List and describe the three mechanisms of heat transfer.

The flow of energy can occur in three ways: conduction, convection, and radiation (Figure 2.11). Although we will present them separately, all three mechanisms of heat transfer can operate simultaneously and, working in tandem, these processes can transfer heat between the Sun and Earth and between Earth’s surface, its atmosphere, and outer space.

Conduction

Anyone who attempts to pick up a metal spoon left in a boiling pot of soup realizes that heat is transmitted along the entire length of the spoon. The transfer of heat in this manner is called conduction. The hot soup causes the molecules at the bowl end of the spoon to vibrate more rapidly. These molecules collide more vigorously with their neighbors and so on up the handle of the spoon. Thus, conduction is the transfer of heat through molecular collisions from one molecule to another. The ability of substances to conduct heat varies considerably. Metals are good conductors, as those of us who have touched a hot metal spoon quickly learned. Air, in contrast, is a very poor conductor of heat. Consequently, conduction is important only between Earth’s surface and the air immediately in contact with the surface. Conduction is the least significant means of heat transfer for the atmosphere as a whole, and we can disregard it when considering most meteorological phenomena.

Objects that are poor conductors, such as air, are called insulators. Most objects that are good insulators, such as cork, plastic foam, or goose down, contain many small air spaces. The poor conductivity of the trapped air gives these materials their insulating value. Snow, like other good insulators, contains numerous air spaces that impair the flow of heat. This is why wild animals may burrow into a snowbank to escape the “cold.” The snow, like a down-filled comforter, does not supply heat; it simply retards the loss of the animal’s own body heat.

Convection

Much of the heat transport in Earth’s atmosphere and oceans occurs by convection. Convection is heat transfer that involves the actual movement or circulation of a substance. It takes place in fluids (liquids such as water and gases such as air) where the material can flow.

The pan of water being heated over a campfire in Figure 2.11 illustrates the nature of a simple convective circulation. The fire warms the bottom of the pan, which conducts heat to the water inside. Because water is a relatively poor conductor of heat, only the water in close proximity to the bottom of the pan is heated by conduction. Heating causes water to expand and become less dense. Thus, the hot, buoyant water near the bottom of the pan rises, while the cooler, denser water above sinks. As long as the water is heated from the bottom and cools near the top, it will continue to “turn over,” producing a convective circulation.

In a similar manner, some of the air in the lowest layer of the atmosphere that is heated by radiation and conduction is then transported by convection to higher layers of the atmosphere. For example, on a hot, sunny day, a dark plowed field becomes warmer than the surrounding croplands, and the air above the plowed field will be heated more than the air above the crops. As warm, less-dense air above the plowed field buoys upward, it is replaced by the cooler air above the croplands (Figure 2.12). In this way, a convective flow is established. The warm parcels of rising air are called thermals, and they are what hang-glider pilots use to keep their crafts soaring. Convection of this type not only transfers heat but also transports moisture (water vapor) aloft. The result is an increase in cloudiness that frequently can be observed on warm summer afternoons.

On a much larger scale is the global convective circulation of the atmosphere, which is driven by the unequal heating of Earth’s surface. These complex movements are responsible for the redistribution of heat between hot equatorial regions and frigid polar latitudes; we will discuss them in detail in Chapter 7.

Atmospheric circulation consists of vertical as well as horizontal components, so energy is transferred both vertically and horizontally. Meteorologists often use the term convection to describe the part of atmospheric circulation that involves upward and downward motion of air. By contrast, the term advection is used to denote the horizontal component of airflow. A common example of advection is wind, a phenomenon we will examine closely in later chapters. Residents of the midlatitudes often experience the effects of heat transfer by advection. For example, when frigid Canadian air invades the U.S. Midwest in January, it brings bitterly cold winter weather, whereas advection of latent heat from the Gulf of Mexico is a primary source of energy for spring thunderstorms.

