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72-396: 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

144-410: 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

216-429: 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 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

288-458: 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

360-465: 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

432-446: 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 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

504-416: 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 the thermal gradient that is causing the convection) or a more viscous (sticky) fluid. The onset of natural convection can be determined by

576-443: 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

648-512: 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 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

720-404: 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

792-451: 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

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864-552: 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 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

936-425: 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

1008-479: Is a similar phenomenon in granular material instead of fluids. Advection is fluid motion created by velocity instead of thermal gradients. Convective heat transfer 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 ,

1080-484: 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

1152-495: 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 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

1224-411: Is caused by a variable composition of the fluid. If the varying property is a concentration gradient, it 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

1296-416: 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

1368-409: 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

1440-585: Is imposed on a ferrofluid with varying magnetic susceptibility . In the presence of 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

1512-471: 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

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1584-422: 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

1656-408: 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 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

1728-518: 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

1800-444: 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

1872-508: 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

1944-478: 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

2016-459: 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

2088-509: 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

2160-477: 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

2232-509: 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 the presence of an environment which experiences g-force ( proper acceleration ). The difference of density in

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2304-430: 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)

2376-432: 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

2448-455: 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

2520-466: 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

2592-533: 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

2664-593: 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 )

2736-634: The Wikimedia System Administrators, please include the details below. Request from 172.68.168.237 via cp1104 cp1104, Varnish XID 212173236 Upstream caches: cp1104 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 07:46:58 GMT Convection Convective flow may be transient (such as when 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

2808-413: 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

2880-438: 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

2952-437: 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

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3024-760: 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) Too Many Requests If you report this error to

3096-435: 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 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,

3168-475: The convection of fluid rock and molten metal within the Earth's interior (see below). Gravitational convection, like natural thermal convection, also requires 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

3240-407: 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

3312-425: 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,

3384-424: 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

3456-490: 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 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

3528-409: 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 : 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

3600-496: 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 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

3672-462: 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

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3744-561: 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

3816-904: 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

3888-567: 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

3960-433: 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,

4032-417: 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

4104-401: The oceans and atmosphere which do not involve heat, or else involve additional compositional density factors other than 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

4176-483: 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

4248-475: 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

4320-629: 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

4392-457: 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

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4464-472: 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

4536-418: 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

4608-481: 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

4680-423: 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 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

4752-557: 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

4824-410: 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 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

4896-518: The term convection , [in footnote: [Latin] Convectio , a carrying or conveying] which not only expresses the leading fact, but also accords very well with 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

4968-601: 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

5040-408: 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

5112-502: 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

5184-503: 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 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

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