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105-396: Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum 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 of a location, in geography

210-402: A g-force environment in order to occur. Ice convection on Pluto is believed to occur in a soft mixture of nitrogen ice and carbon monoxide ice. It has also been proposed for Europa , and other bodies in the outer Solar System. Thermomagnetic convection can occur when an external magnetic field is imposed on a ferrofluid with varying magnetic susceptibility . In the presence of

315-410: A hurricane . On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes , at speeds which may closely approach that of light. Thermal convection in liquids can be demonstrated by placing a heat source (for example, a Bunsen burner ) at the side of a container with a liquid. Adding a dye to the water (such as food colouring) will enable visualisation of

420-477: A multiphase mixture of oil and water separates) or steady state (see convection cell ). The convection may be due to gravitational , electromagnetic or fictitious body forces. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere , its oceans , and its mantle . Discrete convective cells in the atmosphere can be identified by clouds , with stronger convection resulting in thunderstorms . Natural convection also plays

525-458: A candle in a sealed space with an inlet and exhaust port. The heat from the candle will cause a strong convection current which can be demonstrated with a flow indicator, such as smoke from another candle, being released near the inlet and exhaust areas respectively. A convection cell , also known as a Bénard cell , is a characteristic fluid flow pattern in many convection systems. A rising body of fluid typically loses heat because it encounters

630-461: A colder surface. In liquid, this occurs because it exchanges heat with colder liquid through direct exchange. In the example of the Earth's atmosphere, this occurs because it radiates heat. Because of this heat loss the fluid becomes denser than the fluid underneath it, which is still rising. Since it cannot descend through the rising fluid, it moves to one side. At some distance, its downward force overcomes

735-412: A common fire-place very well illustrates. If, for instance, we place a thermometer directly before a fire, it soon begins to rise, indicating an increase of temperature. In this case the heat has made its way through the space between the fire and the thermometer, by the process termed radiation . If we place a second thermometer in contact with any part of the grate, and away from the direct influence of

840-411: A demand for greater precision. This led to technological innovations such as the 1735 Marine chronometer by John Harrison , but also to a reconsideration of the underlying assumptions about the shape of Earth itself. Isaac Newton postulated that the conservation of momentum should make Earth oblate (wider at the equator), while the early surveys of Jacques Cassini (1720) led him to believe Earth

945-424: A few atoms. There are a variety of circumstances in which the forces required for convection arise, leading to different types of convection, described below. In broad terms, convection arises because of body forces acting within the fluid, such as gravity. Natural convection is a flow whose motion is caused by some parts of a fluid being heavier than other parts. In most cases this leads to natural circulation :

1050-617: A few hundred meters depending on where in Europe you look. Mars has no oceans and so no sea level, but at least two martian datums have been used to locate places there. In geodetic coordinates , Earth's surface is approximated by an ellipsoid , and locations near the surface are described in terms of geodetic latitude ( ϕ {\displaystyle \phi } ), longitude ( λ {\displaystyle \lambda } ), and ellipsoidal height ( h {\displaystyle h} ). The ellipsoid

1155-539: A geocentric origin and the Geodetic Reference System 1980 ([[GRS   80]]). "This datum, designated as NAD   83…is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin." NAD   83 may be considered a local referencing system. WGS   84 is the World Geodetic System of 1984. It is the reference frame used by

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1260-478: A global reference frame (such as WGS 84 ) is used, the coordinates of a place on the surface generally will change from year to year. Most mapping, such as within a single country, does not span plates. To minimize coordinate changes for that case, a different reference frame can be used, one whose coordinates are fixed to that particular plate. Examples of these reference frames are " NAD 83 " for North America and " ETRS89 " for Europe. Convection Convection

1365-443: A higher heart rate, and adjusting its blood chemistry. 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. 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

1470-404: A lifting force (heat). All thunderstorms , regardless of type, go through three stages: the developing stage , the mature stage , and the dissipation stage . The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through. Solar radiation affects the oceans: warm water from

1575-527: A major feature of all weather systems. Convection is also seen in the rising plume of hot air from fire , plate tectonics , oceanic currents ( thermohaline circulation ) and sea-wind formation (where upward convection is also modified by Coriolis forces ). In engineering applications, convection is commonly visualized in the formation of microstructures during the cooling of molten metals, and fluid flows around shrouded heat-dissipation fins, and solar ponds. A very common industrial application of natural convection

