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90-629: Lapse rate corresponds to the vertical component of the spatial gradient of temperature . Although this concept is most often applied to the Earth's troposphere , it can be extended to any gravitationally supported parcel of gas . A formal definition from the Glossary of Meteorology is: Typically, the lapse rate is the negative of the rate of temperature change with altitude change: where Γ {\displaystyle \Gamma } (sometimes L {\displaystyle L} )

180-410: 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

270-429: 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

360-458: 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

450-415: A characteristic temperature-pressure curve. As air circulates vertically, the air takes on that characteristic gradient. When the air contains little water, this lapse rate is known as the dry adiabatic lapse rate: the rate of temperature decrease is 9.8 °C/km ( 5.4 °F per 1,000 ft) (3.0 °C/1,000 ft). The reverse occurs for a sinking parcel of air. When the environmental lapse rate

540-413: 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

630-446: 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

720-400: A good approximation. When a parcel of air expands, it pushes on the air around it, doing thermodynamic work . Since the upward-moving and expanding parcel does work but gains no heat, it loses internal energy so that its temperature decreases. Downward-moving and contracting air has work done on it, so it gains internal energy and its temperature increases. Adiabatic processes for air have

810-416: 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

900-512: 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

990-404: 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

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1080-465: A parcel rises to the level of free convection (LFC), after which it enters the free convective layer (FCL) and usually rises to the equilibrium level (EL). If the environmental lapse rate is larger than the dry adiabatic lapse rate, it has a superadiabatic lapse rate, the air is absolutely unstable — a parcel of air will gain buoyancy as it rises both below and above the lifting condensation level or convective condensation level. This often happens in

1170-423: A result of decreasing air pressure) but much more slowly, typically about 2 °C per 1,000 m. If unsaturated air rises far enough, eventually its temperature will reach its dew point , and condensation will begin to form. This altitude is known as the lifting condensation level (LCL) when mechanical lift is present and the convective condensation level (CCL) when mechanical lift is absent, in which case,

1260-451: 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

1350-552: 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

1440-425: 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

1530-473: A temperature gradient will arise in a column of still air in a gravitational field without external energy flows. This issue was addressed by James Clerk Maxwell in 1902, who established that if any temperature gradient forms, then that temperature gradient must be universal (i.e., the gradient must be same for all materials) or the Second Law of Thermodynamics would be violated. Maxwell also concluded that

1620-479: 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 ,

1710-484: 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

1800-420: Is absorbed within the atmosphere, heating the atmosphere directly. Thermal conduction helps transfer heat from the surface to the air; this conduction occurs within the few millimeters of air closest to the surface. However, above that thin interface layer, thermal conduction plays a negligible role in transferring heat within the atmosphere; this is because the thermal conductivity of air is very low. The air

1890-495: 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

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1980-481: Is an important source of energy in the development of thunderstorms. While the dry adiabatic lapse rate is a constant 9.8 °C/km ( 5.4 °F per 1,000 ft, 3 °C/1,000 ft ), the moist adiabatic lapse rate varies strongly with temperature. A typical value is around 5 °C/km , ( 9 °F/km , 2.7 °F/1,000 ft , 1.5 °C/1,000 ft ). The formula for the saturated adiabatic lapse rate (SALR) or moist adiabatic lapse rate (MALR)

2070-468: Is called vertical derivative or vertical gradient ; the remainder is called horizontal gradient component, the vector projection of the full gradient onto the horizontal plane . Examples: Convection Convection 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

2160-411: 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

2250-416: 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

2340-523: Is forced towards the adiabatic lapse rate whenever air is moving vertically. As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.50 °C/km (3.56 °F or 1.98 °C/1,000 ft) from sea level to 11 km (36,090 ft or 6.8 mi) . From 11 km up to 20 km (65,620 ft or 12.4 mi) ,

2430-420: Is given by: where: The SALR or MALR ( Γ w {\displaystyle \Gamma _{\text{w}}} ) is the temperature gradient experienced in an ascending or descending packet of air that is saturated with water vapor, i.e., with 100% relative humidity. The varying environmental lapse rates throughout the Earth's atmosphere are of critical importance in meteorology , particularly within

2520-585: 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

2610-504: Is less than the adiabatic lapse rate the atmosphere is stable and convection will not occur. Only the troposphere (up to approximately 12 kilometres (39,000 ft) of altitude) in the Earth's atmosphere undergoes convection : the stratosphere does not generally convect. However, some exceptionally energetic convection processes, such as volcanic eruption columns and overshooting tops associated with severe supercell thunderstorms , may locally and temporarily inject convection through

2700-422: 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

2790-408: 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

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2880-432: Is not saturated with water vapor, i.e., with less than 100% relative humidity. The presence of water within the atmosphere (usually the troposphere) complicates the process of convection. Water vapor contains latent heat of vaporization . As a parcel of air rises and cools, it eventually becomes saturated ; that is, the vapor pressure of water in equilibrium with liquid water has decreased (as temperature has decreased) to

