CONVECTION

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-thunderhead-is a dramatic example of a convection cell.
A cumulonimbus cloud-thunderhead-is a dramatic example of a convection cell.
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.
Convective cells appear on the Sun's surface as a grainy pattern formed by variations in temperature.
Convective cells appear on the Sun’s surface as a grainy pattern formed by variations in temperature.

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.)

KEY TERMS

Asthenosphere: A region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The asthenosphere lies at a depth of about 60-215 mi. (about 100-350 km).
Atmosphere: In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.
Biosphere: A combination of all living things on Earth—plants, animals, birds, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.
Conduction: The transfer of heat by successive molecular collisions. Conduction is the principal means of heat transfer in solids, particularly metals.
Convection: Vertical circulation that results from differences in density ultimately brought about by differences in temperature. Convection involves the transfer of heat through the motion of hot fluid from one place to another and is of two types, natural and forced. (See natural convection, forced convection.)
Convection current: The flow of material heated by means of convection.
Convective cell: The circular pattern created by the rising of warmed fluid and the sinking of cooled fluid. This is sometimes called a convection cell.
Core: The center of Earth, an area constituting about 16% of the planet’s volume and 32% of its mass. Made primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi. (1,220 km) and a liquid outer core about 1,750 mi. (2,820 km) thick.
Crust: The uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3 to 37 mi. (5 to 60 km). Below the crust is the mantle.
Fluid: In the physical sciences, the term fluid refers to any substance that flows and therefore has no definite shape. Fluids can be both liquids and gases. In the earth sciences, occasionally substances that appear to be solid (for example, ice in glaciers) are, in fact, flowing slowly.
Forced convection: Convection that results from the action of a pump or other mechanism (whether man-made or natural), directing heated fluid toward a particular destination.
Geosphere: The upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
Heat: Internal thermal energy that flows from one body of matter to another.
Hydrosphere: The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
Kinetic energy: The energy that an object possesses by virtue of its motion.
Lithosphere: The upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle.
Mantle: The dense layer of rock, approximately 1,429 mi. (2,300 km) thick, between Earth’s crust and its core.
Natural convection: Convection that results from the buoyancy of heated fluid, which causes it to rise.
Plate tectonics: The name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the litho-sphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement.
Plates: Large, movable segments of the lithosphere.
Radiation: The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun’s energy via the electromagnetic spectrum by means of radiation.
Subsidence: A term that refers 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. Subsidence thus is defined variously as the downward movement of air, the sinking of ground, or a depression in Earth’s crust.
System: Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.
Tectonics: The study of tectonism, including its causes and effects, most notably mountain building.
Tectonism: The deformation of the lithosphere.
Temperature: The direction of internal energy flow between two systems when heat is being transferred. Temperature measures the average molecular kinetic energy in transit between those systems.
Thermal energy: Heat energy, a form of kinetic energy produced by the motion of atomic or molecular particles in relation to one another. The greater the relative motion of these particles, the greater the thermal energy.
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.

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