convection

(Click to enlarge)

convection

Air heated by a space heater rises and is replaced by cool air, creating a convection current that circulates hot air throughout a room. (Precision Graphics)

(kən-vĕk'shən)

1. The act or process of conveying; transmission.
2. Physics.
   1. Heat transfer in a gas or liquid by the circulation of currents from one region to another.
   2. Fluid motion caused by an external force such as gravity.
3. Meteorology. The transfer of heat or other atmospheric properties by massive motion within the atmosphere, especially by such motion directed upward.

[Late Latin convectiō, convectiōn-, from convectus, past participle of convehere, to carry together : Latin com-, com- + Latin vehere, to carry.]

View more Science videos

For more information on convection, visit Britannica.com.

Concept

Convection is the name for a means of heat transfer, as distinguished from conduction and radiation. It is also a term that describes processes affecting the atmosphere, waters, and solid earth. In the atmosphere, hot air rises on convection currents, circulating and creating clouds and winds. Likewise, convection in the hydrosphere circulates water, keeping the temperature gradients of the oceans stable. The term convection generally refers to the movement of fluids, meaning liquids and gases, but in the earth sciences, convection also can be used to describe processes that occur in the solid earth. This geologic convection, as it is known, drives the plate movement that is one of the key aspects of plate tectonics.

How It Works

Introduction to Convection

Some concepts and phenomena cross disciplinary boundaries within the earth sciences, an example being the physical process of convection. It is of equal relevance to scientists working in the geologic, atmospheric, and hydrologic sciences, or the realms of study concerned with the geosphere, atmosphere, and hydrosphere, respectively. The only major component of the earth system not directly affected by convection is the biosphere, but given the high degree of interconnection between different subsystems, convection indirectly affects the biosphere in the air, waters, and solid earth.

Convection can be defined as vertical circulation that results from differences in density ultimately brought about by differences in temperature, and it involves the transfer of heat through the motion of hot fluid from one place to another. In the physical sciences, the term fluid refers to any substance that flows and therefore has no definite shape. This usually means liquids and gases, but in the earth sciences it can refer even to slow-flowing solids. Over the great expanses of time studied by earth scientists, the net flow of solids in certain circumstances (for example, ice in glaciers) can be substantial.

Convection and Heat

As indicated in the preceding paragraph, convection is related closely to heat and temperature and indirectly related to another phenomenon, thermal energy. What people normally call heat is actually thermal energy, or kinetic energy (the energy associated with movement) produced by molecules in motion relative to one another.

Heat, in its scientific meaning, is internal thermal energy that flows from one body of matter to another or from a system at a higher temperature to a system at a lower temperature. Temperature thus can be defined as a measure of the average molecular kinetic energy of a system. Temperature also governs the direction of internal energy flow between two systems. Two systems at the same temperature are said to be in a state of thermal equilibrium; when this occurs, there is no exchange of heat, and therefore heat exists only in transfer between two systems.

There is no such thing as cold, only the absence of heat. If heat exists only in transit between systems, it follows that the direction of heat flow must always be from a system at a higher temperature to a system at a lower temperature. (This fact is embodied in the second law of thermodynamics, which is discussed, along with other topics mentioned here, in Energy and Earth.) Heat transfer occurs through three means: conduction, convection, and radiation.

Conduction and Radiation

Conduction involves successive molecular collisions and the transfer of heat between two bodies in contact. It usually occurs in a solid. Convection requires the motion of fluid from one place to another, and, as we have noted, it can take place in a liquid, a gas, or a near solid that behaves like a slow-flowing fluid. Finally, radiation involves electromagnetic waves and requires no physical medium, such as water or air, for the transfer.

If you put one end of a metal rod in a fire and then touch the "cool" end a few minutes later, you will find that it is no longer cool. This is an example of heating by conduction, whereby kinetic energy is passed from molecule to molecule in the same way as a secret is passed from one person to another along a line of people standing shoulder to shoulder. Just as the original phrasing of the secret becomes garbled, some kinetic energy is inevitably lost in the series of transfers, which is why the end of the rod outside the fire is still much cooler than the one sitting in the flames.

As for radiation, it is distinguished from conduction and convection by virtue of the fact that it requires no medium for its transfer. This explains why space is cold yet the Sun's rays warm Earth: the rays are a form of electromagnetic energy, and they travel by means of radiation through space. Space, of course, is the virtual absence of a medium, but upon entering Earth's atmosphere, the heat from the electromagnetic rays is transferred to various media in the atmosphere, hydrosphere, geosphere, and biosphere. That heat then is transferred by means of convection and conduction.

Heat Transfer Through Convection

Like conduction and unlike radiation, convection requires a medium. However, in conduction the heat is transferred from one molecule to another, whereas in convection the heated fluid itself is actually moving. As it does, it removes or displaces cold air in its path. The flow of heated fluid in this situation is called a convection current.