2970-518: 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

3060-506: Is radiatively cooled by greenhouse gases (water vapor, carbon dioxide, etc.) and clouds emitting longwave thermal radiation to space. If radiation were the only way to transfer energy within the atmosphere, then the lapse rate near the surface would be roughly 40 °C/km and the greenhouse effect of gases in the atmosphere would keep the ground at roughly 333 K (60 °C; 140 °F). However, when air gets hot or humid, its density decreases. Thus, air which has been heated by

3150-436: Is relatively slow and so is negligible for moving air. Thus, when air ascends or descends, there is little exchange of heat with the surrounding air. A process in which no heat is exchanged with the environment is referred to as an adiabatic process . Air expands as it moves upward, and contracts as it moves downward. The expansion of rising air parcels, and the contraction of descending air parcels, are adiabatic processes, to

3240-406: Is stronger in locations where the lapse rate is stronger. In Antarctica, thermal inversions in the atmosphere (so that air at higher altitudes is warmer) sometimes cause the localized greenhouse effect to become negative (signifying enhanced radiative cooling to space instead of inhibited radiative cooling as is the case for a positive greenhouse effect). A question has sometimes arisen as to whether

3330-450: Is the specific heat at constant pressure. Assuming an atmosphere in hydrostatic equilibrium : where g is the standard gravity . Combining these two equations to eliminate the pressure, one arrives at the result for the dry adiabatic lapse rate (DALR), The DALR ( Γ d {\displaystyle \Gamma _{\text{d}}} ) is the temperature gradient experienced in an ascending or descending packet of air that

3420-407: Is the lapse rate given in units of temperature divided by units of altitude, T is temperature, and z is altitude. The environmental lapse rate (ELR), is the actual rate of decrease of temperature with altitude in the atmosphere at a given time and location. The ELR is the observed lapse rate, and is to be distinguished from the adiabatic lapse rate which is a theoretical construct. The ELR

3510-478: 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

3600-459: 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

3690-509: 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

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3780-477: 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

3870-509: 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

3960-430: 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)

4050-406: The tropopause and into the stratosphere. Energy transport in the atmosphere is more complex than the interaction between radiation and dry convection. The water cycle (including evaporation , condensation , precipitation ) transports latent heat and affects atmospheric humidity levels, significantly influencing the temperature profile, as described below. The following calculations derive

4140-441: The troposphere . They are used to determine if the parcel of rising air will rise high enough for its water to condense to form clouds , and, having formed clouds, whether the air will continue to rise and form bigger shower clouds, and whether these clouds will get even bigger and form cumulonimbus clouds (thunder clouds). As unsaturated air rises, its temperature drops at the dry adiabatic rate. The dew point also drops (as

4230-466: 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

4320-533: 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

4410-407: The adiabatic lapse rate decreases to the moist adiabatic lapse rate as the air continues to rise. Condensation is also commonly followed by precipitation on the top and windward sides of the mountain. As the air descends on the leeward side, it is warmed by adiabatic compression at the dry adiabatic lapse rate. Thus, the foehn wind at a certain altitude is warmer than the corresponding altitude on

4500-536: The afternoon mainly over land masses. In these conditions, the likelihood of cumulus clouds , showers or even thunderstorms is increased. Meteorologists use radiosondes to measure the environmental lapse rate and compare it to the predicted adiabatic lapse rate to forecast the likelihood that air will rise. Charts of the environmental lapse rate are known as thermodynamic diagrams , examples of which include Skew-T log-P diagrams and tephigrams . (See also Thermals ). The difference in moist adiabatic lapse rate and

4590-405: The air below cooler than it would otherwise be and the air above warmer. When convection happens, this shifts the environmental lapse rate towards the adiabatic lapse rate , which is a thermal gradient characteristic of vertically moving air packets. Because convection is available to transfer heat within the atmosphere, the lapse rate in the troposphere is reduced to around 6.5 °C/km and

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4680-463: The air is conditionally unstable — an unsaturated parcel of air does not have sufficient buoyancy to rise to the LCL or CCL, and it is stable to weak vertical displacements in either direction. If the parcel is saturated it is unstable and will rise to the LCL or CCL, and either be halted due to an inversion layer of convective inhibition , or if lifting continues, deep, moist convection (DMC) may ensue, as

4770-426: The air, which by itself would lead to a high lapse rate; and (b) convection, which is activated when the lapse rate exceeds a critical value; convection stabilizes the environmental lapse rate and prevents it from substantially exceeding the adiabatic lapse rate. Sunlight hits the surface of the earth (land and sea) and heats them. The warm surface heats the air above it. In addition, nearly a third of absorbed sunlight

