Three mechanisms of energy transfer as heat are conduction, convection, and radiation
Anyone who has touched a metal spoon that was left in a hot pan has experienced the result of heat conducted through the spoon. Conduction is the transfer of heat through matter by molecular activity. The energy of molecules is transferred by collisions from one molecule to another. Heat flows from the higher temperature matter to the lower temperature matter.
The ability of substances to conduct heat varies greatly. Metals are good conductors, as those of us who have touched hot metal have quickly learned. Air, however, is a very poor conductor of heat. Because air is a poor conductor, conduction is important only between Earth’s surface and the air directly in contact with the surface. For the atmosphere as a whole, conduction is the least important mechanism of heat transfer.
All metals are good conductors of electricity. For a similar reason, they are also good conductors of heat. In metals, not only do the atoms vibrate more when heated, but the free electrons charge around more as well. These transfer the energy much faster than just vibrations in bonds.
Much of the heat transfer that occurs in the atmosphere is carried on by convection. Convection is the transfer of heat by mass movement or circulation within a substance. It takes place in fluids, like the ocean and air, where the atoms and molecules are free to move about. Convection also takes place in solids, such as Earth’s mantle, that behave like fluids over long periods of time.
The pan of water in Figure 9 shows circulation by convection. Radiation from the fire warms the bottom of the pan, which conducts heat to the water near the bottom of the container. As the water is heated, it expands and becomes less dense than the water above. The warmer water rises because of its buoyancy. At the same time, cooler, denser water near the top of the pan sinks to the bottom, where it becomes heated. As long as the water is heated unequally, it will continue to circulate. In much the same way, most of the heat acquired by radiation and conduction in the lowest layer of the atmosphere is transferred by convective flow.
Radiation travels out in all directions from its source. Unlike conduction and convection,which need material to travel through, radiant energy can travel through the vacuum of space. Solar energy reaches Earth by radiation.
To understand how the atmosphere is heated, it is useful to think about four laws governing radiation.
1. All objects, at any temperature, emit radiant energy. Not only hot objects like the sun but also Earth—
including its polar ice caps—continually emit energy.
2. Hotter objects radiate more total energy per unit area than colder objects do.
3. The hottest radiating bodies produce the shortest wavelengths of maximum radiation. For example, the sun, with a surface temperature of nearly 6000°C radiates maximum energy at 0.5 micrometers, which is in the visible range. The maximum radiation for Earth occurs at a wavelength of 10 micrometers, well within the infrared range.
4. Objects that are good absorbers of radiation are good emitters as well. Gases are selective absorbers and radiators. The atmosphere does not absorb certain wavelengths of radiation, but it is a good absorber of other wavelengths
What Happens to Solar Radiation?
When radiation strikes an object, there usually are three different results.
1. Some energy is absorbed by the object. When radiant energy is absorbed, it is converted to heat and causes
a temperature increase.
2. Substances such as water and air are transparent to certain wavelengths of radiation. These substances
transmit the radiant energy. Radiation that is transmitted does not contribute energy to the object.
3. Some radiation may bounce off the object without being absorbed or transmitted. Figure 12 shows what
happens to incoming solar radiation, averaged for the entire globe.
Units of Energy and Specific Heat