Convection is of two types: natural and forced. Heated air rising is an example of natural convection. Hot air has a lower density than that of the cooler air in the atmosphere above it and therefore is buoyant; as it rises, however, it loses energy and cools. This cooled air, now denser than the air around it, sinks again, creating a repeating cycle that generates wind.

Forced convection occurs when a pump or other mechanism moves the heated fluid. Examples of forced-convection apparatuses include some types of ovens and even refrigerators or air conditioners. As noted earlier, it is possible to transfer heat only from a high-temperature reservoir to a low-temperature one, and thus these cooling machines work by removing hot air. The refrigerator pulls heat from its compartment and expels it to the surrounding room, while an air conditioner pulls heat from a room or building and releases it to the outside.

Forced convection does not necessarily involve man-made machines: the human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the skin by means of conduction, and at the surface of the skin it is removed from the body in a number of ways, primarily by the cooling evaporation of perspiration.

Real-Life Applications

Convective Cells

One important mechanism of convection, whether in the air, water, or even the solid earth, is the convective cell, sometimes known as the convection cell. The latter may be defined as the circular pattern created by the rising of warmed fluid and the sinking of cooled fluid. Convective cells may be only a few millimeters across, or they may be larger than Earth itself.

These cells can be observed on a number of scales. Inside a bowl of soup, heated fluid rises, and cooled fluid drops. These processes are usually hard to see unless the dish in question happens to be one such as Japanese miso soup. In this case, pieces of soybean paste, or miso, can be observed as they rise when heated and then drop down into the interior to be heated again.

On a vastly greater scale, convective cells are present in the Sun. These vast cells appear on the Sun's surface as a grainy pattern formed by the variations in temperature between the parts of the cell. The bright spots are the top of rising convection currents, while the dark areas are cooled gas on its way to the solar interior, where it will be heated and rise again.

A cumulonimbus cloud, or "thunderhead," is a particularly dramatic example of a convection cell. These are some of the most striking cloud formations one ever sees, and for this reason the director Akira Kurosawa used scenes of rolling thunderheads to add an atmospheric quality (quite literally) to his 1985 epic Ran. In the course of just a few minutes, these vertical towers of cloud form as warmed, moist air rises, then cools and falls. The result is a cloud that seems to embody both power and restlessness, hence Kurosawa's use of cumulonimbus clouds in a scene that takes place on the eve of a battle.

A Sea Breeze

Convective cells, along with convection currents, help explain why there is usually a breeze at the beach. At the seaside, of course, there is a land surface and a water surface, both exposed to the Sun's light. Under such exposure, the temperature of land rises more quickly than that of water. The reason is that water has an extraordinarily high specific heat capacity—that is, the amount of heat that must be added to or removed from a unit of mass for a given substance to change its temperature by 33.8°F (1°C). Thus a lake, stream, or ocean is always a good place to cool down on a hot summer day.

The land, then, tends to heat up more quickly, as does the air above it. This heated air rises in a convection current, but as it rises and thus overcomes the pull of gravity, it expends energy and therefore begins to cool. The cooled air then sinks. And so it goes, with the heated air rising and the cooling air sinking, forming a convective cell that continually circulates air, creating a breeze.

Convective Cells Under Our Feet

Convective cells also can exist in the solid earth, where they cause the plates (movable segments) of the lithosphere—the upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle—to shift. They thus play a role in plate tectonics, one of the most important areas of study in the earth sciences. Plate tectonics explains a variety of phenomena, ranging from continental drift to earthquakes and volcanoes. (See Plate Tectonics for much more on this subject.)

Whereas the Sun's electromagnetic energy is the source of heat behind atmospheric convection, the energy that drives geologic convection is geothermal, rising up from Earth's core as a result of radioactive decay. (See Energy and Earth.) The convective cells form in the asthenosphere, a region of extremely high pressure at a depth of about 60-215 mi. (about 100-350 km), where rocks are deformed by enormous stresses.

In the asthenosphere, heated material rises in a convection current until it hits the bottom of the lithosphere (the upper layer of Earth's interior, comprising the crust and the top of the mantle), beyond which it cannot rise. Therefore it begins moving laterally or horizontally, and as it does so, it drags part of the lithosphere. At the same time, this heated material pushes away cooler, denser material in its path. The cooler material sinks lower into the mantle (the thick, dense layer of rock, approximately 1,429 mi. [2,300 km] thick, between Earth's crust and core) until it heats again and ultimately rises up, thus propagating the cycle.

Subsidence: Fair Weather and Foul

As with convective cells, subsidence can occur in the atmosphere or geosphere. The term subsidence can refer either to the process of subsiding, on the part of air or solid earth, or, in the case of solid earth, to the resulting formation. It thus is defined variously as the downward movement of air, the sinking of ground, or a depression in the earth. In the present context we will discuss atmospheric subsidence, which is more closely related to convection. (For more about geologic subsidence, see the entries Geomorphology and Mass Wasting.)

