For other uses see Altitude (disambiguation).

Altitude or height is defined based on the context in which it is used (aviation geometry geographical survey sport and more). As a general definition altitude is a distance measurement usually in the vertical or "up" direction between a reference datum and a point or object. The reference datum also often varies according to the context. 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.

Vertical distance measurements in the "down" direction are commonly referred to as depth. Contents 1 Altitude in aviation and in spaceflight 2 Altitude regions 3 High altitude and low air pressure 3.1 Relation between temperature and altitude in Earth's atmosphere 3.2 Effects of high altitude on humans 4 References 5 External links 6 See also Altitude in aviation and in spaceflight Vertical Distance Comparison

In aviation the term altitude can have several meanings and is always qualified by either 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.1

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 (30m) is referred to as the flight level and is used above the transition altitude (18000 feet (5500 m) in the US but may be as low as 3000 feet (910 m) in other jurisdictions); so when the altimeter reads 18000 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 inHg / 1013.25 mbar).

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

There are several types of aviation altitude: Indicated altitude is the reading on the altimeter when the altimeter is set to the local barometric pressure at Mean Sea Level. Absolute altitude is the height of the aircraft above the terrain over which it is flying. Also referred to feet/metres Above Ground Level (AGL). 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)2 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)2 Pressure altitude is the elevation above a standard datum air-pressure plane (typically 1013.25 millibars or 29.92" Hg and 15C). Pressure altitude and indicated altitude are the same when the altimeter is set to 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 -- what the altimeter says Absolute altitude -- altitude in terms of the distance above the ground directly below it True altitude -- altitude in terms of elevation above sea level Height -- altitude in terms of the distance above a certain point Pressure altitude -- altitude in terms of the air pressure Density altitude -- altitude in terms of the density of the air Altitude regions

