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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 Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often use ...
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. The combination of unit of measurement and the physical quantity ( ...
of a location, in
geography Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and Solar System, planets. The first person t ...

geography
the term
elevation The elevation of a geographic Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and Solar Sy ...

elevation
is often preferred for this usage. Vertical distance measurements in the "down" direction are commonly referred to as
depth Depth(s) may refer to: Science * Three-dimensional space * Depth (ring theory), an important invariant of rings and modules in commutative and homological algebra * Depth in a well, the measurement between two points in an oil well * Color 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 (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contain ...
(MSL) or local ground level (above ground level, or AGL) as the reference datum.
Pressure altitudePressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth ...
divided by 100 feet (30 m) is the
flight level In aviation Aviation is the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, l ...

flight level
, and is used above the
transition altitude In aviation Aviation is the activities surrounding mechanical flight Flight or flying is the process by which an object (physics), object motion (physics), moves through a space without contacting any planetary surface, either within an at ...
( in the US, but may be as low as in other jurisdictions); so when the altimeter reads 18,000 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 Inch of mercury (inHg and ″Hg) is a unit of measurement for pressure. It is used for barometric pressure in Weather forecasting, weather reports, refrigeration and aviation in the United States customary units, United States. It is the pressure ...
or 1013.25 
hPa HPA may refer to: Organizations * Harry Potter Alliance The Harry Potter Alliance (also known as the HPA) is a nonprofit organization run primarily by Harry Potter fandom, ''Harry Potter'' fans. It was founded by Andrew Slack in 2005 to draw att ...
). 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 Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference and a point or objec ...

altimeter
, which is an
aneroid barometer Aneroid may mean: * Something devoid of liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluid ...
with a front face indicating distance (feet or metres) instead of
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer A barometer is a scientific instrument that is used to measure air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the ...
. 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 The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph Wireless telegraphy or radiotelegraphy is transmission of electric ...
) * 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), radio altimeter (RALT), electronic altimeter, or reflection altimeter measures altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between ...
(or "absolute altimeter"). Also referred to as "radar height" or feet/metres
above ground levelIn aviation Aviation is the activities surrounding mechanical flight Flight or flying is the process by which an object (physics), object motion (physics), moves through a space without contacting any planetary surface, either within an atmos ...
(AGL). * True altitude is the actual elevation above
mean sea level There are several kinds of mean in mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contain ...
. It is indicated altitude corrected for non-standard temperature and pressure. * Height is the vertical distance above a reference point, commonly the terrain elevation.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 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 Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference and a point or object. The exact definition and reference dat ...
is the altitude corrected for non-ISA
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model A reference atmospheric model describes how the ideal gas An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to i ...

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 pressure Atmospheric pressure, also known as barometric pressure (after the 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 ...

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 atmospheric studies


Atmospheric layers

The
Earth's atmosphere The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...

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: *
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 Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
: surface to at the poles, at the
Equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

Equator
, ending at the Tropopause *
Stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above 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 atmosphe ...

Stratosphere
: Troposphere to *
Mesosphere upright=0.5, Diagram showing the five primary layers of the Earth's atmosphere: exosphere, thermosphere, mesosphere, stratosphere, and troposphere. The layers are to scale. From Earth's surface to the top of the stratosphere (50 km) is just u ...
: Stratosphere to *
Thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, causes /photodissociation of molecules, creating ions; the thermosphere thus constitutes the larger part of the . Taking its name from ...
: 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 density ...
: Thermosphere to The
Kármán line The Kármán line is an attempt to define a boundary between Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose ab ...
, at an altitude of above
sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in th ...

sea level
, by convention defines represents the demarcation between the atmosphere and
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...
. 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 harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

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 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 Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

radiation
and
convection Convection is single or multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

convection
. Sunlight in the
visible spectrum The visible spectrum is the portion of the that is to the . in this range of s is called ' or simply . A typical will respond to wavelengths from about 380 to about 750 . In terms of frequency, this corresponds to a band in the vicinity of ...
hits the ground and heats it. The ground then heats the air at the surface. If
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

radiation
were the only way to transfer heat from the ground to space, the
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., greenhouse gas A greenhou ...

greenhouse effect
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 Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

convection
. 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 Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these qu ...

adiabatic 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 Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the o ...
, 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 Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and ...
. As air rises and cools, it eventually becomes saturated 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 minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

cloud
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 Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the o ...
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. It corresponds to the vertical component o ...
(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 A reference atmospheric model describes how the ideal gas An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to i ...

international standard atmosphere
(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 9 ...
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 Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
(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 The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphe ...

stratosphere
, 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 if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mix ...
of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. 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. Symptom ...
,
high altitude pulmonary edema High-altitude pulmonary edema (HAPE) is a life-threatening form of non-cardiogenic pulmonary edema (fluid accumulation in the lungs) that occurs in otherwise healthy people at altitudes typically above . However, cases have also been reported betwe ...
, 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 diso ...
. 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 Track and field is a sport Sport pertains to any form of Competition, competitive physical activity or game that aims to use, maintain or improve physical ability and skills while providing enjo ...

long jump
,
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 Track and field is a sport Sport pertains to any form of Competition, competitive physical activity or game that aims ...

triple jump
) 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 trainingImage:Swiss Olympic training base.jpg, Altitude training in the Swiss Olympic Training Base in the Alps (elevation ) in St. Moritz. Altitude training is the practice by some endurance Sportsperson, athletes of training for several weeks at high altit ...
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 (graph theory), tree showing the evolutionary relationships among va ...

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 File:Global Amphibian Richness Grids, 2015 Release, All Amphibians (28794889801).jpg, 300px, Global amphibian richness (2015) Species richness is the number of different species represented in an community (ecology), ecological community, landscape ...
at high altitudes, likely due to lower oxygen partial pressures. These factors may decrease
productivity Productivity is the efficiency Efficiency is the (often measurable) ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do thin ...
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

*
Near space File:EarthAtmosphereBig.jpg, upright=0.5, Diagram showing the five primary layers of the Earth's atmosphere: exosphere, thermosphere, mesosphere, stratosphere, and troposphere. The layers are to scale. From Earth's surface to the top of the stratosp ...

Near space
*
Atmosphere of Earth The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (ph ...
*
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 In science Science (from th ...
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. * Vertical metre


References


External links

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



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