In the atmosphere, subsidence results from a disturbance in the normal upward flow of convection currents. These currents may act to set up a convective cell, as we have seen, resulting in the flow of breeze. The water vapor in the air may condense as it cools, changing state to a liquid and forming clouds. Convection can create an area of low pressure, accompanied by converging winds, near Earth's surface, a phenomenon known as a cyclone. On the other hand, if subsidence occurs, it results in the creation of a high-pressure area known as an anticyclone.

Air parcels continue to rise in convective currents until the density of their upper portion is equal to that of the surrounding atmosphere, at which point the column of air stabilizes. On the other hand, subsidence may occur if air at an altitude of several thousand feet becomes denser than the surrounding air without necessarily being cooler or moister. In fact, this air is unusually dry, and it may be warm or cold. Its density then makes it sink, and, as it does, it compresses the air around it. The result is high pressure at the surface and diverging winds just above the surface.

The form of atmospheric subsidence described here produces pleasant results, explaining why high-pressure systems usually are associated with fair weather. On the other hand, if the subsiding air settles onto a cooler lay of air, it creates what is known as a subsidence inversion, and the results are much less beneficial. In this situation a warm air layer becomes trapped between cooler layers above and below it, at a height of several hundred or even several thousand feet. This means that air pollution is trapped as well, creating a potential health hazard. Subsidence inversions occur most often in the far north during the winter and in the eastern United States during the late summer.

When a Non-Fluid Acts Like a Fluid

Up to this point we have spoken primarily of convection in the atmosphere and the geosphere, but it is of importance also in the oceans. The miso soup example given earlier illustrates the movement of fluid, and hence of particles, that can occur when a convective cell is set up in a liquid.

Likewise, in the ocean convection—driven both by heat from the surface and, to a greater extent, by geothermal energy at the bottom—keeps the waters in constant circulation. Oceanic convection results in the transfer of heat throughout the depths and keeps the ocean stably stratified. In other words, the strata, or layers, corresponding to various temperature levels are kept stable and do not wildly fluctuate.

Ocean waters fit the most common, everyday definition of fluid, but as noted at the beginning of this essay, a fluid can be anything that flows—including a gas or, in special circumstances, a solid. Solid rocks or solid ice, in the form of glaciers, can be made to flow if the materials are deformed sufficiently. This occurs, for instance, when the weight of a glacier deforms ice at the bottom, thus causing the glacier as a whole to move. Likewise, geothermal energy can heat rock and cause it to flow, setting into motion the convective process of plate tectonics, described earlier, which literally moves the earth.

Where to Learn More

Educator's Guide to Convection (Web site). <http://www.solarviews.com/eng/edu/convect.htm>.

Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts on File, 1992.

Hess, Harry. "History of Ocean Basins" (Web site). <http://www-geology.ucdavis.edu/~GEL102/hess/jesse.htm>.

Jones, Helen. Open-Ocean Deep Convection: A Field Guide (Web site). <http://puddle.mit.edu/~helen/oodc.html>.

Ocean Oasis Teacher's Guide Activity 4 (Web site). <http://www.oceanoasis.org/teachersguide/activity4.html>.

Santrey, Laurence, and Lloyd Birmingham. Heat. Mahwah, NJ: Troll Associates, 1985.

Scorer, R. S., and Arjen Verkaik. Spacious Skies. Newton Abbot, England: David and Charles, 1989.

Sigurdsson, Haraldur. Melting the Earth: The History of Ideas on Volcanic Eruptions. New York: Oxford University Press, 1999.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 5th ed. New York: John Wiley and Sons, 1999.

Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.

The transfer of thermal energy by actual physical movement from one location to another of a substance in which thermal energy is stored. A familiar example is the free or forced movement of warm air throughout a room to provide heating. Technically, convection denotes the nonradiant heat exchange between a surface and a fluid flowing over it. Although heat flow by conduction also occurs in this process, the controlling feature is the energy transfer by flow of the fluid—hence the name convection. Convection is one of the three basic methods of heat transfer, the other two being conduction and radiation. See also Conduction (heat); Heat radiation; Heat transfer.

Natural convection is exemplified by the cooling of a vertical surface in a large quiescent body of air of temperature t_∞. The lower-density air next to a hot vertical surface moves upward because of the buoyant force of the higher-density cool air farther away from the surface. At any arbitrary vertical location x, the actual variation of velocity u with distance y from the surface will be similar to that in the illustration b, increasing from zero at the surface to a maximum, and then decreasing to zero as ambient surrounding conditions are reached. In contrast, the temperature t of the air decreases from the heated wall value t's to the surrounding air temperature. These temperature and velocity distributions are clearly interrelated, and the distances from the wall through which they exist are coincident because, when the temperature approaches that of the surrounding air, the density difference causing the upward flow approaches zero.

