Convection
Convection in the most general terms refers to the internal movement of currents within fluids (i.e. liquids and gases). It cannot occur in solids due to the atoms not being able to flow freely.
Convection may cause a related phenomenon called advection, in which either mass or heat is transported by the currents or motion in the fluid.
In difference to diffusion (driven by an internal force) convection is fluid motion caused by an external force such as gravity or heating.
A common use of the term convection relates to the special case in which advected (carried) substance is heat. In this case, the heat itself may be an indirect cause of the fluid motion even while being transported by it. In this case, the problem of heat transport (and related transport of other substances in the fluid due to it) may become especially complicated.
Mechanism of the special case of heat-driven heat convection
The mechanism of heat-driven convection is that uneven heating of fluids may cause uneven densities due to temperature driven expansion or contraction. In a gravity field (or other equivalent acceleration situation), such differences cause forces due to buoyancy of the less-dense parcels of fluid.
Purely heat-driven convection in gravity fields, especially that which itself carries heat, is sometimes referred to as "natural heat convection." A familiar example is the process that carries heated air upward from a fire or hot object.
Atmospheric heat-driven convection
In the case of Earth's atmosphere, solar radiation heats the Earth's surface, and this heat is then transferred to the atmosphere by processes that are mostly convective. When a parcel of air is heated, it expands, becoming less dense and is pushed upward by buoyancy, carrying the heat energy upward with it. The air then cools, so it contracts, and sinks. The cycle then repeats with the cold air reheating and rising again. Since it cannot sink through the rising air beneath it, it moves laterally and then begins to sink. These convection currents cause local breezes, winds, thermals, cyclones and thunderstorms, and at a larger scale, produce the global atmospheric circulation features.
A single region of air with a falling and rising current is called a convection cell.
Forced convection
Natural heat convection (also called free convection) is distinguished from various types of forced heat convection, which refer to heat advection by a fluid which is not due to the natural forces of buoyancy induced by heating. In forced heat convection, transfer of heat is due to movement in the fluid which results from many other forces, such as (for example) a fan or pump. A convection oven thus 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.
Buoyancy induced convection not due to heat
Buoyancy forces which cause convection in gravity fields may result from sources of density variations in fluids other than those produced by heat, such as variable composition. For example, 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. Similarly variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (densest parts deepest) continues to cause a faction of the convection of fluid rock and molten metal within the Earth's interior (see below).
Oceanic convection
Solar radiation also affects the oceans. Warm water from the Equator tends to circulate toward the poles, while cold polar water heads towards the Equator. Oceanic convection is also frequently driven by density differences due to varying salinity, known as thermohaline convection, and is of crucial importance in the global thermohaline circulation. In this case it is quite possible for relatively warm, saline water to sink, and colder, fresher water to rise, reversing the normal transport of heat.
Mantle convection
Convection within Earth's mantle is the driving force for plate tectonics. There are actually two convection currents occuring within the Earth. The outer core has an extremely rapid convective turnover of fluid metals (primarily iron and nickel) which are responsible for the Earth's magnetic field. The movement of metals forms electrical currents, which in turn generate magnetic fields.
As heat from the inner and outer core heat the lower portion of the mantle, a second set of convective currents form. This mantle convection is extremely slow, as the mantle is a thick semi-solid with the consistency of a very thick paste. This slow convection can take millions of years to complete one cycle.
The source of the heat in the inner core is the radioactive decay of 40K, 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.
Vibration convection in gravity fields
Vibration-induced convection occurs in powders and granulated materials in containers subject to vibration, in a gravity field. 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 larger particles. Thus, larger particles tend to become sorted to the top of such a mixture.
Scale and rate of convection
Convection may happen in fluids at all scales larger than a few atoms. Convection occurs on a large scale in atmospheres, oceans, and planetary mantles. Current 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.
Pattern formation
 | This December 2006 may be confusing or unclear to readers. |
Convection, especially Rayleigh-Bénard convection, where the convecting fluid is contained by two rigid horizontal plates, is a convenient example of a pattern forming system.
When heat is fed into the system from one direction (usually below), at small values it merely difuses (conducts) from below upward, without causing fluid flow. As the heat flow is increased, above a critical value of the Rayleigh number, the system undergoes a bifurcation from the stable conducting state to the convecting state, where bulk motion of the fluid due to heat begins. If fluid parameters other than density do not depend significantly on temperature, the flow profile is symmetric, with the same volume of fluid rising as falling. This is known as Boussinesq convection.
As the temperature difference between the top and bottom of the fluid becomes higher, significant differences in fluid parameters other than density may develop in the fluid due to temperature. An example of such a parameter is viscosity, which may begin to significantly vary horizontally across layers of fluid. This breaks the symmetry of the system, and generally changes the pattern of up- and down-moving fluid from stripes to hexagons, as seen at right. Such hexagons are one example of a convection cell.
As the Rayleigh number is increased even further above the value where convection cells first appear, the system may undergo other bifurcations, and other more complex patters, such as spirals, may begin to appear. These may be familiar as examples from systems in which viscosity is relatively low and heat through-put high, such as the spiraling upward flow of gases in a fire.