Radiation

The third mechanism of heat transfer is radiation. Unlike conduction and convection, radiation is the only mechanism that can transfer thermal energy through the vacuum of space and thus is responsible for solar energy reaching Earth.

Solar Radiation

The Sun is the ultimate source of energy that drives the weather machine. We know the Sun emits light of varying energy—including visible light, infrared radiation, and ultraviolet radiation. Although these forms of energy constitute a major portion of the total energy that radiates from the Sun, they are only a part of a large array of energy called radiation, or electromagnetic radiation. This array or spectrum of electromagnetic energy is shown in Figure 2.13.

All types of radiation from the Sun travel through the vacuum of space at 300,000 kilometers (186,000 miles) per second, a value known as the speed of light. To help visualize radiant energy, imagine ripples made in a calm pond when a pebble is tossed in. Like the waves produced in the pond, waves of electromagnetic radiation come in various sizes, or wavelengths—the distance from one crest to the next (Figure 2.13). Radio waves have the longest wavelengths, up to thousands of meters in length. Gamma waves are the shortest, at less than one-billionth of a centimeter. Shortwave radiation is usually measured in micrometers (abbreviated μm), which are one-millionth of a meter.

Radiation is often identified by the effect that it produces when it interacts with an object. The retinas of our eyes, for instance, are sensitive to a range of wavelengths called visible light. We often refer to visible light as white light because it appears white in color. It is easy to show, however, that white light is an array of colors, each color corresponding to a specific range of wavelengths. When it is passed through a prism, white light can be divided into the colors of the rainbow, from violet (with the shortest wavelength, 0.4 micrometer [μm]) to red (with the longest wavelength, 0.7 micrometer). See Figure 2.13.

Located adjacent to the color red, and having a longer wavelength, is infrared radiation (IR), which cannot be seen by the human eye but is detected as heat. On the opposite side of the visible range, located next to violet, the energy emitted is called ultraviolet (UV) radiation and consists of shorter wavelengths that may cause skin to become sunburned.

Although we divide radiant energy into categories based on our ability to perceive them, all wavelengths of radiation behave similarly. When an object absorbs any form of electromagnetic energy, the waves excite subatomic particles (electrons). This results in an increase in molecular motion and a corresponding increase in temperature. Thus, electromagnetic waves from the Sun travel through space and, upon being absorbed, increase the molecular motion of other molecules—including those that make up the atmosphere, Earth’s surface, and human bodies.

One important difference among the various wavelengths of radiant energy is that shorter wavelengths are more energetic. This accounts for the fact that exposure to relatively short (high-energy) ultraviolet waves can cause significant sunburn, while similar exposure to longer-wavelength radiation cannot. Extended exposure to UV radiation can result in skin cancer and cataracts (see Severe & Hazardous Weather Box 2.3).

It is important to note that the Sun emits all forms of radiation, but in varying quantities. Over 95 percent of all solar radiation is emitted in a narrow band between 0.1 and 2.5 micrometers, much of which is concentrated in the visible and near-infrared parts of the electromagnetic spectrum (Figure 2.14). Visible light represents over 43 percent of the total energy, infrared accounts for 49 percent, and ultraviolet, 7 percent. Less than 1 percent of solar radiation is emitted as X-rays, gamma rays, microwaves, and radio waves.

Laws of Radiation

To better appreciate how the Sun’s radiant energy interacts with Earth’s atmosphere and surface, it is helpful to have a general understanding of the basic radiation laws. Although the mathematics of these laws is beyond the scope of this text, the fundamental concepts are straightforward:

• All objects continually emit radiant energy over a range of wavelengths. Not only do hot objects such as the Sun continually emit energy, but cooler objects such as Earth’s polar ice caps emit energy as well.

• Hotter objects radiate more total energy per unit area than do colder objects. The Sun, which has a surface temperature of 6000 K (10,000°F), emits about 160,000 times more energy per unit area than does Earth, which has an average surface temperature of 288 K (59°F).