Temperature and velocity distributions in air near a heated vertical surface at arbitary vertical location. The distance δ is that distance at which the velocity and the temperature reach ambient surrounding conditions.

The region in which these velocity and temperature changes occur is called the boundary layer. Because velocity and temperature gradients both approach zero at the outer edge, there will be no heat flow out of the boundary layer by conduction or convection. See also Boundary-layer flow.

When air is blown across a heated surface, forced convection results. Although the natural convection forces are still present in this latter case, they are clearly negligible compared with the imposed forces. The process of energy transfer from the heated surface to the air is not, however, different from that described for natural convection. The major distinguishing feature is that the maximum fluid velocity is at the outer edge of the boundary layer. This difference in velocity profile and the higher velocities provide more fluid near the surface to carry along the heat conducted normal to the surface. Consequently, boundary layers are very thin.

Heat convection in turbulent flow is interpreted similarly to that in laminar flow. Rates of heat transfer are higher for comparable velocities, however, because the fluctuating velocity components of the fluid in a turbulent flow stream provide a macroscopic exchange mechanism which greatly increases the transport of energy normal to the main flow direction. Because of the complexity of this type of flow, most of the information regarding heat transfer has been obtained experimentally. See also Laminar flow; Turbulent flow.

Convection heat transfer which occurs during high-speed flight or high-velocity flow over a surface is known as aerodynamic heating. This heating effect results from the conversion of the kinetic energy of the fluid as it approaches a body to internal energy as it is slowed down next to the surface. In the case of a gas, its temperature increases, first, because of compression as it passes through a shock and approaches the stagnation region, and second, because of frictional dissipation of kinetic energy in the boundary layer along the surface.

The phenomena of condensation and boiling are important phase-change processes involving heat release or absorption. Because vapor and liquid movement are present, the energy transfer is basically by convection. Local and average heat-transfer coefficients are determined and used in the Newton cooling-law equation for calculating heat rates which include the effects of the latent heat of vaporization.

The process whereby heat is transferred from one part of a liquid or gas to another, by movement of the fluid itself. (Because, in meteorology, the most striking weather events are due to upward convection, it is possible to forget that all convection currents also have a downward component.) Convection carries excess heat from the earth's surface and distributes it through the troposphere.

In the atmosphere, warmer, lighter air moves upward and is replaced by colder, heavier air. This is free convection, or thermal convection, propelled by buoyancy. The upward movement of an air parcel over mountains, at fronts, or because of turbulence is known as forced convection, or mechanical convection. See also cumuliform convection, slope convection.

Heat transmission, either natural or forced (by means of a fan), by currents of air resulting from differences in density due to temperature differences in the heated space.

The transfer of heat from one place to another by the motion of a gas or a liquid across the heated surface.

Movements of mantle material, laterally or in upward-downward directions, due to heat variations.

The motion of warm material that rises, cools off, and sinks again, producing a continuous circulation of material and transfer of heat. Some examples of processes involving convection are boiling water, in which heat is transferred from the stove to the air; the circulation of the atmosphere of the Earth, transferring heat from the equator to the North Pole and South Pole; and plate tectonics, in which heat is transferred from the interior of the Earth to its surface.

The act of conveying or transmission; specifically, transmission of heat in a liquid or gas by circulation of heated particles.

• Heat - convection: transfer of heat in a fluid, either by absorption or by rejection once the fluid has moved

This figure shows a calculation for thermal convection in the Earth's mantle. Colors closer to red are hot areas and colors closer to blue are cold areas. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and likewise, cold material from the top moves downwards.

Convection is the concerted, collective movement of ensembles of molecules within fluids (i.e. liquids, gases) and rheids. Convection of mass cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids. Diffusion of heat can take place in solids, but is referred to separately in that case as heat conduction.

Convective heat transfer is one of the major modes of heat transfer and convection is also a major mode of mass transfer in fluids. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.^[1] Note that in common use the term convection may refer loosely to heat transfer by convection, as opposed to mass transfer by convection, or the convection process in general. Sometimes "convection" is even used to refer specifically to "free heat convection" (natural heat convection), as opposed to forced heat convection. However, in mechanics the correct use of the word is the general sense, and different types of convection should be properly qualified for clarity.

Convection can be qualified in terms of being natural, forced, gravitational, granular, or thermomagnetic. It may also be said to be due to combustion, capillary action, or Marangoni and Weissenberg effects. Due to its role in heat transfer, 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 seen as clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics.