The Earth's atmosphere is divided into several altitude regions:3 Troposphere surface to 8000 metres (5.0 mi) at the poles 18000 metres (11 mi) at the equator ending at the Tropopause. Stratosphere Troposphere to 50 kilometres (31 mi) Mesosphere Stratosphere to 85 kilometres (53 mi) Thermosphere Mesosphere to 675 kilometres (419 mi) Exosphere Thermosphere to 10000 kilometres (6200 mi) High altitude and low air 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 2400 metres (8000 ft) above sea level.4 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.5 Because of the lower pressure the air expands as it rises which causes it to cool.67 Thus high altitude air is cold which causes a characteristic alpine climate. This climate dramatically affects the ecology at high altitude. Relation between temperature and altitude in Earth's atmosphere Main article: Lapse rate The environmental lapse rate (ELR) is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location. As an average the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 K(C)/1000 m (3.56 F or 1.98 K(C)/1000 Ft) from sea level to 11 kilometres (36000 ft). From 11 to 20 kilometres (36000 to 66000 ft) the constant temperature is 56.5 C (69.7 F) which is the lowest assumed temperature in the ISA. The standard atmosphere contains no moisture. Unlike the idealized ISA the temperature of the actual atmosphere does not always fall at a uniform rate with height. For example there can be an inversion layer in which the temperature increases with height. Effects of high altitude on humans Main article: Effects of high altitude on humans The lower atmospheric pressure affects humans (and other animals) due to the decrease in the partial pressure of oxygen.8 This lack of oxygen can cause potentially fatal illnesses such as altitude sickness High altitude pulmonary edema and High altitude cerebral edema.9 The human body can adapt to high altitude by breathing faster having a higher heart rate and adjusting its blood chemistry.1011 It can take days or weeks to adapt to high altitude. However above 8000 metres (26000 ft) (in the "death zone") the human body cannot adapt and will eventually die.12 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 means there is less resistance from the atmosphere and the athlete's performance will generally be better at high altitude.13 For endurance events (races of 5000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at high altitude. Sports organisations acknowledge the effects of altitude on performance: the International Association of Athletic Federations (IAAF) for example have ruled that performances achieved at an altitude greater than 1000 metres (3300 ft) will not be approved for record purposes. 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.1415 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. References Air Navigation. Department of the Air Force. 1 December 1989. AFM 51-40.  a b Radiotelephony Manual. UK Civil Aviation Authority. 1 January 1995. ISBN 0-86039-601-0. CAP413.  "Layers of the Atmosphere". JetStream the National Weather Service Online Weather School. National Weather Service. http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm. Retrieved 22 December 2005.  Webster's New World Medical Dictionary. Wiley. 2008. ISBN 978-0-470-18928-3. http://www.medterms.com/script/main/art.asparticlekey8578.  "Atmospheric pressure". NOVA Online Everest. Public Broadcasting Service. http://www.pbs.org/wgbh/nova/everest/exposure/pressure.html. Retrieved January 23 2009.  Mark Zachary Jacobson (2005). Fundamentals of Atmospheric Modelling (2nd ed.). Cambridge University Press. ISBN 0-521-83970-X.  C. Donald Ahrens (2006). Meteorology Today (8th ed.). Brooks/Cole Publishing. ISBN 0-495-01162-2.  Peacock Andrew J (October 17 1998). "Oxygen at high altitude". British Medical Journal 317 (7165): 10631066. PMC 1114067. PMID 9774298.  Cymerman A; Rock PB. Medical Problems in High Mountain Environments. A Handbook for Medical Officers. USARIEM-TN94-2. US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report. http://archive.rubicon-foundation.org/7976. Retrieved 2009-03-05.  Young Andrew J. and Reeves John T. (2002). "21". Human Adaptation to High Terrestrial Altitude. In: Medical Aspects of Harsh Environments. 2. Washington DC. http://www.bordeninstitute.army.mil/publishedvolumes/harshEnv2/harshEnv2.html. Retrieved 2009-01-05.  Muza SR; Fulco CS; Cymerman A (2004). "Altitude Acclimatization Guide.". US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report (USARIEM-TN-04-05). http://archive.rubicon-foundation.org/7616. Retrieved 2009-03-05.  "Everest:The Death Zone". Nova. PBS. 1998-02-24. http://www.pbs.org/wgbh/nova/transcripts/2506everest.html.  Ward-Smith AJ (1983). "The influence of aerodynamic and biomechanical factors on long jump performance". Journal of Biomechanics 16 (8): 655658. doi:10.1016/0021-9290(83)90116-1. PMID 6643537.  Wehrlin JP Zuest P Halln J Marti B (June 2006). "Live hightrain low for 24 days increases hemoglobin mass and red cell volume in elite endurance athletes". J. Appl. Physiol. 100 (6): 193845. doi:10.1152/japplphysiol.01284.2005. PMID 16497842. http://jap.physiology.org/cgi/pmidlookupviewlong&pmid16497842. Retrieved 2009-03-05.  Gore CJ Clark SA Saunders PU (September 2007). "Nonhematological mechanisms of improved sea-level performance after hypoxic exposure". Med Sci Sports Exerc 39 (9): 16009. doi:10.1249/mss.0b013e3180de49d3. PMID 17805094. http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htman00005768-200709000-00023. Retrieved 2009-03-05.  External links "Altitude pressure calculator". Apex (altitude physiology expeditions). http://www.altitude.org/airpressure.php. Retrieved 2006-08-08.  "The Race to the Stratosphere". U.S. Centennial of Flight Commission. http://www.centennialofflight.gov/essay/Lighterthanair/racetostrato/LTA11.htm. Retrieved 25 January 2006.  Downloadable ETOPO2 Raw Data Database (2 minute grid) Downloadable ETOPO5 Raw Data Database (5 minute grid) Calculate true altitude with these JavaScript applications Find the altitude of any place See also Acronyms and abbreviations in avionics Flight altitude record High altitude wind power