1680-456: A more accurate representation of some specific area of coverage than WGS   84 can. OSGB36 , for example, is a better approximation to the geoid covering the British Isles than the global WGS   84 ellipsoid. However, as the benefits of a global system outweigh the greater accuracy, the global WGS   84 datum has become widely adopted. The spherical nature of Earth was known by

1785-451: A result of physical rearrangement of denser portions of the Earth's interior toward the center of the planet (that is, a type of prolonged falling and settling). The Stack effect or chimney effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers due to buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The greater

1890-482: A role in stellar physics . Convection is often categorised or described by the main effect causing the convective flow; for example, thermal convection. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Granular convection is a similar phenomenon in granular material instead of fluids. Advection is fluid motion created by velocity instead of thermal gradients. Convective heat transfer

1995-602: A series of physically monumented geodetic control points of known location. Global datums, such as WGS 84 and ITRF , are typically bound to the center of mass of the Earth (making them useful for tracking satellite orbits and thus for use in satellite navigation systems. A specific point can have substantially different coordinates, depending on the datum used to make the measurement. For example, coordinates in NAD   83 can differ from NAD   27 by up to several hundred feet. There are hundreds of local horizontal datums around

2100-425: A square cavity. It is differentially heated between the two vertical walls, where the left and right walls are held at 10 °C and 0 °C, respectively. The density anomaly manifests in its flow pattern. As the water is cooled at the right wall, the density increases, which accelerates the flow downward. As the flow develops and the water cools further, the decrease in density causes a recirculation current at

2205-496: A temperature gradient this results in a nonuniform magnetic body force, which leads to fluid movement. A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field . In a zero-gravity environment, there can be no buoyancy forces, and thus no convection possible, so flames in many circumstances without gravity smother in their own waste gases. Thermal expansion and chemical reactions resulting in expansion and contraction gases allows for ventilation of

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2310-484: A traditional standard horizontal or vertical datum. A standard datum specification (whether horizontal, vertical, or 3D) consists of several parts: a model for Earth's shape and dimensions, such as a reference ellipsoid or a geoid ; an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude); and multiple control points or reference points that have been precisely measured from

2415-607: Is "the horizontal control datum for the United States that was defined by a location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) Meades Ranch (Kansas) ." ... The geoidal height at Meades Ranch was assumed to be zero, as sufficient gravity data was not available, and this was needed to relate surface measurements to the datum. "Geodetic positions on the North American Datum of 1927 were derived from

2520-499: Is a model used to precisely measure positions on Earth; it is thus a crucial component of any spatial reference system or map projection . A horizontal datum binds a specified reference ellipsoid , a mathematical model of the shape of the earth, to the physical earth. Thus, the geographic coordinate system on that ellipsoid can be used to measure the latitude and longitude of real-world locations. Regional horizontal datums, such as NAD 27 and NAD 83 , usually create this binding with

2625-484: Is a vertical section of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation. The Sun warms the ground, which in turn warms the air directly above it. The warmer air expands, becoming less dense than the surrounding air mass, and creating a thermal low . The mass of lighter air rises, and as it does, it cools by expansion at lower air pressures. It stops rising when it has cooled to

2730-512: Is completely parameterised by the semi-major axis a {\displaystyle a} and the flattening f {\displaystyle f} . From a {\displaystyle a} and f {\displaystyle f} it is possible to derive the semi-minor axis b {\displaystyle b} , first eccentricity e {\displaystyle e} and second eccentricity e ′ {\displaystyle e'} of

2835-416: Is divided into a number of tectonic plates that are continuously being created and consumed at their opposite plate boundaries. Creation ( accretion ) occurs as mantle is added to the growing edges of a plate. This hot added material cools down by conduction and convection of heat. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in

2940-404: Is drawn downward by gravity. Together, these effects create a flow of fluid from the heat source to the heat sink and back again. Gravitational convection is a type of natural convection induced by buoyancy variations resulting from material properties other than temperature. Typically this is caused by a variable composition of the fluid. If the varying property is a concentration gradient, it

3045-409: 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. 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

