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 ellipsoid (HAE, as reported by a GPS receiver), or height above average terrain (AAT or HAAT, in broadcast engineering ). In other words, these expressions (AGL, AMSL, HAE, AAT) indicate where the "zero level" or "reference altitude" – the vertical datum – is located.
33-398: HAGL may refer to: Height above ground level Hoang Anh Gia Lai Group , a Vietnamese company Hoàng Anh Gia Lai F.C. , an association football club Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title HAGL . If an internal link led you here, you may wish to change
66-709: A moon of Saturn, and Triton , a moon of Neptune, have atmospheres mainly of nitrogen . When in the part of its orbit closest to the Sun, Pluto has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when it is farther from the Sun. Other bodies within the Solar System have extremely thin atmospheres not in equilibrium. These include the Moon ( sodium gas), Mercury (sodium gas), Europa (oxygen), Io ( sulfur ), and Enceladus ( water vapor ). The first exoplanet whose atmospheric composition
99-502: A particular point (e.g. the airport, runway threshold , or ground at present location), and "elevation" describes a feature of the terrain itself in terms of distance above MSL. In weather and climate studies, measurements or simulations often need to refer to a specific height or altitude, which is naturally AGL. However, the values of geophysical variables measured in various places on the natural (ground) surface may not be easily compared in hilly or mountainous terrain, because part of
132-508: A planet from atmospheric escape and that for some magnetizations the presence of a magnetic field works to increase the escape rate. Other mechanisms that can cause atmosphere depletion are solar wind -induced sputtering, impact erosion, weathering , and sequestration—sometimes referred to as "freezing out"—into the regolith and polar caps . Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that
165-467: A reference. During approaches to landing, there are several other references that are used, including AFE (above field elevation) which is height referencing the highest point on the airfield, TDZE (touchdown zone elevation) or TH (threshold height) which both refer to the elevation of the landing end of the runway measured AMSL and AGL respectively. In general, "altitude" refers to distance above mean sea level (MSL or AMSL), "height" refers to distance above
198-451: A variable amount of water vapor is also present, on average about 1% at sea level. The low temperatures and higher gravity of the Solar System's giant planets — Jupiter , Saturn , Uranus and Neptune —allow them more readily to retain gases with low molecular masses . These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds. Two satellites of the outer planets possess significant atmospheres. Titan ,
231-464: A wide range of velocities, there will always be some fast enough to produce a slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermal kinetic energy , and so gases of low molecular weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have lost much of their water when, after being photodissociated into hydrogen and oxygen by solar ultraviolet radiation,
264-423: Is covered in craters . Without an atmosphere, the planet has no protection from meteoroids , and all of them collide with the surface as meteorites and create craters. For planets with a significant atmosphere, most meteoroids burn up as meteors before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind. Wind erosion is a significant factor in shaping
297-625: Is originally determined by the stellar nebula's chemistry and temperature, but can also by a product processes within the astronomical body outgasing a different atmosphere. The atmospheres of the planets Venus and Mars are principally composed of carbon dioxide and nitrogen , argon and oxygen . The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally
330-401: Is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the stratosphere . The troposphere contains 75–80% of the mass of the atmosphere, and is the atmospheric layer wherein the weather occurs; the height of the troposphere varies between 17 km at the equator and 7.0 km at the poles. The stratosphere extends from the top of the troposphere to
363-420: Is the only important measurement for aviation authorities, which require that some tall towers have proper aircraft warning paint and lights to avoid collisions . Atmosphere An atmosphere (from Ancient Greek ἀτμός ( atmós ) 'vapour, steam' and σφαῖρα ( sphaîra ) 'sphere') is a layer of gases that envelop an astronomical object , held in place by
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#1732794050044396-499: Is used to make nucleotides and amino acids ; plants , algae , and cyanobacteria use carbon dioxide for photosynthesis . The layered composition of the atmosphere minimises the harmful effects of sunlight , ultraviolet radiation, solar wind , and cosmic rays and thus protects the organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of
429-425: The broadcast range of a station. Rather, it is HAAT (the height above the average terrain (in the surrounding area)) which is used to determine how far a broadcast station (or any other sort of VHF or higher radio-frequency ) transmission will travel. From aviation safety perspective though, the more important aspect is the height of the radio tower used to support the radio antenna . In this case, height AGL
462-596: The gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere ; stars of low temperature might have outer atmospheres containing compound molecules . The atmosphere of Earth is composed of nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.04%) and trace gases. Most organisms use oxygen for respiration ; lightning and bacteria perform nitrogen fixation which produces ammonia that
495-477: The paleoatmosphere by living organisms. Atmospheres are clouds of gas bound to and engulfing an astronomical focal point of sufficiently dominating mass , adding to its mass, possibly escaping from it or collapsing into it. Because of the latter, such planetary nucleus can develop from interstellar molecular clouds or protoplanetary disks into rocky astronomical objects with varyingly thick atmospheres, gas giants or fusors . Composition and thickness
528-399: The aircraft above ground. This is done by communicating with the control tower of the airport (to get the current surface pressure) and setting the altimeter so as to read zero on the ground of that airport. Confusion between AGL and AMSL, or improper calibration of the altimeter, may result in controlled flight into terrain , a crash of a fully functioning aircraft under pilot control. While
