Altitude

Source: Wikipedia, the free encyclopedia.

Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference

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
.

In aviation

air density there is roughly 11000 of that in the troposphere.[1]
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.

inHg or 1013.25 hPa
).

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:

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 satellite orbits

Low (cyan) and Medium (yellow) Earth orbit regions to scale. The black dashed line is the geosynchronous orbit. The green dashed line is the 20,230 km orbit used for GPS satellites.
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

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:[6]

  • 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

adiabatic lapse rate, which is approximately 9.8 °C per kilometer (or 5.4 °F [3.0 °C] per 1000 feet) of altitude.[12]

The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent

moist adiabatic lapse rate (5.5 °C per kilometer or 3 °F [1.7 °C] per 1000 feet).[13]
As an average, the International Civil Aviation Organization (ICAO) defines an
international standard atmosphere (ISA) with a temperature lapse rate of 6.49 °C per kilometer (3.56 °F per 1,000 feet).[14]
The actual lapse rate can vary by altitude and by location.

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,

adapt to high altitude by breathing faster, having a higher heart rate, and adjusting its blood chemistry.[19][20] It can take days or weeks to adapt to high altitude. However, above 8,000 metres (26,000 ft), (in the "death zone"), altitude acclimatization becomes impossible.[21]

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

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.[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

References

  1. ^ "The Stratosphere - overview". scied.ucar.edu. University Corporation for Atmospheric Research. Retrieved 6 February 2021.
  2. ^ . CAP413.
  3. ^ a b c Air Navigation. Department of the Air Force. 1 December 1989. AFM 51-40.
  4. ^ 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)
  5. ^ Definitions of geocentric orbits from the Goddard Space Flight Center Archived May 27, 2010, at the Wayback Machine
  6. ^ "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.
  7. ^ 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.
  8. ISBN 978-0-470-18928-3. Archived from the original
    on 8 December 2011. Retrieved 27 April 2010.
  9. ^ "An Altitude Tutorial". International Society for Mountain Medicine. Archived from the original on 19 July 2011. Retrieved 22 June 2011.
  10. ^ 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)
  11. ^ "Atmospheric pressure". NOVA Online Everest. Public Broadcasting Service. Archived from the original on 25 January 2009. Retrieved 23 January 2009.
  12. ^ 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.
  13. ^ "Dry Adiabatic Lapse Rate". tpub.com. Archived from the original on 3 June 2016. Retrieved 2 May 2016.
  14. . Doc 7488-CD.
  15. ^ "The stratosphere: overview". UCAR. Retrieved 2 May 2016.
  16. ^ "Non-Physician Altitude Tutorial". International Society for Mountain Medicine. Archived from the original on 23 December 2005. Retrieved 22 December 2005.
  17. PMID 12631426
    .
  18. .
  19. ^ 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)
  20. ^ 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)
  21. ^ "Everest:The Death Zone". Nova. PBS. 24 February 1998.
  22. PMID 21452955
    .
  23. .
  24. .
  25. ^ .
  26. .
  27. ^ "IAAF World Indoor Lists 2012" (PDF). IAAF Statistics Office. 9 March 2012. Archived from the original (PDF) on 22 October 2013.
  28. PMID 16497842
    .
  29. .
  30. .
  31. .
  32. .

External links