The universe is made up of a combination of matter and energy. The concept of matter is easy to grasp because it is the “stuff” we can see, smell, and touch. Energy, on the other hand, is abstract and, therefore, more difficult to describe. For our purposes, we define energy simply as the capacity to do work. We can think of work as being accomplished whenever matter is moved. You are likely familiar with some of the common forms of energy, such as thermal, chemical, nuclear, radiant (light), and gravitational energy. One type of energy is described as kinetic energy, which is energy of motion. Recall that matter is composed of atoms or molecules that are constantly in motion and therefore possesses kinetic energy.

Heat is a term that is commonly used synonymously with thermal energy. In this usage, heat is energy possessed by a material arising from the internal motions of its atoms or molecules. Whenever a substance is heated, its atoms move faster and faster, which leads to an increase in its heat content. Temperature, on the other hand, is related to the average kinetic energy of a material’s atoms or molecules. Stated another way, the term heat generally refers to the quantity of energy present, whereas the word temperature refers to the intensity—that is, the degree of “hotness.”

Heat and temperature are closely related concepts. Heat is the energy that flows because of temperature differences. In all situations, heat is transferred from warmer to cooler objects. Thus, if two objects of different temperatures are in contact, the warmer object will become cooler and the cooler object will become warmer until they both reach the same temperature.

The flow of energy can occur in three ways: conduction, convection, and radiation ▼. Although they are presented in this section separately, all three mechanisms of heat transfer can operate simultaneously. In addition, working in tandem, these processes may transfer heat between the Sun and Earth and between Earth’s surface, the atmosphere, and outer space.

Mechanism of Heat Transfer: Conduction

Conduction is familiar to all of us. Anyone who has touched a metal spoon that was left in a hot pan has discovered that heat was conducted through the spoon. Conduction is the transfer of heat through matter by molecular activity. The energy of molecules is transferred through collisions from one molecule to another, with the heat flowing from the area of higher temperature to that of lower temperature.

The ability of substances to conduct heat varies considerably. Metals are good conductors, as those of us who have touched hot metal have quickly learned. Air, conversely, is a very poor conductor of heat. Consequently, conduction is important only between Earth’s surface and the air directly in contact with the surface. As a means of heat transfer for the atmosphere as a whole, conduction is the least significant.

Mechanism of Heat Transfer: Convection

Much of the heat transport that occurs in the atmosphere occurs via convection. Convection is the transfer of heat by mass movement or circulation within a substance. It takes place in fluids (e.g., liquids like the ocean and gases like air) where the atoms and molecules are free to move about.

The pan of water in the figure above ▲ illustrates the nature of simple convective circulation. 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. Because of this new buoyancy, the warmer water rises. 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—that is, from the bottom up—the water will continue to “turn over,” producing a convective circulation.

In a similar manner, most of the heat acquired in the lowest portion of the atmosphere by way of radiation and conduction is transferred upward by convection. For example, on a hot, sunny, day, the air above a plowed field will be heated more than the air above the surrounding croplands. As warm, less dense air above the plowed field buoys upward, it is replaced by the cooler air above the croplands ▼. In this way, a convective flow is established. The warm parcels of rising air, called thermals, 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, which may condense into clouds (at the condensation level labeled in the figure). The result is the increase in cloudiness that frequently can be observed on warm summer afternoons.

On a much larger scale, the global convective circulation of the atmosphere 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 and will be discussed in detail later this semester.

Mechanism of Heat Transfer: Radiation

The third mechanism of heat transfer is radiation. As shown in ▲, radiation travels out in all directions from its source. Unlike conduction and convection, which need a medium to travel through, radiant energy readily travels through the vacuum of space. Thus, radiation is the heat-transfer mechanism by which solar energy reaches our planet.

Solar Radiation

From our everyday experience, we know that the Sun emits light and heat as well as the ultraviolet rays that cause suntan. Although these forms of energy comprise a major portion of the total energy that radiates from the Sun, they are only part of a large array of energy called radiation, or electromagnetic radiation. This array or spectrum of electromagnetic energy is shown in the figure below ▼. All radiation, whether x-rays, radio waves, or heat waves, travels through the vacuum of space at 300,000 kilometers (186,000 miles) per second and only slightly more slowly through our atmosphere.

Nineteenth-century physicists were so puzzled by the seemingly impossible phenomenon of energy traveling through the vacuum of space without a medium to transmit it that they assumed that a material, which they named ether, existed between the Sun and Earth. This medium was thought to transmit radiant energy in much the same way that air transmits sound waves. Of course, this was incorrect. We now know that, like gravity, radiation requires no material for transmission.

In some respects, the transmission of radiant energy parallels the motion of the gentle swells in the open ocean. Like ocean swells, electromagnetic waves come in various sizes. For our purpose, the most important characteristic is their wavelength, or the distance from one crest to the next. Radio waves have the longest wavelengths, ranging to tens of kilometers, whereas gamma waves are the shortest, being less than one-billionth of a centimeter long.