• Hotter objects radiate energy in the form of shorter-wavelength radiation than do cooler objects. We can visualize this law by imagining a piece of metal that, when heated sufficiently (as occurs in a blacksmith’s shop), produces a white glow. As it cools, the metal emits more of its energy in longer wavelengths, and the glow turns a reddish color. Eventually, no light is given off, but if you place your hand near the metal, you will detect longer-wavelength infrared radiation as heat. The Sun radiates its peak energy at 0.5 micrometer, which is in the visible range (Figure 2.14). The peak radiation emitted from Earth occurs at a wavelength of 10 micrometers, well within the infrared (heat) range. Because the peak terrestrial radiation is roughly 20 times longer than the peak solar radiation, radiation emitted by Earth is often referred to as longwave radiation, whereas solar radiation is called shortwave radiation.

• Objects that are good absorbers of radiation are also good emitters. Earth’s surface and the Sun are nearly perfect radiators because they absorb and radiate with nearly 100 percent efficiency. Bodies that absorb and radiate all wavelengths well are called blackbodies. By contrast, the gases that compose our atmosphere are selective absorbers and emitters of radiation. For some wavelengths, the atmosphere is nearly transparent, allowing most of the energy of that wavelength to pass through. For other wavelengths, however, it is nearly opaque, absorbing most of the radiation that strikes it. Experience tells us that the atmosphere is quite transparent to visible light because these wavelengths readily reach Earth’s surface.

Although the Sun is the ultimate source of radiant energy, all objects continually radiate energy over a range of wavelengths. Hot objects, such as the Sun, emit mostly shortwave (high-energy) radiation, but cooler objects, such as Earth, emit longwave (low-energy) radiation. Objects that are good absorbers of radiation, such as Earth’s surface, are also good emitters. By contrast, most atmospheric gases are good absorbers (emitters) of radiation only in certain wavelengths but are poor absorbers (emitters) in other wavelengths.

• advection: Horizontal convective motion, such as wind.

• conduction: The transfer of heat through matter by molecular activity. Energy is transferred during collisions among molecules.

• convection: The transfer of heat by the movement of a mass or substance. It can take place only in fluids.

• infrared radiation (IR): Radiation with a wavelength from 0.7 to 200 micrometers.

• longwave radiation: A reference to radiation emitted by Earth. Wavelengths are roughly 20 times longer than those emitted by the Sun.

• micrometer (μm): A unit of length equal to one millionth of a meter.

• radiation (electromagnetic radiation): 

• shortwave radiation: The wavelike energy emitted by any substance that possesses heat. This energy travels through space at 300,000 kilometers per second (the speed of light).

• thermal: An example of convection that involves the upward movements of warm, less dense air. In this manner, heat is transported to greater heights.

• ultraviolet (UV) radiation: Radiation with a wavelength from 0.2 to 0.4 micrometer.

• visible light: Radiation with a wavelength from 0.4 to 0.7 micrometer.

• wavelength: The horizontal distance separating successive crests or troughs.

• Conduction is the transfer of heat through matter by collisions between molecules and atoms. Because air is a poor conductor, conduction is significant only between Earth’s surface and the air in immediate contact with the surface.

• Convection is heat transfer that involves the actual movement or circulation of a substance. Convection is an important mechanism of heat transfer in the atmosphere, where warm air rises and cooler air descends.

• Radiation (electromagnetic radiation) is the only mechanism to transfer heat through the vacuum of space. It consists of a large array of light energy that includes X-rays, visible light, heat waves, microwaves, and radio waves. Shorter wavelengths of radiation have greater energy.

• These are four basic laws of radiation: (1) All objects emit radiant energy; (2) hotter objects radiate more total energy per unit area than colder objects; (3) the hotter the radiating body, the shorter the wavelength of maximum radiation; and (4) objects that are good absorbers of radiation are also good emitters.