High Altitude
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Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference
datum In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...
and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometry, geographical survey, sport, or atmospheric pressure). Although the term ''altitude'' is commonly used to mean the
height above sea level Height above mean sea level is a measure of the vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as ''orthometric heights''. The com ...
of a location, in
geography Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and ...
the term
elevation The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
is often preferred for this usage. Vertical distance measurements in the "down" direction are commonly referred to as depth.


In aviation

In aviation, 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 There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
(MSL) or local ground level (above ground level, or AGL) as the reference datum.
Pressure altitude Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question. The National Oceanic and Atmospheric Administration (NOAA) published the follow ...
divided by 100 feet (30 m) is the
flight level In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa ...
, and is used above the
transition altitude In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa ...
( in the US, but may be as low as in other jurisdictions). So when the altimeter reads the country-specific flight level on the standard pressure setting the aircraft is said to be at "Flight level XXX/100" (where XXX is the transition altitude). When flying at a flight level, the altimeter is always set to standard pressure (29.92 
inHg Inch of mercury (inHg and ″Hg) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury in heigh ...
or 1013.25 
hPa HPA may refer to: Organizations * Harry Potter Alliance, a charity * Halifax Port Authority, Canada * Hamburg Port Authority, Germany * Hawaii Preparatory Academy, a school in Hawaii, US * Health Protection Agency, UK * Heerespersonalamt, the Ger ...
). On the flight deck, the definitive instrument for measuring altitude is the pressure
altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
, which is an
aneroid barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
with a front face indicating distance (feet or metres) instead of
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
. There are several types of altitude in aviation: * Indicated altitude is the reading on the altimeter when it is set to the local barometric pressure at 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) * Absolute altitude is the vertical distance of the aircraft above the terrain over which it is flying. It can be measured using a
radar altimeter A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
(or "absolute altimeter"). Also referred to as "radar height" or feet/metres
above ground level In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL or HAGL) is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level (AMSL or HAMSL), height above elli ...
(AGL). * True altitude is the actual elevation above
mean sea level There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
. 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) * 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 The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as ...
is the altitude corrected for non-ISA
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...
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 pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The Standard atmosphere (unit), standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equival ...
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


In atmospheric studies


Atmospheric layers

The
Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
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: *
Troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
: surface to at the poles, at the
Equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
, ending at the Tropopause *
Stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
: Troposphere to *
Mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
: Stratosphere to *
Thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...
: Mesosphere to *
Exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densit ...
: Thermosphere to The
Kármán line The Kármán line (or von Kármán line ) is an attempt to define a boundary between Earth's atmosphere and outer space, and offers a specific definition set by the Fédération aéronautique internationale (FAI), an international record-keeping ...
, at an altitude of above
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
, by convention defines represents the demarcation between the atmosphere and
space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
. 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 Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
'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 above sea level. 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.


Temperature profile

The temperature profile of the atmosphere is a result of an interaction between
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
and
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. Sunlight in the
visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
hits the ground and heats it. The ground then heats the air at the surface. If
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
were the only way to transfer heat from the ground to space, the
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
of gases in the atmosphere would keep the ground at roughly , and the temperature would decay exponentially with height. 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
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an
adiabatic process In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, ...
, which has a characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the
adiabatic lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
, which is approximately 9.8 °C per kilometer (or per 1000 feet) of altitude. Note that the presence of water in the atmosphere complicates the process of convection. Water vapor contains latent
heat of vaporization The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
. As air rises and cools, it eventually becomes
saturated Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds ** Saturated and unsaturated compounds **Degree of unsaturation ** Saturated fat or fatty ac ...
and cannot hold its quantity of water vapor. The water vapor condenses (forming
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may co ...
s), and releases heat, which changes the lapse rate from the
dry adiabatic lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
to the
moist adiabatic lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
(5.5 °C per kilometer or per 1000 feet). As an average, the International Civil Aviation Organization (ICAO) defines an
international standard atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...
(ISA) with a temperature
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
of 6.49 °C per kilometer (3.56 °F per 1,000 feet). The actual lapse rate can vary by altitude and by location. Finally, note that only the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
(up to approximately of altitude) in the Earth's atmosphere undergoes notable convection; in the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
, there is little vertical convection.


Effects on organisms


Humans

Medicine recognizes that altitudes above start to affect humans, and there is no record of humans living at extreme altitudes above for more than two years. As the altitude increases, atmospheric pressure decreases, which affects humans by reducing the
partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
. The lack of oxygen above can cause serious illnesses such as
altitude sickness Altitude sickness, the mildest form being acute mountain sickness (AMS), is the harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. People can respond to high altitude in different ways. Sympt ...
,
high altitude pulmonary edema High-altitude pulmonary edema (HAPE) is a life-threatening form of non-cardiogenic pulmonary edema that occurs in otherwise healthy people at altitudes typically above . However, cases have also been reported between in more vulnerable subjects. ...
, and
high altitude cerebral edema High-altitude cerebral edema (HACE) is a medical condition in which the brain swells with fluid because of the physiological effects of traveling to a high altitude. It generally appears in patients who have acute mountain sickness and involves di ...
. The higher the altitude, the more likely are serious effects. The human body can adapt to high altitude by breathing faster, having a higher heart rate, and adjusting its blood chemistry. It can take days or weeks to adapt to high altitude. However, above , (in the "
death zone In mountaineering, the death zone refers to altitudes above a certain point where the pressure of oxygen is Effects of high altitude on humans, insufficient to sustain human life for an extended time span. This point is generally tagged as , less ...
"), altitude acclimatization becomes impossible. There is a significantly lower overall mortality rate for permanent residents at higher altitudes. Additionally, there is a dose response relationship between increasing elevation and decreasing obesity prevalence in the United States. 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. However, people living at higher elevations have a statistically significant higher rate of suicide. The cause for the increased suicide risk is unknown so far.


Athletes

For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres,
long jump The long jump is a track and field event in which athletes combine speed, strength and agility in an attempt to leap as far as possible from a takeoff point. Along with the triple jump, the two events that measure jumping for distance as a gr ...
,
triple jump The triple jump, sometimes referred to as the hop, step and jump or the hop, skip and jump, is a track and field event, similar to the long jump. As a group, the two events are referred to as the "horizontal jumps". The competitor runs down th ...
) the reduction in atmospheric pressure signifies less atmospheric resistance, which generally results in improved athletic performance. 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 with the letter "A". 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. These changes are the basis of
altitude training Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-altitude locations. A ...
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 adapted at high altitudes. Animals have developed physiological adaptations to enhance oxygen uptake and delivery to tissues which can be used to sustain metabolism. The strategies used by animals to adapt to high altitude depend on their
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
and
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
. 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 Species richness is the number of different species represented in an ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the abundances of the species or their relative a ...
at high altitudes, likely due to lower oxygen partial pressures. These factors may decrease
productivity Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...
in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity. However, some species, such as birds, thrive at high altitude. Birds thrive because of physiological features that are advantageous for high-altitude flight.


See also

*
Atmosphere of Earth The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
*
Coffin corner (aerodynamics) Coffin corner (also known as the aerodynamic ceiling or Q corner) is the region of flight where a fast but subsonic fixed-wing aircraft's stall speed is near the critical Mach number, at a given gross weight and G-force loading. In this regio ...
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 In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, and ...
*
Near space The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...


References


External links

* *
Downloadable ETOPO2 Raw Data Database (2 minute grid)



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