Altitude
Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference
is often preferred for this usage.Vertical distance measurements in the "down" direction are commonly referred to as
In aviation
This section needs additional citations for verification. (December 2018) |
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
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 altitude in aviation:
- Indicated altitude is the reading on the altimeter when it is set to the QNH)[2]
- Absolute altitude is the vertical distance of the aircraft above the terrain over which it is flying.above ground level(AGL).
- True altitude is the actual elevation above mean sea level.[3]: 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 QFE)[2]
- 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 pressurein 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 satellite orbits
- Transatmospheric orbit (TAO)
- Geocentric orbits with altitudes at perigee that intersects with the defined atmosphere.[4]
- Low Earth orbit (LEO)
- Geocentric orbits ranging in altitude from 160 km (100 mi) to 2,000 km (1,200 mi) above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8 km/s (26,000 ft/s).
- Medium Earth orbit (MEO)
- Geocentric orbits with altitudes at apogee ranging between 2,000 km (1,200 mi) and that of the geosynchronous orbit at 35,786 km (22,236 mi).
- Geosynchronous orbit (GEO)
- Geocentric circular orbit with an altitude of 35,786 km (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3 km/s (9,800 ft/s).
- High Earth orbit (HEO)
- Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit, where altitude at perigee is less than 2,000 km (1,200 mi).[5]
In atmospheric studies
Atmospheric layers
The
- Troposphere: surface to 8,000 metres (5.0 mi) at the poles, 18,000 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 10,000 kilometres (6,200 mi)
The Kármán line, at an altitude of 100 kilometres (62 mi) above sea level, by convention defines represents the demarcation between the atmosphere and space.[7] 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 2,400 meters (8,000 ft) above sea level.[8][9][10]
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.[11]
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 333 K (60 °C; 140 °F), and the temperature would decay exponentially with height.[12]
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
The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent
Finally, only the troposphere (up to approximately 11 kilometres (36,000 ft) of altitude) in the Earth's atmosphere undergoes notable convection; in the stratosphere, there is little vertical convection.[15]
Effects on organisms
Humans
Medicine recognizes that altitudes above 1,500 metres (4,900 ft) start to affect humans,
There is a significantly lower overall mortality rate for permanent residents at higher altitudes.[22] Additionally, there is a dose response relationship between increasing elevation and decreasing obesity prevalence in the United States.[23] 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.[24] However, people living at higher elevations have a statistically significant higher rate of suicide.[25] The cause for the increased suicide risk is unknown so far.[25]
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.[26] 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 1,000 metres (3,300 ft) with the letter "A".[27]
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.[28][29] 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
There is also a general trend of smaller body sizes and lower species richness at high altitudes, likely due to lower oxygen partial pressures.[30] These factors may decrease productivity in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity.[31]
However, some species, such as birds, thrive at high altitude.[32] 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
References
- ^ "The Stratosphere - overview". scied.ucar.edu. University Corporation for Atmospheric Research. Retrieved 6 February 2021.
- ^ ISBN 978-0-86039-601-7. CAP413.
- ^ a b c Air Navigation. Department of the Air Force. 1 December 1989. AFM 51-40.
- ^ McDowell, Jonathan (24 May 1998). "Jonathan's Space Report".
Transatmospheric orbit (TAO): orbital flight with perigee less than 80 km but more than zero. Potentially used by aerobraking missions and transatmospheric vehicles, also in some temporary phases of orbital flight (e.g. STS pre OMS-2, some failures when no apogee restart)
- ^ Definitions of geocentric orbits from the Goddard Space Flight Center Archived May 27, 2010, at the Wayback Machine
- ^ "Layers of the Atmosphere". JetStream, the National Weather Service Online Weather School. National Weather Service. Archived from the original on 19 December 2005. Retrieved 22 December 2005.
- ^ Dr. S. Sanz Fernández de Córdoba (24 June 2004). "The 100 km Boundary for Astronautics". Fédération Aéronautique Internationale. Archived from the original on 9 August 2011.
- ISBN 978-0-470-18928-3. Archived from the originalon 8 December 2011. Retrieved 27 April 2010.
- ^ "An Altitude Tutorial". International Society for Mountain Medicine. Archived from the original on 19 July 2011. Retrieved 22 June 2011.
- ^ a b c Cymerman, A; Rock, PB (1994). "Medical Problems in High Mountain Environments. A Handbook for Medical Officers". USARIEM-TN94-2. U.S. Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report. Archived from the original on 23 April 2009. Retrieved 5 March 2009.
{{cite journal}}
: Cite journal requires|journal=
(help)CS1 maint: unfit URL (link) - ^ "Atmospheric pressure". NOVA Online Everest. Public Broadcasting Service. Archived from the original on 25 January 2009. Retrieved 23 January 2009.
- ^ a b Goody, Richard M.; Walker, James C.G. (1972). "Atmospheric Temperatures" (PDF). Atmospheres. Prentice-Hall. Archived from the original (PDF) on 29 July 2016. Retrieved 2 May 2016.
- ^ "Dry Adiabatic Lapse Rate". tpub.com. Archived from the original on 3 June 2016. Retrieved 2 May 2016.
- ISBN 978-92-9194-004-2. Doc 7488-CD.
- ^ "The stratosphere: overview". UCAR. Retrieved 2 May 2016.
- ^ "Non-Physician Altitude Tutorial". International Society for Mountain Medicine. Archived from the original on 23 December 2005. Retrieved 22 December 2005.
- PMID 12631426.
- PMID 9774298.
- ^ Young, Andrew J.; Reeves, John T. (2002). "21". Human Adaptation to High Terrestrial Altitude. In: Medical Aspects of Harsh Environments. Vol. 2. Borden Institute, Washington, DC. Archived from the original on 11 January 2009.
{{cite book}}
: CS1 maint: location missing publisher (link) - ^ Muza, SR; Fulco, CS; Cymerman, A (2004). "Altitude Acclimatization Guide". U.S. Army Research Inst. Of Environmental Medicine Thermal and Mountain Medicine Division Technical Report (USARIEM–TN–04–05). Archived from the original on 23 April 2009. Retrieved 5 March 2009.
{{cite journal}}
: CS1 maint: unfit URL (link) - ^ "Everest:The Death Zone". Nova. PBS. 24 February 1998.
- PMID 21452955.
- PMID 23357956.
- S2CID 32245379.
- ^ PMID 21214344.
- PMID 6643537.
- ^ "IAAF World Indoor Lists 2012" (PDF). IAAF Statistics Office. 9 March 2012. Archived from the original (PDF) on 22 October 2013.
- PMID 16497842.
- PMID 17805094.
- S2CID 484645.
- .
- S2CID 16820157.
External links
- "Altitude pressure calculator". Apex (altitude physiology expeditions). Retrieved 8 August 2006.
- "The Race to the Stratosphere". U.S. Centennial of Flight Commission. Archived from the original on 9 March 2006. Retrieved 25 January 2006.
- Downloadable ETOPO2 Raw Data Database (2 minute grid)
- Downloadable ETOPO5 Raw Data Database (5 minute grid)