Visible light, as the name implies, is the only portion of the spectrum we can see. We often refer to visible light as “white” light because it appears “white” in color. However, it is easy to show that white light is really a mixture of colors, each corresponding to a specific wavelength. Using a prism, white light can be divided into a rainbow color array. The figure above ▲ shows that violet has the shortest wavelength of visible light—0.4 micrometer (1 micrometer is 0.0001 centimeter)—and red has the longest wavelength—0.7 micrometer.

Located adjacent to red, and having a longer wavelength, is infrared radiation, which we cannot see but can detect as heat. The closest invisible waves to violet are called ultraviolet (UV) rays. They are responsible for the sunburn that can occur after intense exposure to the Sun. Although we divide radiant energy into groups based on our ability to perceive the different types, all forms of radiation are basically the same. When any form of radiant energy is absorbed by an object, the result is an increase in molecular motion, which causes a corresponding increase in temperature.

Laws of Radiation

To obtain a better understanding of how the Sun’s radiant energy interacts with Earth’s atmosphere and land–sea surface, it is helpful to have a general understanding of the basic laws governing radiation:

1. All objects, at whatever temperature, emit radiant energy. Hence, not only hot objects, such as the Sun, but also Earth, including its polar ice caps, continually emit energy.

2. Hotter objects radiate more total energy per unit area than do colder objects. The Sun, which has a surface temperature of nearly 6000°C (10,000°F), emits about 160,000 times more energy per unit area than does Earth, which has an average surface temperature of about 15°C (59°F).

3. Hotter objects radiate more energy in the form of short-wavelength radiation than do cooler objects. We can visualize this law by imagining a metal bar heated to white hot in a forge and allowed to cool. As it cools, it gradually dims, and its color changes from white through yellow to red as progressively more of its energy is radiated at longer wavelengths. Even when it no longer glows visibly, if you place your hand near the metal, the still-longer infrared radiation will be detected as heat. The Sun radiates maximum energy at 0.5 micrometer, which is in the visible range. The maximum radiation for Earth occurs at a wavelength of 10 micrometers, well within the infrared (heat) range. Because the maximum Earth radiation is roughly 20 times longer than the maximum solar radiation, Earth radiation is often called long-wave radiation, and solar radiation is called short-wave radiation.

4. Objects that are good absorbers of radiation are good emitters as well. Earth’s surface and the Sun approach being perfect radiators because they absorb and radiate with nearly  percent efficiency for their respective temperatures. On the other hand, gases are selective absorbers and radiators. Thus, the atmosphere, which is nearly transparent (does not absorb) to certain wavelengths of radiation, is nearly opaque (a good absorber) to others. Our experience tells us that the atmosphere is transparent to visible light; that is why visible light readily reaches Earth’s surface. The atmosphere is much less transparent to the longer-wavelength radiation emitted by Earth.

• Heat refers to the quantity of energy present in a material, whereas temperature refers to intensity, or the degree of “hotness.”

• The three mechanisms of heat transfer are (1) conduction, the transfer of heat through matter by molecular activity; (2) convection, the transfer of heat by the movement of a mass or substance from one place to another; and (3) radiation, the transfer of heat by electromagnetic waves.

• Electromagnetic radiation is energy emitted in the form of rays, or waves, called electromagnetic waves. All radiation is capable of transmitting energy through the vacuum of space. One of the most important differences between electromagnetic waves is their wavelengths, which range from very long for radio waves to very short for gamma rays. Visible light is the only portion of the electromagnetic spectrum we can see.

• Some basic laws that relate to radiation are (1) all objects emit radiant energy; (2) hotter objects radiate more total energy than do colder objects; (3) the hotter the radiating body, the shorter the wavelengths of maximum radiation; and (4) objects that are good absorbers of radiation are good emitters as well.

• conduction: The transfer of heat through matter by molecular activity. Energy is transferred through collisions from one molecule to another. Along with convection and radiation, it is a mechanism of heat transfer.

• convection: The transfer of heat by the movement of a fluid mass or substance.

• electromagnetic radiation (EMR): Transfer of energy in the form of light and related types of radiation, including gamma rays, x-ray, ultraviolet light, infrared light, microwaves, and radio waves.

• heat: The kinetic energy of random molecular motion; often used synonymously with thermal energy.

• infrared radiation (IR): Radiation with a wavelength from 0.7 to 200 micrometers. Cannot be seen but can be detected as heat.

• radiation: The transfer of energy (heat) through space by electromagnetic waves. Along with conduction and convection, it is a mechanism of heat transfer. The term can also refer generally to electromagnetic radiation.

• temperature: A measure of the degree of hotness or coldness of a substance; a measure of the average kinetic energy of individual atoms or molecules in a substance.

• ultraviolet radiation (UV): (UV) Non-visible radiation with a wavelength of 0.01–0.4 micrometer.

• visible light: Radiation with a wavelength from 0.4 to 0.7 micrometer; detectable by the human eye.