3150-463: Is free air cooling without the aid of fans: this can happen on small scales (computer chips) to large scale process equipment. Natural convection will be more likely and more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection or a larger distance through the convecting medium. Natural convection will be less likely and less rapid with more rapid diffusion (thereby diffusing away

3255-425: Is known as solutal convection . For example, gravitational convection can be seen in the diffusion of a source of dry salt downward into wet soil due to the buoyancy of fresh water in saline. Variable salinity in water and variable water content in air masses are frequent causes of convection in the oceans and atmosphere which do not involve heat, or else involve additional compositional density factors other than

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3360-471: Is known as the adiabatic lapse rate , which is approximately 9.8 °C per kilometer (or 5.4 °F [3.0 °C] per 1000 feet) of altitude. The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent heat of vaporization . As air rises and cools, it eventually becomes saturated and cannot hold its quantity of water vapor. The water vapor condenses (forming clouds ), and releases heat, which changes

3465-422: Is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. A typical granule has a diameter on the order of 1,000 kilometers and each lasts 8 to 20 minutes before dissipating. Below the photosphere is a layer of much larger "supergranules" up to 30,000 kilometers in diameter, with lifespans of up to 24 hours. Water is a fluid that does not obey

3570-507: Is not unlike that of a lava lamp .) This downdraft of heavy, cold and dense water becomes a part of the North Atlantic Deep Water , a south-going stream. Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. It is one of 3 driving forces that causes tectonic plates to move around the Earth's surface. The Earth's surface

3675-492: Is single or multiphase fluid flow that occurs spontaneously through 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 convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow. Convective flow may be transient (such as when

3780-490: Is the intentional use of convection as a method for heat transfer . Convection is a process in which heat is carried from place to place by the bulk movement of a fluid and gases. In the 1830s, in The Bridgewater Treatises , the term convection is attested in a scientific sense. In treatise VIII by William Prout , in the book on chemistry , it says: [...] This motion of heat takes place in three ways, which

3885-568: Is the only world referencing system in place today. WGS   84 is the default standard datum for coordinates stored in recreational and commercial GPS units. Users of GPS are cautioned that they must always check the datum of the maps they are using. To correctly enter, display, and to store map related map coordinates, the datum of the map must be entered into the GPS map datum field. Examples of map datums are: The Earth's tectonic plates move relative to one another in different directions at speeds on

3990-444: Is the process of 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 , which has a characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation

4095-508: Is the transition altitude). When flying at a flight level, the altimeter is always set to standard pressure (29.92  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

4200-478: Is to use two identical jars, one filled with hot water dyed one colour, and cold water of another colour. One jar is then temporarily sealed (for example, with a piece of card), inverted and placed on top of the other. When the card is removed, if the jar containing the warmer liquid is placed on top no convection will occur. If the jar containing colder liquid is placed on top, a convection current will form spontaneously. Convection in gases can be demonstrated using

4305-456: Is transported outward from the core region primarily by convection rather than radiation . This occurs at radii which are sufficiently opaque that convection is more efficient than radiation at transporting energy. Granules on the photosphere of the Sun are the visible tops of convection cells in the photosphere, caused by convection of plasma in the photosphere. The rising part of the granules

Altitude - Misplaced Pages Continue

4410-509: Is wind driven: wind moving over water cools the water and also causes evaporation , leaving a saltier brine. In this process, the water becomes saltier and denser. and decreases in temperature. Once sea ice forms, salts are left out of the ice, a process known as brine exclusion. These two processes produce water that is denser and colder. The water across the northern Atlantic Ocean becomes so dense that it begins to sink down through less salty and less dense water. (This open ocean convection

4515-516: The Greenwich Observatory for longitude, from the Equator for latitude, or from the nearest coast for sea level. Astronomical and chronological methods have limited precision and accuracy, especially over long distances. Even GPS requires a predefined framework on which to base its measurements, so WGS   84 essentially functions as a datum, even though it is different in some particulars from

4620-721: The Anglo-French Survey (1784–1790) , by the end of the 18th century, survey control networks covered France and the United Kingdom . More ambitious undertakings such as the Struve Geodetic Arc across Eastern Europe (1816-1855) and the Great Trigonometrical Survey of India (1802-1871) took much longer, but resulted in more accurate estimations of the shape of the Earth ellipsoid . The first triangulation across

