17.2 Earth Science, Tarbuck and Lutgens: Heating the Atmosphere
How are heat and temperature related?
What are the three major mechanisms of heat transfer?
How is the atmosphere affected by each of the heat transfer mechanisms?
The concepts of heat and temperature often are confused. The phrase “in the heat of the day” is one common expression in which the word “heat” is misused to describe the concept of temperature.
Heat is the energy transferred from one object to another because of a difference in their temperatures. Recall that all matter is composed of atoms or molecules that possess kinetic energy, or the energy of motion.
Temperature is a measure of the average kinetic energy of the individual atoms or molecules in a substance. When energy is transferred to the gas atoms and molecules in air, those particles move faster and air temperature rises. When air transfers energy to a cooler object, its particles move slower, and air temperature drops.
Energy Transfer as Heat
Three mechanisms of energy transfer as heat are conduction, convection, and radiation.
All three processes, illustrated in Figure 9, happen simultaneously in the atmosphere. These mechanisms operate to transfer energy between Earth’s surface (both land and water) and the atmosphere.
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.
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 (by Electromagnetic Waves)
The sun is the ultimate source of energy that creates our weather. You know that the sun emits light and heat as well as the ultraviolet rays that cause a suntan. These forms of energy are only part of a large array of energy called the electromagnetic spectrum. This spectrum of electromagnetic energy is shown in Figure 10. All radiation, whether X-rays, radio waves, or heat waves, travel through the vacuum of space at 300,000 kilometers per second. They travel only slightly slower through our atmosphere.
“Radiation” is a word with many meanings – it doesn’t necessarilly mean nuclear radiation, or anything that could give you cancer
Dropping a stone in water – waves radiate out from where the stone hits. This is physical radiation.
People doing the wave in a crowd – the motion radiates from one side to another
Visible light is a form of electromagnetic radiation, and light radiates from the Sun, down to the Earth; light radiates from the classroom light-bulb throughout our room. (And again, this radiation can not cause cancer.)
The third mechanism of heat transfer is radiation. As shown in Figure 9, 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.