Describe what happens to incoming solar radiation.

When solar radiation enters Earth’s atmosphere, three different things may occur simultaneously. First, air, which is transparent to certain wavelengths of radiation, may simply transmit energy—allowing it to pass through without redirecting or absorbing it. Second, some of the energy may be absorbed. Recall that when radiant energy is absorbed, the molecules begin to vibrate faster, which causes an increase in temperature. Third, some radiation may “bounce off” gas molecules or dust particles in the atmosphere, without being absorbed or transmitted.

What determines whether solar radiation will be transmitted to the Earth’s surface, absorbed by the gases and particles in the atmosphere, or scattered or reflected by these gases and particles? As you will see, it depends greatly upon the wavelength of the radiation, as well as the size and nature of the intervening material.

Transmission

Transmission is the process by which energy passes though the atmosphere (or any transparent media) without interacting with the gases or other particles in the atmosphere. About half of the incoming shortwave (solar) energy that reaches Earth’s surface is transmitted through the atmosphere. The remainder is redirected by gas molecules and particles in the atmosphere and arrives as diffused light. Figure 2.15 illustrates what happens to incoming solar radiation, averaged for the entire globe. Notice that on average, about 55 percent of incoming solar energy reaches Earth’s surface—about 50 percent is absorbed at the surface, and the remaining 5 percent is reflected back toward space.

Absorption

The amount of energy absorbed by an object depends on the wavelength of the radiation and the object’s absorptivity. In the visible range, the degree of absorptivity is largely responsible for the brightness of an object. Surfaces that are good absorbers of all wavelengths of visible light appear black in color, whereas light-colored surfaces have a much lower absorptivity. That is why wearing light-colored clothing on a sunny summer day may help keep you cooler.

Although Earth’s surface is a relatively good absorber (effectively absorbing most wavelengths of solar radiation), the atmosphere is not. As a result, gases in the atmosphere absorb only 20 percent of the solar radiation that reaches Earth (Figure 2.15). The atmosphere is a less effective absorber because gases are selective absorbers (and emitters) of radiation.

Freshly fallen snow is another example of a selective absorber. Snow is a poor absorber of visible light (reflecting up to 90 percent) and therefore the temperature directly above a snow-covered surface is colder than it would otherwise be because the snow reflected away much of the incoming radiation. By contrast, snow is a very good absorber (absorbing up to 95 percent) of longwave (infrared) radiation that is emitted from Earth’s surface. As the ground radiates heat upward, the lowest layer of snow absorbs this energy and radiates some of the energy back downward. Thus, a winter’s frost cannot penetrate very deeply into snow-covered ground compared to an equally cold region without snow—giving credence to the statement “The ground is blanketed with snow.” Farmers who plant winter wheat desire a deep snow cover because it insulates their crops from bitter winter temperatures.

Reflection and Scattering

Reflection is the process whereby light bounces back from an object at about the same angle and intensity at which it was received. By contrast, scattering is a general process in which radiation bounces off an obstacle in many directions. Atoms, molecules, or tiny particles in the atmosphere cause incoming sunlight to scatter (Figure 2.16). 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

The fraction of radiation that is reflected by an object is called its albedo. Figure 2.17 gives the albedos for various surfaces. Fresh snow and thick clouds have high albedos (that is, they are good reflectors). You can observe the high reflectivity of clouds when you look down on bright clouds during an airline flight. By contrast, dark soils and parking lots have low albedos and thus absorb much of the radiation they receive. In the case of a lake or the ocean, the angle at which the Sun’s rays strike the water surface greatly affects its albedo.

Earth’s total albedo, called planetary albedo, is 30 percent (see Figure 2.15). This energy is lost to Earth and does not play a role in heating the atmosphere or Earth’s surface. The amount of light reflected from Earth’s surface represents a small percentage of the total planetary albedo. Not surprisingly, thick clouds, which have high albedos, are largely responsible for most of Earth’s “brightness” as seen from space.

Scattering and 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 different directions. The result, called diffused light, explains how light reaches the area under the limbs of a tree and how a room is lit in the absence of direct sunlight. In contrast, bodies without atmospheres, such as the Moon and Mercury, have dark skies and “pitch-black” shadows, even during daylight hours. Overall, about one-half of the solar radiation that is absorbed at Earth’s surface arrives as diffused (scattered) light.

Blue Skies and Red Sunsets

The two factors that produce Earth’s blue skies and red sunsets are the selective scattering of solar radiation by atmospheric gases and the amount of atmosphere through which the Sun’s rays travel before reaching Earth. Recall that sunlight appears white but is composed of all the colors of the rainbow. Atmospheric gases scatter shorter-wavelength (blue/violet) light more effectively than they scatter longer-wavelength (red/orange) light. Because shortwave radiation is selectively scattered, when you look in any direction away from the direct Sun, you observe the short-wavelength (blue) light (Figure 2.18). Scattering of visible light by atmospheric gases is called Rayleigh scattering.

The Sun appears reddish when viewed near Earth’s horizon at sunrise or sunset because solar radiation must travel a greater distance through the atmosphere before it reaches your eyes. During its travel, shorter-wavelength blue and violet wavelengths are preferentially scattered away, so the light that reaches your eyes consists mostly of red and orange hues. In other words, the sky and clouds are illuminated by light from which the blue color has been preferentially scattered away.

On Earth, the most spectacular sunsets occur when large quantities of tiny dust or smoke particles penetrate the stratosphere. (Scattering of visible light by dust or smoke is called Mei scattering.) For 3 years after the great eruption of the Indonesian volcano Krakatau in 1883, brilliant sunsets occurred worldwide. In addition, the European summer that followed this colossal explosion was cooler than normal, which has been attributed to the loss of incoming solar radiation due to an increase in backscattering.

Crepuscular Rays and White Clouds

Large particles associated with haze, fog, and cloud droplets scatter light more equally at all wavelengths. Because no color predominates over any other, the sky appears white or gray on days when large particles are abundant. Scattering of sunlight by haze, water droplets, or dust particles makes it possible for us to observe bands (or rays) of sunlight called crepuscular rays. These bright fan-shaped bands are most commonly seen when the Sun shines through a break in the clouds, as shown in Figure 2.19.

The color of the sky gives an indication of the size of particles present. Numerous small particles produce red sunsets, whereas large particles produce white (gray) skies. Thus, the bluer the sky, the less polluted the air.

• absorptivity: A measure of the amount of radiant energy absorbed by a substance.

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

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

• reflection: The process where light bounces back from an object at the same angle and intensity at which it was received.

• scattering: The process in which light bounces off of an object in many directions.

• transmission: Passage of light through a substance without being absorbed or scattered.

• Approximately 50 percent of the solar radiation that strikes the atmosphere reaches Earth’s surface. About 30 percent is reflected back to space. Clouds and the atmosphere’s gases absorb the remaining 20 percent of the incoming solar energy.