modifier (e.g. "true altitude"), or implicitly through the context of the communication. Parties exchanging information concerning this topic must be clear which definition is being used.<ref name="AFM_51-40> {{cite book | date=[[1 December]] [[1989]] | title=Air Navigation |publisher=Department of the Air Force| id=AFM 51-40 }}</ref>
*'''True altitude''' is the elevation above [[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=0860396010 }}</ref>
*'''[[height]]''' is the elevation above a ground 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 [[QFE]])<ref name="CAP413" />
*'''Absolute altitude''' is the height of the aircraft above the terrain over which it is flying.
*'''Indicated altitude''' is the reading on the altimeter.
*'''[[Pressure altitude]]''' is the elevation above a standard datum plane (typically, 1013.25 millibars or 29.92" Hg and 15°C). Pressure altitude divided by 100 feet is referred to as the [[flight level]], and is used above the [[transition altitude]] (18,000 feet in the US, but may be as low as 3,000ft in other countries); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". When flying at a Flight Level, the altimeter is always set to standard pressure (29.92 / 1013.2). Below the [[transition level]], altitudes are read in thousands, pronounced "one three thousand" for 13,000, "seven thousand" for 7,000 etc.
*'''[[Density altitude]]''' is the altitude corrected for non-ISA [[International Standard Atmosphere]] (ISA) conditions at which the air density is unequal to ISA 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 may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.
==Altitude regions==
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.
Mountain medicine recognizes three altitude regions:<ref>{{cite web | title=Non-Physician Altitude Tutorial | publisher=International Society for Mountain Medicine | url=http://www.ismmed.org/np_altitude_tutorial.htm | accessmonthday=22 December | accessyear=2005}}</ref>
* High altitude = 1500 m – 3500 m (5000 – 11,500 ft)
* Very High altitude = 3500 m – 5500 m (11,500 – 18,000 ft)
* Extreme altitude = 5500 m – above
Travel to high altitudes can lead to medical problems, from the mild symptoms of [[acute mountain sickness]] to the potentially fatal [[high altitude pulmonary oedema]] ([[HAPE]]) and [[high altitude cerebral oedema]] ([[HACE]]). These conditions are caused by the profound hypoxia associated with travel to high altitudes.
The [[Earth's atmosphere]] is divided into several altitude regions:<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 | accessmonthday=22 December | accessyear=2005}}</ref>
* [[Troposphere]] — surface to 8000 m / 5 miles at poles – 18.000 m / 11 miles at [[equator]], ending at the Tropopause.
* [[Stratosphere]] — Tropopause to 50 km /31 miles
* [[Mesosphere]] — Stratopause to 85 km /53 miles
* [[Thermosphere]] — Mesopause to 675 km / 420 miles
* [[Exosphere]] — Thermopause to 10,000 km /6200 miles
==References==
<references />
==External links==
*{{cite web| title=Altitude pressure calculator | publisher=Apex (altitude physiology expeditions) | url=http://www.altitude.org/calculators/airpressure.htm | accessdate=8th August | accessyear=2006}}
*
*{{cite web | title=The Race to the Stratosphere | publisher=U.S. Centennial of Flight Commission | url=http://www.centennialofflight.gov/essay/Lighter_than_air/race_to_strato/LTA11.htm | accessmonthday=25 January | accessyear=2006 }}
*[http://www.ngdc.noaa.gov/mgg/fliers/01mgg04.html Downloadable ETOPO2 Raw Data Database (2 minute grid)]
*[http://www.ngdc.noaa.gov/mgg/fliers/93mgg01.html Downloadable ETOPO5 Raw Data Database (5 minute grid)]
=See also=
* [[Altitude sickness]]
* [[Death zone]]
* [[Flight altitude record]]
* [[High altitude pulmonary edema]]
* [[High altitude cerebral edema]]
[[Category:Physical geography]]
[[Category:Topography]]
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