Contents 1 Terminology 2 Examples and applications of convection 2.1 Heat transfer 2.2 Convection cells 2.3 Atmospheric circulation 2.4 Weather 2.5 Oceanic circulation 2.6 Mantle convection 2.7 Stack effect 2.8 Stellar physics 3 Convection mechanisms 3.1 Natural convection 3.2 Forced convection 3.3 Gravitational or buoyant convection 3.4 Granular convection 3.5 Thermomagnetic convection 3.6 Capillary action 3.7 Marangoni effect 3.8 Weissenberg effect 3.9 Combustion 4 Mathematical models of convection 4.1 Quantifying natural versus forced convection 5 See also 6 References 7 External links

Terminology

The term convection may have slightly different but related usages in different scientific or engineering contexts or applications. The broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause.^{[2] [3]} However in thermodynamics "convection" often refers specifically to heat transfer by convection.^[4]

Additionally, convection includes fluid movement both by bulk motion (advection) and by the motion of individual particles (diffusion). However in some cases, convection is taken to mean only advective phenomena. For instance, in the transport equation, which describes a number of different transport phenomena, terms are separated into "convective" and "diffusive" effects, with "convective" meaning purely advective in context. A similar differentiation is made in the Navier–Stokes equations. In such cases the precise meaning of the term may be clear only from context.

Examples and applications of convection

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 our sun and all stars. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in 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.

Heat transfer

A heat sink provides a large surface area for convection to efficiently carry away heat.

Main article: Convective heat transfer

Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids.^[5] Heat is the entity of interest being advected (carried), and diffused (dispersed). This can be contrasted with conductive heat transfer, which is the transfer of energy by vibrations at a molecular level through a solid or fluid, and radiative heat transfer, the transfer of energy through electromagnetic waves.

Heat is transferred by convection in numerous examples of naturally occurring fluid flow, such as: wind, oceanic currents, and movements within the Earth's mantle. Convection is also used in engineering practices to provide desired temperature changes, as in heating of homes, industrial processes, cooling of equipment, etc.

The rate of convective heat transfer may be improved by the use of a heat sink, often in conjunction with a fan. For instance, a typical computer CPU will have a purpose-made fan to ensure its operating temperature is kept within tolerable limits.

Convection cells

Convection cells in a gravity field

Main article: Convection cell

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 a cold surface; because it exchanges heat with colder liquid through direct exchange; or in the example of the Earth's atmosphere, 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 the rising force beneath it, and the fluid begins to descend. As it descends, it warms again and the cycle repeats itself.

Atmospheric circulation

Idealised depiction of the global circulation on Earth

Main article: Atmospheric circulation

Atmospheric circulation is the large-scale movement of air, and the means by which thermal energy is distributed on the surface of the Earth, together with 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 is the consequence of the fact that incident solar radiation per unit area is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at the poles. It consists of two primary convection cells, the Hadley cell and the polar vortex, with the Hadley cell experiencing stronger convection as a consequence of the release of latent heat energy upon condensation at higher altitudes.

Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases more heat, but the temperature changes less than land. This effect is noticeable; it is what 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.

Weather

How Foehn is produced

See also: Cloud, Thunderstorm, and Wind

More localized phenomena than global atmospheric movement are also due to convection, including wind and some of the hydrologic cycle. For example, a foehn wind is a type of down-slope wind which occurs in 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.^[6] As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes.

A thermal column (or thermal) 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.^[7][8] The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude pressures. It stops rising when it has cooled to 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.^[9][10]

Stages of a thunderstorm's life.

Warm air has a lower density than cool air, so warm air rises within cooler air,^[11] similar to hot air balloons.^[12] Clouds form as relatively warmer air carrying moisture rises within cooler air. As the moist air rises, it cools causing some of the water vapor in the rising packet of air to condense.^[13] When the moisture condenses, it releases energy known as latent heat of fusion which allows the rising packet of air to cool less than its surrounding air,^[14] 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 a lifting force (heat).

All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage.^[15] 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.^[16]

Oceanic circulation

Ocean currents

Main articles: Gulf Stream and Thermohaline circulation

Solar radiation affects the oceans: warm water from 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,^[17] and the westerlies blow eastward at mid-latitudes.^[18] This wind pattern applies a stress to the subtropical ocean surface with negative curl across the Northern Hemisphere,^[19] and the reverse across the Southern Hemisphere. The resulting Sverdrup transport is equatorward.^[20] 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 the ocean basin, outweighing the effects of friction with the cold western boundary current which originates from high latitudes.^[21] 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.^[22]

As it travels poleward, warm water transported by the strong warm water current undergoes evaporative cooling. The cooling is wind driven: wind moving over the water cools it and also causes evaporation, leaving a saltier brine. In this process, the water increases in salinity and density, and decreases in temperature. Once sea ice forms, salts are left out of the ice, a process known as brine exclusion.^[23] These two processes produce water that is denser and colder (or, more precisely, water that is still liquid at a lower temperature). The water across the northern Atlantic ocean becomes so dense that it begins to sink down through less salty and less dense water. (The convective action 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 southgoing stream.^[24]

Mantle convection

An oceanic plate is added to by upwelling (left) and consumed at a subduction zone (right).

Main article: Mantle convection

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.^[25] It is the driving force that causes tectonic plates to move around the Earth's surface.^[26]

The Earth's surface 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 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.