4725-473: The Hadley cell and the polar vortex , with the Hadley cell experiencing stronger convection due to the release of latent heat energy by condensation of water vapor at higher altitudes during cloud formation. Longitudinal circulation, on the other hand, comes about because the ocean has a higher specific heat capacity than land (and also thermal conductivity , allowing the heat to penetrate further beneath

4830-736: The U.S. Department of Defense (DoD) and is defined by the National Geospatial-Intelligence Agency (NGA) (formerly the Defense Mapping Agency, then the National Imagery and Mapping Agency). WGS   84 is used by the DoD for all its mapping, charting, surveying, and navigation needs, including its GPS "broadcast" and "precise" orbits. WGS   84 was defined in January 1987 using Doppler satellite surveying techniques. It

4935-450: The elevations of Earth features including terrain , bathymetry , water level , and human-made structures. An approximate definition of sea level is the datum WGS 84 , an ellipsoid , whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least 2,159 spherical harmonics . Other datums are defined for other areas or at other times; ED50 was defined in 1950 over Europe and differs from WGS   84 by

5040-430: The hydrologic cycle . For example, a foehn wind is a down-slope wind which occurs on the downwind side of a mountain range. It results from the adiabatic warming of air which has dropped most of its moisture on windward slopes. Because of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than at the same height on the windward slopes. A thermal column (or thermal)

5145-432: 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. Medicine recognizes that altitudes above 1,500 metres (4,900 ft) start to affect humans, and there is no record of humans living at extreme altitudes above 5,500–6,000 metres (18,000–19,700 ft) for more than two years. As

5250-455: The "down" direction are commonly referred to as depth . 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

5355-455: The (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used." NAD   27 is a local referencing system covering North America. The North American Datum of 1983 (NAD 83) is "The horizontal control datum for the United States, Canada, Mexico, and Central America, based on

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5460-460: The Boussinesq approximation. This is because its density varies nonlinearly with temperature, which causes its thermal expansion coefficient to be inconsistent near freezing temperatures. The density of water reaches a maximum at 4 °C and decreases as the temperature deviates. This phenomenon is investigated by experiment and numerical methods. Water is initially stagnant at 10 °C within

5565-519: The Equator tends to circulate toward the poles , while cold polar water heads towards the Equator. The surface currents are initially dictated by surface wind conditions. The trade winds blow westward in the tropics, and the westerlies blow eastward at mid-latitudes. This wind pattern applies a stress to the subtropical ocean surface with negative curl across the Northern Hemisphere , and

5670-547: The United States was not completed until 1899. The U.S. survey resulted in the North American Datum (horizontal) of 1927 (NAD   27) and the Vertical Datum of 1929 (NAVD29), the first standard datums available for public use. This was followed by the release of national and regional datums over the next several decades. Improving measurements, including the use of early satellites , enabled more accurate datums in

5775-530: 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. For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump , triple jump )

5880-457: The ability of a fluid in a system to circulate continuously under gravity, with transfer of heat energy. The driving force for natural convection is gravity. In a column of fluid, pressure increases with depth from the weight of the overlying fluid. The pressure at the bottom of a submerged object then exceeds that at the top, resulting in a net upward buoyancy force equal to the weight of the displaced fluid. Objects of higher density than that of

5985-413: 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. However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This

6090-438: The altitude increases, atmospheric pressure decreases, which affects humans by reducing the partial pressure of oxygen . The lack of oxygen above 2,400 metres (8,000 ft) can cause serious illnesses such as altitude sickness , high altitude pulmonary edema , and high altitude cerebral edema . The higher the altitude, the more likely are serious effects. The human body can adapt to high altitude by breathing faster, having

6195-437: The altitude: 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: 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 . The thermosphere and exosphere (along with

6300-462: The ancient Greeks, who also developed the concepts of latitude and longitude, and the first astronomical methods for measuring them. These methods, preserved and further developed by Muslim and Indian astronomers, were sufficient for the global explorations of the 15th and 16th Centuries. However, the scientific advances of the Age of Enlightenment brought a recognition of errors in these measurements, and

6405-719: 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. These factors may decrease productivity 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. Datum (geodesy) Since

