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For other uses see Altitude (disambiguation)
Altitude is the elevation of an object from a known level or datum. Common datums are mean sea level and the surface of the WGS-84 geoid, used by GPS. In the United States and the UK aviation altitude is usually measured in feet. Everywhere else in the world the altitude is measured in metres.
Atmospheric pressure decreases as altitude increases. This principle is the basis of operation of the pressure altimeter, which is an aneroid barometer calibrated to indicate altitude instead of pressure. It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude. (Altitude pressure calculator)
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Altitude in aviation
In aviation, the term altitude can have several meanings. It is a fundamental tenet of flight safety that both parties exchanging information concerning this topic are absolutely clear which definition is being used. (AFM 51-40)
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) (CAP413)
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) (CAP413)
Indicated altitude is the reading on the altimeter.
Pressure altitude is the elevation above a standard datum plane (typically, 1013.2 millibars). Pressure altitude divided by 100 feet is referred to as the flight level; so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". Below FL180, 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 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.
Absolute altitude is the height of the aircraft above the terrain over which it is flying.
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Altitude regions
Mountain medicine recognizes three altitude regions: (Non-Physician Altitude Tutorial)
High altitude = 1500 m – 3500 m
Very High altitude = 3500 m – 5500 m
Extreme altitude = 5500 m – above
Travel to high altitudes leads to a range of medical problems, from the relatively 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: (Layers of the Atmosphere)
Troposphere — surface to 8 km (above poles) – 18 km (above equator), ending at the Tropopause.
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Altitude records
1783 August — 24 m Jean Francois Pilatre in a hot-air balloon.
1783 1 December, 1783 — 610 m Professor Charles and assistant Robert in Charliere, his hydrogen-filled balloon.
1783 1 December, 1783 — 2.7 km Professor Charles in Charliere, his hydrogen-filled balloon.
1862 5 September, 1862 — 11.887 km Coxwell and English physicist Glaisher in a balloon.
1927 November 1927 — 13.222 km Captain Hawthorne C. Gray of the US Army Air Corps. in a balloon.
1931 27 May 1931 — 15.787 km Auguste Piccard & Paul Kipfer in a hydrogen balloon.
1932 — 16.2 km Auguste Piccard & Max Cosyns in a hydrogen balloon.
1933 30 September — 18.501 km USSR balloon.
1933 20 November — 18.592 km Auguste & Jean Piccard in Century of Progress balloon.
1934 30 January — 21.946 km USSR balloon.
1935 10 November — 22.066 km Anderson & Stevens in Explorer II.
1961 4 May — 34.668 km Victor Prather & Malcolm Ross of the US Navy in Strato-Lab V, a zero-pressure balloon.
1962 17 July — 95.94 km Robert White in an X-15 aircraft.
1963 19 July — 106.01 km Joseph Walker in an X-15 aircraft.
1963 22 August — 107.96 km Joseph Walker in an X-15 aircraft.
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Reference
Washington Post; June 18, 1910; Indianapolis, Indiana, June 17, 1910. Walter Brookins, in a Wright biplane, broke the world's aeroplane record for altitude today, when he soared to a height of 4,603 feet, according to the measurement of the altimeter. His motor stopped as he was descending, and he made a glide of 2 miles, landing easily in a wheat field.
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See also
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