Stack effect

Main article: Stack effect

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 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.

Stellar physics

An illustration of the structure of the Sun and a red giant star, showing their convective zones. These are the granular zones in the outer layers of these stars.

Granules—the tops or upper visible sizes of convection cells, seen on the photosphere of the Sun. These are caused by the convection in the upper photosphere of the Sun. North America is superimposed on the same scale, to indicate scale.

Main articles: Convection zone and granule (solar physics)

The convection zone of a star is the range of radii in which energy is transported primarily by convection.

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 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.

Convection mechanisms

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 natural and forced 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 (buoyancy), or surface forces acting at a boundary of the fluid.

The causes of convection are generally described as one of either "natural" ("free") or "forced", although other mechanisms also exist (discussed below). However the distinction between natural and forced convection is particularly important for convective heat transfer.

Natural convection

Main article: Natural convection

Natural convection, or free convection, occurs due to temperature differences which affect the density, and thus relative buoyancy, of the fluid. Heavier (more dense) components will fall, while lighter (less dense) components rise, leading to bulk fluid movement. Natural convection can only occur, therefore, in a gravitational field. A common example of natural convection is the rise of smoke from a fire. it can be seen in a pot of boiling water in which the hot and less-dense water on the bottom layer moves upwards in plumes, and the cool and more dense water near the top of the pot likewise sinks.

Natural convection will be more likely and/or more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection, and/or a larger distance through the convecting medium. Natural convection will be less likely and/or less rapid with more rapid diffusion (thereby diffusing away the thermal gradient that is causing the convection) and/or a more viscous (sticky) fluid.

The onset of natural convection can be determined by the Rayleigh number (Ra).

Note that 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).

Forced convection

Main article: Forced convection

In forced convection, also called heat advection, fluid movement results from external surface forces such as a fan or pump. Forced convection is typically used to increase the rate of heat exchange. Many types of mixing also utilize forced convection to distribute one substance within another. Forced convection also occurs as a by-product to other processes, such as the action of a propeller in a fluid or aerodynamic heating. Fluid radiator systems, and also heating and cooling of parts of the body by blood circulation, are other familiar examples of forced convection.

Forced convection may happen by natural means, such as when the heat of a fire causes expansion of air and bulk air flow by this means. In microgravity, such flow (which happens in all directions) along with diffusion is the only means by which fires are able to draw in fresh oxygen to maintain themselves. The shock wave that transfers heat and mass out of explosions is also a type of forced convection.

Although forced convection from thermal gas expansion in zero-g does not fuel a fire as well as natural convection in a gravity field, some types of artificial forced convection are far more efficient than free convection, as they are not limited by natural mechanisms. For instance, a convection oven works by forced convection, as a fan which rapidly circulates hot air forces heat into food faster than would naturally happen due to simple heating without the fan.

Gravitational or buoyant convection

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 is known as solutal convection.^[27] 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.^[28]

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 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 a g-force environment in order to occur.

Granular convection

Main article: Granular convection

Vibration-induced convection occurs in powders and granulated materials in containers subject to vibration where an axis of vibration is parallel to the force of gravity. When the container accelerates upward, the bottom of the container pushes the entire contents upward. In contrast, when the container accelerates downward, the sides of the container push the adjacent material downward by friction, but the material more remote from the sides is less affected. The net result is a slow circulation of particles downward at the sides, and upward in the middle.

If the container contains particles of different sizes, the downward-moving region at the sides is often narrower than the largest particles. Thus, larger particles tend to become sorted to the top of such a mixture. This is one possible explanation of the Brazil nut effect.

Thermomagnetic convection

Main article: Thermomagnetic convection

Thermomagnetic convection can occur when an external magnetic field 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.

This form of heat transfer can be useful for cases where conventional convection fails to provide adequate heat transfer, e.g., in miniature microscale devices or under reduced gravity conditions.

Capillary action

Main article: Capillary action

Capillary action is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials. This effect can cause liquids to flow against the force of gravity. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surfaces; If the diameter of the tube is sufficiently small, then the combination of surface tension and forces of adhesion between the liquid and container act to lift the liquid.

Marangoni effect

Main article: Marangoni effect

The Marangoni effect is the convection of fluid along an interface between dissimilar substances because of variations in surface tension. Surface tension can vary because of inhomogeneous composition of the substances, and/or the temperature-dependence of surface tension forces. In the latter case the effect is known as thermo-capillary convection.

A well-known phenomenon exhibiting this type of convection is the "tears of wine".

Weissenberg effect

Main article: Weissenberg effect

See also: Reptation

The Weissenberg effect is a phenomenon that occurs when a spinning rod is placed into a solution of liquid polymer. Entanglements cause the polymer chains to be drawn towards the rod instead of being thrown outward as would happen with an ordinary fluid (i.e., water).