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6510-422: The coordinates of a point from one datum system to another. Because the survey networks upon which datums were traditionally based are irregular, and the error in early surveys is not evenly distributed, datum conversion cannot be performed using a simple parametric function. For example, converting from NAD 27 to NAD 83 is performed using NADCON (later improved as HARN), a raster grid covering North America, with

6615-514: The datum used to make the measurement. There are hundreds of locally developed reference datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The most common reference Datums in use in North America are NAD   27, NAD   83, and WGS 84 . The North American Datum of 1927 (NAD   27)

6720-442: The density changes from thermal expansion (see thermohaline circulation ). Similarly, variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (in other words, with densest parts deepest) continues to cause a fraction of the convection of fluid rock and molten metal within the Earth's interior (see below). Gravitational convection, like natural thermal convection, also requires

6825-408: The disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums. This phenomenon is called datum shift or, more generally, datum transformation , as it may involve rotation and scaling, in addition to displacement. Because Earth is an imperfect ellipsoid, local datums can give

6930-553: The displaced fluid then sink. For example, regions of warmer low-density air rise, while those of colder high-density air sink. This creates a circulating flow: convection. Gravity drives natural convection. Without gravity, convection does not occur, so there is no convection in free-fall ( inertial ) environments, such as that of the orbiting International Space Station. Natural convection can occur when there are hot and cold regions of either air or water, because both water and air become less dense as they are heated. But, for example, in

7035-983: The ellipsoid The two main reference ellipsoids used worldwide are the GRS   80 and the WGS   84. A more comprehensive list of geodetic systems can be found here . The Global Positioning System (GPS) uses the World Geodetic System 1984 (WGS   84) to determine the location of a point near the surface of Earth. The difference in co-ordinates between datums is commonly referred to as datum shift . The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters (or several kilometers for some remote islands). The North Pole , South Pole and Equator will be in different positions on different datums, so True North will be slightly different. Different datums use different interpolations for

7140-417: The fire, has become heated, and has carried up the chimney the temperature acquired from the fire. There is at present no single term in our language employed to denote this third mode of the propagation of heat; but we venture to propose for that purpose, the term convection , [in footnote: [Latin] Convectio , a carrying or conveying] which not only expresses the leading fact, but also accords very well with

7245-404: The fire, we shall find that this thermometer also denotes an increase of temperature; but here the heat must have travelled through the metal of the grate, by what is termed conduction . Lastly, a third thermometer placed in the chimney, away from the direct influence of the fire, will also indicate a considerable increase of temperature; in this case a portion of the air, passing through and near

7350-407: The first type, plumes rise from the lower mantle, and corresponding unstable regions of lithosphere drip back into the mantle. In the second type, subducting oceanic plates (which largely constitute the upper thermal boundary layer of the mantle) plunge back into the mantle and move downwards towards the core-mantle boundary . Mantle convection occurs at rates of centimeters per year, and it takes on

7455-425: The flame, as waste gases are displaced by cool, fresh, oxygen-rich gas. moves in to take up the low pressure zones created when flame-exhaust water condenses. Systems of natural circulation include tornadoes and other weather systems , ocean currents , and household ventilation . Some solar water heaters use natural circulation. The Gulf Stream circulates as a result of the evaporation of water. In this process,

7560-424: The flow. Another common experiment to demonstrate thermal convection in liquids involves submerging open containers of hot and cold liquid coloured with dye into a large container of the same liquid without dye at an intermediate temperature (for example, a jar of hot tap water coloured red, a jar of water chilled in a fridge coloured blue, lowered into a clear tank of water at room temperature). A third approach

7665-401: The fluid in the system, but not all of it. The heat source is positioned lower than the heat sink. Most fluids expand when heated, becoming less dense , and contract when cooled, becoming denser. At the heat source of a system of natural circulation, the heated fluid becomes lighter than the fluid surrounding it, and thus rises. At the heat sink, the nearby fluid becomes denser as it cools, and

7770-411: The higher parts of the mesosphere) are regions of the atmosphere that are conventionally defined as space. 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. At high altitude, atmospheric pressure is lower than that at sea level. This

7875-545: The lapse rate from the dry adiabatic lapse rate to the moist adiabatic lapse rate (5.5 °C per kilometer or 3 °F [1.7 °C] per 1000 feet). 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). The actual lapse rate can vary by altitude and by location. Finally, only

