The goal of this section is to describe how energy from the Sun heats Earth’s surface and atmosphere. It is important to know the paths taken by incoming solar radiation and the factors that cause the amount of solar radiation taking each path to vary.

What Happens to Incoming Solar Radiation?

When radiation strikes an object, three different results usually occur. First, some of the energy is absorbed by the object. Recall that when radiant energy is absorbed, it is converted to heat, which causes an increase in temperature. Second, substances such as water and air are transparent to certain wavelengths of radiation. Such materials simply transmit this energy. Radiation that is transmitted does not contribute energy to the object. Third, some radiation may “bounce off” the object without being absorbed or transmitted. Reflection and scattering are responsible for redirecting incoming solar radiation. In summary, radiation may be absorbed, transmitted, or redirected (reflected or scattered).

The figure below ▼ shows the fate of incoming solar radiation averaged for the entire globe. Notice that the atmosphere is quite transparent to incoming solar radiation. On average, about 50 percent of the solar energy that reaches the top of the atmosphere is absorbed at Earth’s surface. Another 30 percent is reflected back to space by the atmosphere, clouds, and reflective surfaces. The remaining 20 percent is absorbed by clouds and the atmosphere’s gases. What determines whether solar radiation will be transmitted to the surface, scattered, reflected outward, or absorbed by the atmosphere? As you will see, the answer depends greatly on the wavelength of the energy being transmitted, as well as on the nature of the intervening material.

Reflection and Scattering

Reflection is the process whereby light bounces back from an object at the same angle at which it encounters a surface and with the same intensity. By contrast, scattering is a general process in which radiation is forced to deviate from a straight trajectory. When a beam of light strikes an atom, a molecule, or a tiny particle in the atmosphere, it can spread out in all directions ▼. Scattering disperses light both forward and backward. Whether solar radiation is reflected or scattered depends largely on the size of the intervening particles and the wavelength of the light.

Reflection and Earth’s Albedo

Energy is returned to space from Earth in two ways: reflection and emission of radiant energy. The portion of solar energy that is reflected back to space leaves in the same short wavelengths in which it came to Earth. About 30 percent of the solar energy that reaches the outer atmosphere is reflected back to space. Included in the figure below is the amount sent skyward by backscattering. This energy is lost to Earth and does not play a role in heating the atmosphere.

The fraction of the total radiation that is reflected by a surface is called its albedo. Thus, the albedo for Earth as a whole (the planetary albedo) is 30 percent. However, the albedo varies considerably both from place to place and from one time to another, depending on the amount of cloud cover and particulate matter in the air, on the angle of the Sun’s rays, and on the nature of the surface. A lower Sun angle means that more atmosphere must be penetrated, thus making the “obstacle course” longer and the loss of solar radiation greater. The figure below ▼ shows the albedos for various surfaces. Note that the angle at which the Sun’s rays strike a water surface greatly affects its albedo.

Diffused Light

Although incoming solar radiation travels in a straight line, small dust particles and gas molecules in the atmosphere scatter some of this energy in all directions. The result, called diffused light, explains how light reaches into the area beneath a shade tree and how a room is lit in the absence of direct sunlight. Further, scattering accounts for the brightness and even the blue color of the daytime sky. In contrast, bodies such as the Moon and Mercury, which are without atmospheres, have dark skies and “pitch-black” shadows, even during daylight hours. Overall, about half of the solar radiation that is absorbed at Earth’s surface arrives as diffused (scattered) light.

Absorption

As stated earlier, gases are selective absorbers, meaning that they absorb some wavelengths strongly, some moderately, and some only slightly. When a gas molecule absorbs radiation, the energy is transformed into internal molecular motion, which is detectable as a rise in temperature.

Nitrogen, the most abundant constituent in the atmosphere, is a poor absorber of all types of incoming solar radiation. Oxygen and ozone are efficient absorbers of ultraviolet radiation. Oxygen removes most of the shorter ultraviolet radiation high in the atmosphere, and ozone absorbs most of the remaining UV rays in the stratosphere. The absorption of UV radiation in the stratosphere accounts for the high temperatures experienced there. The only other significant absorber of incoming solar radiation is water vapor, which, along with oxygen and ozone, accounts for most of the solar radiation absorbed directly by the atmosphere.