561-405: The aircraft's altimeter to decide when to deploy the undercarriage and prepare for landing. Therefore, the pilot needs reliable information on the height of the plane with respect to the landing area (usually an airport). The altimeter, which is usually a barometer calibrated in units of distance instead of atmospheric pressure , can therefore be set in such a way as to indicate the height of
594-423: The altitudes of these locations are set above the simulated ground level. This is often implemented using the so-called sigma coordinate system, which is the ratio of the pressure at a location (latitude, longitude, altitude) divided by the pressure at the nadir of that location on ground surface (same latitude, same longitude, altitude AGL = 0). In broadcasting, altitude AGL has relatively little direct bearing on
627-438: The atmosphere can transport thermal energy from the higher temperature interior up to the surface. From the perspective of a planetary geologist , the atmosphere acts to shape a planetary surface. Wind picks up dust and other particles which, when they collide with the terrain, erode the relief and leave deposits ( eolian processes). Frost and precipitations , which depend on the atmospheric composition, also influence
660-411: The base of the exosphere at 690 km and contains the ionosphere , where solar radiation ionizes the atmosphere. The density of the ionosphere is greater at short distances from the planetary surface in the daytime and decreases as the ionosphere rises at night-time, thereby allowing a greater range of radio frequencies to travel greater distances. The exosphere begins at 690 to 1,000 km from
693-428: The bottom of the mesosphere , and contains the ozone layer , at an altitude between 15 km and 35 km. It is the atmospheric layer that absorbs most of the ultraviolet radiation that Earth receives from the Sun. The mesosphere ranges from 50 km to 85 km and is the layer wherein most meteors are incinerated before reaching the surface. The thermosphere extends from an altitude of 85 km to
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#1732794050044726-426: The diminishing mass of the gas above the point of barometric measurement. The units of air pressure are based upon the standard atmosphere (atm), which is 101,325 Pa (equivalent to 760 Torr or 14.696 psi ). The height at which the atmospheric pressure declines by a factor of e (an irrational number equal to 2.71828) is called the scale height ( H ). For an atmosphere of uniform temperature,
759-431: The distance from the Sun determines the energy available to heat atmospheric gas to the point where some fraction of its molecules' thermal motion exceed the planet's escape velocity , allowing those to escape a planet's gravitational grasp. Thus, distant and cold Titan , Triton , and Pluto are able to retain their atmospheres despite their relatively low gravities. Since a collection of gas molecules may be moving at
792-442: The hydrogen escaped. Earth's magnetic field helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen. The net effect, taking the most important escape processes into account, is that an intrinsic magnetic field does not protect
825-482: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=HAGL&oldid=908511916 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Above ground level A pilot flying an aircraft under instrument flight rules (typically under poor visibility conditions) must rely on
858-424: The observed variability is due to changes in the altitude of the surface. For this reason, variables such as pressure or temperature are sometimes 'reduced' to mean sea level. In general circulation models and global climate models , the state and properties of the atmosphere are specified or computed at a number of discrete locations and heights. When the topography of the continents is explicitly represented,
891-406: The past. The circulation of the atmosphere occurs due to thermal differences when convection becomes a more efficient transporter of heat than thermal radiation . On planets where the primary heat source is solar radiation, excess heat in the tropics is transported to higher latitudes. When a planet generates a significant amount of heat internally, such as is the case for Jupiter , convection in
924-416: The relief. Climate changes can influence a planet's geological history. Conversely, studying the surface of the Earth leads to an understanding of the atmosphere and climate of other planets. For a meteorologist , the composition of the Earth's atmosphere is a factor affecting the climate and its variations. For a biologist or paleontologist , the Earth's atmospheric composition is closely dependent on
957-468: The scale height is proportional to the atmospheric temperature and is inversely proportional to the product of the mean molecular mass of dry air, and the local acceleration of gravity at the point of barometric measurement. Surface gravity differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter retains light gases such as hydrogen and helium that escape from objects with lower gravity. Secondly,
990-413: The surface, and extends to roughly 10,000 km, where it interacts with the magnetosphere of Earth. Atmospheric pressure is the force (per unit-area) perpendicular to a unit-area of planetary surface, as determined by the weight of the vertical column of atmospheric gases. In said atmospheric model, the atmospheric pressure , the weight of the mass of the gas, decreases at high altitude because of
1023-417: The terrain of rocky planets with atmospheres, and over time can erase the effects of both craters and volcanoes . In addition, since liquids cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes , rivers and oceans . Earth and Titan are known to have liquids at their surface and terrain on the planet suggests that Mars had liquid on its surface in
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1056-474: The use of a barometric altimeter setting that provides a zero reading on the ground of the airport is a reference available to pilots, in commercial aviation it is a country-specific procedure that is not often used (it is used, e.g., in Russia, and a few other countries ). Most countries (Far East, North and South America, all of Europe, Africa, Australia) use the airport's AMSL (above mean sea level) elevation as
1089-461: Was determined is HD 209458b , a gas giant with a close orbit around a star in the constellation Pegasus . Its atmosphere is heated to temperatures over 1,000 K, and is steadily escaping into space. Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere. The atmosphere of Earth is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure. The troposphere
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