7980-689: The later 20th century, such as NAD 83 in North America, ETRS89 in Europe, and GDA94 in Australia. At this time global datums were also first developed for use in satellite navigation systems, especially the World Geodetic System (WGS   84) used in the U.S. global positioning system (GPS), and the International Terrestrial Reference System and Frame (ITRF) used in the European Galileo system. A horizontal datum

8085-904: 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 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. 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 and phylogeny . For example, small mammals face

8190-567: The moist air rises, it cools, causing some of the water vapor in the rising packet of air to condense . When the moisture condenses, it releases energy known as latent heat of condensation which allows the rising packet of air to cool less than its surrounding air, continuing the cloud's ascension. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form, which support lightning and thunder. Generally, thunderstorms require three conditions to form: moisture, an unstable airmass, and

8295-433: The much slower (lagged) ocean circulation system. The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains fairly constant. Latitudinal circulation occurs because incident solar radiation per unit area is highest at the heat equator , and decreases as the latitude increases, reaching minima at the poles. It consists of two primary convection cells,

8400-417: The ocean basin, outweighing the effects of friction with the cold western boundary current which originates from high latitudes. The overall process, known as western intensification, causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary. As it travels poleward, warm water transported by strong warm water current undergoes evaporative cooling. The cooling

8505-642: The order of 50 to 100 mm (2.0 to 3.9 in) per year. Therefore, locations on different plates are in motion relative to one another. For example, the longitudinal difference between a point on the equator in Uganda, on the African Plate , and a point on the equator in Ecuador, on the South American Plate , increases by about 0.0014 arcseconds per year. These tectonic movements likewise affect latitude. If

8610-483: The order of hundreds of millions of years to complete a cycle of convection. Neutrino flux measurements from the Earth's core (see kamLAND ) show the source of about two-thirds of the heat in the inner core is the radioactive decay of K , uranium and thorium. This has allowed plate tectonics on Earth to continue far longer than it would have if it were simply driven by heat left over from Earth's formation; or with heat produced from gravitational potential energy , as

8715-409: The origin and physically monumented. Then the coordinates of other places are measured from the nearest control point through surveying . Because the ellipsoid or geoid differs between datums, along with their origins and orientation in space, the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated. Using local datums,

8820-513: The precise shape and size of Earth ( reference ellipsoids ). For example, in Sydney there is a 200 metres (700 feet) difference between GPS coordinates configured in GDA (based on global standard WGS   84) and AGD (used for most local maps), which is an unacceptably large error for some applications, such as surveying or site location for scuba diving . Datum conversion is the process of converting

8925-452: The prefixed variant Natural Convection is used to distinguish the fluid mechanics concept of Convection (covered in this article) from convective heat transfer. Some phenomena which result in an effect superficially similar to that of a convective cell may also be (inaccurately) referred to as a form of convection; for example, thermo-capillary convection and granular convection . Convection may happen in fluids at all scales larger than

9030-404: The presence of an environment which experiences g-force ( proper acceleration ). The difference of density in the fluid is the key driving mechanism. If the differences of density are caused by heat, this force is called as "thermal head" or "thermal driving head." A fluid system designed for natural circulation will have a heat source and a heat sink . Each of these is in contact with some of

9135-475: The process of subduction at an ocean trench. This subducted material sinks to some depth in the Earth's interior where it is prohibited from sinking further. The subducted oceanic crust triggers volcanism. Convection within Earth's mantle is the driving force for plate tectonics . Mantle convection is the result of a thermal gradient: the lower mantle is hotter than the upper mantle , and is therefore less dense. This sets up two primary types of instabilities. In

9240-618: 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 1,000 metres (3,300 ft) with

9345-454: The reference datum. Pressure altitude divided by 100 feet (30 m) is the flight level , and is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 m) 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

9450-463: The reverse across the Southern Hemisphere . The resulting Sverdrup transport is equatorward. Because of conservation of potential vorticity caused by the poleward-moving winds on the subtropical ridge 's western periphery and the increased relative vorticity of poleward moving water, transport is balanced by a narrow, accelerating poleward current, which flows along the western boundary of

9555-461: The rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications. The WGS   84 is intended for global use, unlike most earlier datums. Before GPS, there was no precise way to measure the position of a location that was far from reference points used in the realization of local datums, such as from the Prime Meridian at