Combustion

In a zero-gravity environment, there can be no buoyancy forces, and thus no natural (free) convection possible, so flames in many circumstances without gravity smother in their own waste gases. However, flames may be maintained with any type of forced convection (breeze); or (in high oxygen environments in "still" gas environments) entirely from the minimal forced convection that occurs as heat-induced expansion (not buoyancy) of gases allows for ventilation of the flame, as waste gases move outward and cool, and fresh high-oxygen gas moves in to take up the low pressure zones created when flame-exhaust water condenses.^[29]

Mathematical models of convection

Mathematically, convection can be described by the convection–diffusion equation or the generic scalar transport equation.

Quantifying natural versus forced convection

In cases of mixed convection (natural and forced occurring together) one would often like to know how much of the convection is due to external constraints, such as the fluid velocity in the pump, and how much is due to natural convection occurring in the system.

The relative magnitudes of the Grashof and Reynolds number squared determine which form of convection dominates. if forced convection may be neglected, whereas if natural convection may be neglected. If the ratio is approximately one, then both forced and natural convection need to be taken into account.

See also

• Atmospheric convection
• Bénard cells
• Churchill-Bernstein Equation
• Double diffusive convection
• Fluid dynamics
• Heat transfer
  • Heat conduction
  • Thermal radiation
• Heat pipe
• Laser-heated pedestal growth
• Nusselt number
• Thermomagnetic convection
• Vortex tube

References

1. ^ Frank P. Incropera; David P. De Witt and D. P. Dewitt (1990). Fundamentals of Heat and Mass Transfer (3rd ed.). John Wiley & Sons. ISBN 0-471-51729-1. 
2. ^ Munson, Bruce R.. Fundamentals of Fluid Mechanics. John Wiley & Sons. ISBN 047185526X. 
3. ^ * Falkovich, G. (2011). Fluid Mechanics, a short course for physicists. Cambridge University Press. p. 8 and Exercise 2.2. ISBN 978-1-107-00575-4. http://www.cambridge.org/gb/knowledge/isbn/item6173728/?site_locale=en_GB. 
4. ^ Çengel, Yunus A.; Boles, Michael A.. Thermodynamics:An Engineering Approach. McGraw-Hill Education. ISBN 007121688X. 
5. ^ Yugnus A Cengel (2003), “Heat transfer-A Practical Approach” 2nd ed. Publisher McGraw Hill Professional, p26 by ISBN 0072458933, 9780072458930, Google Book Search. Accessed 20-04.-09
6. ^ Dr. Michael Pidwirny (2008). "CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes". Physical Geography. http://www.physicalgeography.net/fundamentals/8e.html. Retrieved 2009-01-01. 
7. ^ National Weather Service Forecast Office in Tucson, Arizona (2008). "What is a monsoon?". National Weather Service Western Region Headquarters. http://www.wrh.noaa.gov/twc/monsoon/monsoon_whatis.php. Retrieved 2009-03-08. 
8. ^ Douglas G. Hahn and Syukuro Manabe (1975). "The Role of Mountains in the South Asian Monsoon Circulation". Journal of Atmospheric Sciences 32 (8): 1515–1541. Bibcode ...32.1515H 1975JAtS ...32.1515H. doi:10.1175/1520-0469(1975)032<1515:TROMIT>2.0.CO;2. ISSN 1520-0469. 
9. ^ University of Wisconsin. Sea and Land Breezes. Retrieved on 2006-10-24.
10. ^ JetStream: An Online School For Weather (2008). The Sea Breeze. National Weather Service. Retrieved on 2006-10-24.
11. ^ Albert Irvin Frye (1913). Civil engineers' pocket book: a reference-book for engineers, contractors. D. Van Nostrand Company. p. 462. http://books.google.com/?id=PDtIAAAAIAAJ&pg=PA462&lpg=PA462&dq=density+varies+by+temperature+book&q=. Retrieved 2009-08-31. 
12. ^ Yikne Deng (2005). Ancient Chinese Inventions. Chinese International Press. pp. 112–13. ISBN 9787508508375. http://books.google.com/?id=ssO_19TRQ9AC&pg=PA112&dq=Kongming+balloon. Retrieved 2009-06-18. 
13. ^ FMI (2007). "Fog And Stratus – Meteorological Physical Background". Zentralanstalt für Meteorologie und Geodynamik. http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/FgStr/backgr.htm. Retrieved 2009-02-07. 
14. ^ Chris C. Mooney (2007). Storm world: hurricanes, politics, and the battle over global warming. Houghton Mifflin Harcourt. p. 20. ISBN 9780151012879. http://books.google.com/?id=RRSzR4NQdGkC&pg=PA20&lpg=PA20&dq=cloud+formation+latent+heat+book&q=. Retrieved 2009-08-31. 
15. ^ Micheal H. Mogil (2007). Extreme Weather. New York: Black Dog & Leventhal Publisher. pp. 210–211. ISBN 978-1-57912-743-5. 
16. ^ National Severe Storms Laboratory (2006-10-15). "A Severe Weather Primer: Questions and Answers about Thunderstorms". National Oceanic and Atmospheric Administration. http://www.nssl.noaa.gov/primer/tstorm/tst_basics.html. Retrieved 2009-09-01. 
17. ^ Glossary of Meteorology (2009). "trade winds". Glossary of Meteorology. American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?id=trade-winds1. Retrieved 2008-09-08. 
18. ^ Glossary of Meteorology (2009). Westerlies. American Meteorological Society. Retrieved on 2009-04-15.
19. ^ Matthias Tomczak and J. Stuart Godfrey (2001). Regional Oceanography: an Introduction. Matthias Tomczak, pp. 42. ISBN 81-7035-306-8. Retrieved on 2009-05-06.
20. ^ Earthguide (2007). Lesson 6: Unraveling the Gulf Stream Puzzle - On a Warm Current Running North. University of California at San Diego. Retrieved on 2009-05-06.
21. ^ Angela Colling (2001). Ocean circulation. Butterworth-Heinemann, pp. 96. Retrieved on 2009-05-07.
22. ^ National Environmental Satellite, Data, and Information Service (2009). Investigating the Gulf Stream. North Carolina State University. Retrieved on 2009-05-06.
23. ^ Russel, Randy. "Thermohaline Ocean Circulation". University Corportation for Atmospheric Research. http://www.windows.ucar.edu/tour/link=/earth/Water/thermohaline_ocean_circulation.html. Retrieved 2009-01-06. 
24. ^ Behl, R.. "Atlantic Ocean water masses". California State University Long Beach. Archived from the original on May 23, 2008. http://web.archive.org/web/20080523170145/http://seis.natsci.csulb.edu/rbehl/NADW.htm. Retrieved 2009-01-06. 
25. ^ Kobes, Randy and Gabor Kunstatter (2002-12-16). "Mantle Convection". Physics Department, University of Winnipeg. http://theory.uwinnipeg.ca/mod_tech/node195.html. Retrieved 2010-01-03. 
26. ^ Kent C. Condie (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 5. ISBN 0-7506-3386-7. http://books.google.com/books?id=HZrA6OQzsvgC&pg=PA5. 
27. ^ "CiteSeerX — Pattern Formation in Solutal Convection: Vermiculated Rolls and Isolated Cells". Citeseerx.ist.psu.edu. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.15.8288. Retrieved 2010-09-12. 
28. ^ Raats, P. A. C. (1969). "Steady Gravitational Convection Induced by a Line Source of Salt in a Soil". Soil Science Society of America Proceedings 33 (4): 483. doi:10.2136/sssaj1969.03615995003300040005x. 
29. ^ Does a candle burn in zero-g?