For the atmosphere as a whole, none of the gases are effective absorbers of visible radiation. This explains why most visible radiation reaches Earth’s surface and why we say that the atmosphere is transparent to incoming solar radiation. Thus, the atmosphere does not acquire the bulk of its energy directly from the Sun. Rather, it is heated chiefly by energy that is first absorbed by Earth’s surface and then reradiated to the sky.

Heating the Atmosphere:
The Greenhouse Effect

Approximately 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth’s surface and is absorbed. Most of this energy is then reradiated skyward. Because Earth has a much lower surface temperature than the Sun, the radiation that it emits has longer wavelengths than solar radiation.

The atmosphere as a whole is an efficient absorber of the longer wavelengths emitted by Earth (terrestrial radiation). Water vapor and carbon dioxide are the principal absorbing gases. Water vapor absorbs roughly five times more terrestrial radiation than do all the other gases combined and accounts for the warm temperatures found in the lower troposphere, where it is most highly concentrated. Because the atmosphere is quite transparent to shorter-wavelength solar radiation and more readily absorbs longer-wavelength radiation emitted by Earth, the atmosphere is heated from the ground up rather than vice versa. This explains the general drop in temperature with increasing altitude experienced in the troposphere. The farther from the “radiator,” the colder it becomes.

When the gases in the atmosphere absorb radiation emitted by Earth, they warm, but they eventually radiate this energy away. Some energy travels skyward, where it may be reabsorbed by other gas molecules, although that happens progressively less often with increasing height as the atmosphere thins and the concentration of water vapor decreases. The remainder travels Earthward and is again absorbed by Earth. For this reason, Earth’s surface is continually being supplied with heat from the atmosphere as well as from the Sun. Without these absorptive gases in our atmosphere, Earth would be a truly frigid place.

This very important phenomenon, illustrated in the figure below ▼, received the name greenhouse effect because people at the time thought it was the main way greenhouses work. Like Earth’s atmosphere, the glass of a greenhouse is largely transparent to visible light but somewhat opaque to the longer-wavelength radiation emitted by materials in the greenhouse. Greenhouse glass allows short-wavelength light to enter and heat plants and soil but prevents much of the longer-wavelength radiation these objects emit from leaving. In the case of greenhouses, we now know that this effect is much less important than the simple fact that the greenhouse prevents the warmer interior air from mixing with cooler outside air. Nevertheless, in the case of the atmosphere, the term greenhouse effect is still used.

• About 50 percent of the solar radiation that strikes the atmosphere reaches Earth’s surface. About 30 percent is reflected back to space. The remaining 20 percent of incoming solar energy is absorbed by clouds and the atmosphere’s gases. The fraction of radiation reflected by a surface is called the albedo of that surface.

• Radiant energy absorbed at Earth’s surface is eventually radiated skyward. Because Earth has a much lower surface temperature than the Sun, its radiation is in the form of long-wave infrared radiation. Because atmospheric gases, primarily water vapor and carbon dioxide, are more efficient absorbers of long-wave radiation than of short-wave radiation, the atmosphere is heated from the ground up.

• Greenhouse effect is the term for the selective absorption of Earth’s long-wave radiation by water vapor and carbon dioxide, which results in Earth’s average temperature being warmer than it would be otherwise.

• albedo: The reflectivity of a substance, usually expressed as a percentage of the incident radiation reflected.

• diffused light: Solar energy scattered and reflected in the atmosphere that reaches Earth’s surface in the form of diffuse blue light from the sky.

• greenhouse effect: The trapping of heat energy in the lower atmosphere due to the transmission of short-wave solar radiation coupled with the selective absorption of longer-wavelength terrestrial radiation, especially by water vapor and carbon dioxide.

• reflection: The process whereby light bounces back from an object at the same angle at which it encounters a surface and with the same intensity.

• scattering: The redirecting (in all directions) of light by small particles and gas molecules in the atmosphere. The result is diffused light.

• selective absorbers: Gases in the atmosphere, such as carbon dioxide and water vapor, that absorb and emit radiation only in certain wavelengths.