9660-418: The rising force beneath it, and the fluid begins to descend. As it descends, it warms again and the cycle repeats itself. Additionally, convection cells can arise due to density variations resulting from differences in the composition of electrolytes. Atmospheric circulation is the large-scale movement of air, and is a means by which thermal energy is distributed on the surface of the Earth , together with

9765-481: The same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal. Another convection-driven weather effect is the sea breeze . Warm air has a lower density than cool air, so warm air rises within cooler air, similar to hot air balloons . Clouds form as relatively warmer air carrying moisture rises within cooler air. As

9870-553: The surface ) and thereby absorbs and releases more heat , but the temperature changes less than land. This brings the sea breeze, air cooled by the water, ashore in the day, and carries the land breeze, air cooled by contact with the ground, out to sea during the night. Longitudinal circulation consists of two cells, the Walker circulation and El Niño / Southern Oscillation . Some more localized phenomena than global atmospheric movement are also due to convection, including wind and some of

9975-601: The term elevation is often preferred for this usage. In aviation, altitude is typically measured relative to mean sea level or above ground level to ensure safe navigation and flight operations. In geometry and geographical surveys, altitude helps create accurate topographic maps and understand the terrain's elevation. For high-altitude trekking and sports, knowing and adapting to altitude is vital for performance and safety. Higher altitudes mean reduced oxygen levels, which can lead to altitude sickness if proper acclimatization measures are not taken. Vertical distance measurements in

10080-408: The thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation and infiltration. Some cooling towers operate on this principle; similarly the solar updraft tower is a proposed device to generate electricity based on the stack effect. The convection zone of a star is the range of radii in which energy

10185-529: The thermal gradient that is causing the convection) or a more viscous (sticky) fluid. The onset of natural convection can be determined by the Rayleigh number ( Ra ). Differences in buoyancy within a fluid can arise for reasons other than temperature variations, in which case the fluid motion is called gravitational convection (see below). However, all types of buoyant convection, including natural convection, do not occur in microgravity environments. All require

10290-551: The two other terms. Later, in the same treatise VIII, in the book on meteorology , the concept of convection is also applied to "the process by which heat is communicated through water". Today, the word convection has different but related usages in different scientific or engineering contexts or applications. In fluid mechanics , convection has a broader sense: it refers to the motion of fluid driven by density (or other property) difference. In thermodynamics , convection often refers to heat transfer by convection , where

10395-470: The value of each cell being the average adjustment distance for that area in latitude and longitude. Datum conversion may frequently be accompanied by a change of map projection . A geodetic reference datum is a known and constant surface which is used to describe the location of unknown points on Earth. Since reference datums can have different radii and different center points, a specific point on Earth can have substantially different coordinates depending on

10500-493: The water increases in salinity and density. In the North Atlantic Ocean, the water becomes so dense that it begins to sink down. Convection occurs on a large scale in atmospheres , oceans, planetary mantles , and it provides the mechanism of heat transfer for a large fraction of the outermost interiors of the Sun and all stars. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in

10605-410: The world's oceans it also occurs due to salt water being heavier than fresh water, so a layer of salt water on top of a layer of fresher water will also cause convection. Natural convection has attracted a great deal of attention from researchers because of its presence both in nature and engineering applications. In nature, convection cells formed from air raising above sunlight-warmed land or water are

10710-524: The world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The WGS 84 datum, which is almost identical to the NAD 83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum. A vertical datum is a reference surface for vertical positions , such as

10815-421: Was prolate (wider at the poles). The subsequent French geodesic missions (1735-1739) to Lapland and Peru corroborated Newton, but also discovered variations in gravity that would eventually lead to the geoid model. A contemporary development was the use of the trigonometric survey to accurately measure distance and location over great distances. Starting with the surveys of Jacques Cassini (1718) and

10920-412: Was redefined again and was more closely aligned with International Earth Rotation Service (IERS) frame ITRF 94. It was then formally called WGS   84 (G873). WGS   84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997. Another update brought it to WGS   84 (G1674). The WGS   84 datum, within two meters of the NAD   83 datum used in North America,

11025-448: Was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000 GMT January 2, 1994, WGS   84 was upgraded in accuracy using GPS measurements. The formal name then became WGS   84 (G730), since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994. At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS   84

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