External links

• Correlations for Convective Heat Transfer
• Fluid Mechanics webpage with video, Q&A etc

v d e Meteorological data and variables General Adiabatic processes Lapse rate Lightning Surface solar radiation Surface weather analysis Visibility Vorticity Wind Condensation Cloud Cloud condensation nuclei Fog Lifted condensation level (LCL) Precipitation Water vapor Convection Convective available potential energy (CAPE) Convective inhibition (CIN) Convective instability Convective temperature (Tc) Equilibrium level (EL) Helicity Level of free convection (LFC) Lifted index (LI) Bulk Richardson number (BRN) Temperature Dew point (Td) Equivalent temperature (Te) Forest fire weather index Haines Index Heat index Humidex Humidity Potential temperature (θ) Equivalent potential temperature (θe) Sea surface temperature (SST) Wet-bulb temperature Wet-bulb potential temperature Wind chill Pressure Atmospheric pressure Baroclinity Barotropicity

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - varmestrømning, konvektion

Nederlands (Dutch)
convectie

Français (French)
n. - (Phys, Élec) convection

Deutsch (German)
n. - (Phys.) Konvektion (Hitzetransfer)

Ελληνική (Greek)
n. - (φυσ.) μετάδοση/διάδοση (θερμότητας) με ρεύματα
attrib. - (φυσ.) μεταφοράς, εκ μεταφοράς

Italiano (Italian)
convezione

Português (Portuguese)
n. - convecção (f), transporte (m), propagação (f)
attrib. - aquele que propaga

Русский (Russian)
конвекция

Español (Spanish)
n. - convección

Svenska (Swedish)
n. - konvektion
attr. - konvektions-

中文（简体）(Chinese (Simplified))
传送, 对流

中文（繁體）(Chinese (Traditional))
n. - 傳送, 對流

한국어 (Korean)
n. - 대류, 전달

日本語 (Japanese)
n. - 伝達, 対流

العربيه (Arabic)
‏(الاسم) انتقال الحارة بواسطه الحمل (صفه)‏

עברית (Hebrew)
n. - ‮זרימת חום‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.