4860-434: The atmosphere is a result of the interaction between radiative heating from sunlight , cooling to space via thermal radiation , and upward heat transport via natural convection (which carries hot air and latent heat upward). Above the tropopause , convection does not occur and all cooling is radiative. Within the troposphere , the lapse rate is a essentially the consequence of a balance between (a) radiative cooling of

4950-501: 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 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

5040-435: 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,

5130-459: The constant temperature is −56.5 °C (−69.7 °F) , which is the lowest assumed temperature in the ISA. The standard atmosphere contains no moisture. Unlike the idealized ISA, the temperature of the actual atmosphere does not always fall at a uniform rate with height. For example, there can be an inversion layer in which the temperature increases with altitude. The temperature profile of

5220-475: 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

5310-432: The difference by 125 m/°C. If the environmental lapse rate is less than the moist adiabatic lapse rate, the air is absolutely stable — rising air will cool faster than the surrounding air and lose buoyancy . This often happens in the early morning, when the air near the ground has cooled overnight. Cloud formation in stable air is unlikely. If the environmental lapse rate is between the moist and dry adiabatic lapse rates,

5400-412: The dry rate is the cause of foehn wind phenomenon (also known as " Chinook winds " in parts of North America). The phenomenon exists because warm moist air rises through orographic lifting up and over the top of a mountain range or large mountain. The temperature decreases with the dry adiabatic lapse rate, until it hits the dew point, where water vapor in the air begins to condense. Above that altitude,

5490-407: 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

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5580-425: 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,

5670-424: 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

5760-490: 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

5850-409: 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

5940-459: The greenhouse effect is reduced to a point where Earth has its observed surface temperature of around 288 K (15 °C; 59 °F). As convection causes parcels of air to rise or fall, there is little heat transfer between those parcels and the surrounding air. Air has low thermal conductivity , and the bodies of air involved are very large; so transfer of heat by conduction is negligibly small. Also, intra-atmospheric radiative heat transfer

6030-496: 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

6120-567: 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

6210-433: 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,

6300-417: 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

6390-401: 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

6480-483: 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

6570-407: The parcel must be heated from below to its convective temperature . The cloud base will be somewhere within the layer bounded by these parameters. The difference between the dry adiabatic lapse rate and the rate at which the dew point drops is around 4.5 °C per 1,000 m. Given a difference in temperature and dew point readings on the ground, one can easily find the LCL by multiplying

6660-416: The point where it is equal to the actual vapor pressure of water. With further decrease in temperature the water vapor in excess of the equilibrium amount condenses, forming cloud , and releasing heat (latent heat of condensation). Before saturation, the rising air follows the dry adiabatic lapse rate. After saturation, the rising air follows the moist (or wet ) adiabatic lapse rate. The release of latent heat

6750-526: 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

6840-472: 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

6930-418: 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

7020-481: 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

7110-412: The same thing, just that the lapse rate is a prerequisite for the greenhouse effect. The presence of greenhouse gases on a planet causes radiative cooling of the air, which leads to the formation of a non-zero lapse rate. So, the presence of greenhouse gases leads to there being a greenhouse effect at a global level. However, this need not be the case at a localized level. The localized greenhouse effect

7200-423: 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

7290-557: 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

7380-437: The surface tends to rise and carry internal energy upward, especially if the air has been moistened by evaporation from water surfaces. This is the process of convection . Vertical convective motion stops when a parcel of air at a given altitude has the same density as the other air at the same elevation. Convection carries hot, moist air upward and cold, dry air downward, with a net effect of transferring heat upward. This makes

7470-587: The temperature as a function of altitude for a packet of air which is ascending or descending without exchanging heat with its environment. Thermodynamics defines an adiabatic process as: the first law of thermodynamics can be written as Also, since the density ρ = m / V {\displaystyle \rho =m/V} and γ = c p / c v {\displaystyle \gamma =c_{\text{p}}/c_{\text{v}}} , we can show that: where c p {\displaystyle c_{\text{p}}}

7560-410: 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

7650-518: 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

7740-408: 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

7830-474: The universal result must be one in which the temperature is uniform, i.e., the lapse rate is zero. Spatial gradient A spatial gradient is a gradient whose components are spatial derivatives , i.e., rate of change of a given scalar physical quantity with respect to the position coordinates in physical space . Homogeneous regions have spatial gradient vector norm equal to zero. When evaluated over vertical position (altitude or depth), it

7920-502: 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

8010-503: 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

8100-419: The windward side of the mountain range. In addition, because the air has lost much of its original water vapor content, the descending air creates an arid region on the leeward side of the mountain. If the environmental lapse rate was zero, so that the atmosphere was the same temperature at all elevations, then there would be no greenhouse effect . This doesn't mean the lapse rate and the greenhouse effect are

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