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Depletion of Solar Radiation

If the Sun’s radiation was not filtered or depleted in some manner, our planet would soon be too hot for life to exist. We must now consider how the Sun’s heat energy is both dispersed and depleted. This is accomplished through dispersion, scattering, reflection, and absorption.

DISPERSION.— Earlier it was learned that Earth’s axis is inclined at an angle of 23 1/2°. This inclination causes the Sun’s rays to be received on the surface of Earth at varying angles of incidence, depending on the position of Earth. When the Sun’s rays are not perpendicular to the surface of Earth, the energy becomes DIS-PERSED or spread out over a greater area

Figure 1-2-5.—Electromagnetic spectrum.

Figure 1-2-6.-Dispersion of insolation.

(fig. 1-2-6). If the available energy reaching the atmosphere is constant and is dispersed over a greater area, the amount of energy at any given point within the area decreases, and therefore the temperature is lower. Dispersion of insolation in the atmosphere is caused by the rotation of Earth. Dispersion of insolation also takes place with the seasons in all latitudes, but especially in the latitudes of the polar areas.

SCATTERING.— About 25 percent of the incoming solar radiation is scattered or diffused by the atmosphere. Scattering is a phenomenon that occurs when solar radiation passes through the air and some of the wavelengths are deflected in all directions by molecules of gases, suspended particles, and water vapor. These suspended particles then act like a prism and produce a variety of colors. Various wavelengths and particle sizes result in complex scattering affects that produce the blue sky. Scattering is also responsible for the red Sun at sunset, varying cloud colors at sunrise and sunset, and a variety of optical phenomena (discussed in Unit 5, Lesson 3).

Scattering always occurs in the atmosphere, but does not always produce dramatic settings. Under certain radiation wavelength and particle size conditions, all that can be seen are white clouds and a whitish haze. This occurs when there is a high moisture content (large particle size) in the air and is called diffuse reflection. About two-thirds of the normally scattered radiation reaches Earth as diffuse sky radiation. Diffuse sky radiation may account for almost 100 percent of the radiation received by polar stations during winter.

REFLECTION.— Reflection is the process whereby a surface turns a portion of the incident radiation back into the medium through which the radiation came.

Some insolation is reflected by a substance. This means that the electromagnetic waves simply bounce back into space. Earth reflects an average of 36 percent of the insolation. The percent of reflectivity of all wavelengths on a surface is known as its ALBEDO. Earth’s average albedo is from 36 to 43 percent. That is, Earth reflects 36 to 43 percent of insolation back into space. In calculating the albedo of Earth, the assumption is made that the average cloudiness over Earth is 52 percent.

All surfaces do not have the same degree of reflectivity; consequently, they do not have the same albedo. Some examples are as follows:

1. Upper surfaces of clouds reflect from 40 to 80 percent, with an average of about 55 percent.

2. Snow surfaces reflect over 80 percent of incoming sunlight for cold, fresh snow and as low as 50 percent for old, dirty snow.

3. Land surfaces reflect from 5 percent of incoming sunlight for dark forests to 30 percent for dry land.

4. Water surfaces (smooth) reflect from 2 percent, when the Sun is directly overhead, to 100 percent when, the Sun is very low on the horizon. This increase is not linear. When the Sun is more than 25° above the horizon, the albedo is less than 10 percent. In general, the albedo of water is quite low. When Earth as a whole is considered, cloud surfaces are most important in determining Earth’s albedo.

ABSORPTION.— Earth and its atmosphere absorb about 64 percent of the insolation. Land and water surfaces of Earth absorb 51 percent of this insolation. The remaining 13 percent is directly absorbed by ozone, carbon dioxide, and water vapor. These gases absorb the insolation at certain wavelengths. For example, ozone absorbs only a small percentage of the insolation. The portion or type the ozone does absorb is critical since it reduces ultraviolet radiation to a level where animal life can safely exist. The most important absorption occurs with carbon dioxide and water vapor which absorb strongly over a broader wavelength band. Clouds are by far the most important absorbers of radiation at essentially all wavelengths. In sunlight, clouds reflect a high percentage of the incident solar radiation and account for most of the brightness of Earth as seen from space. 

There are regions, such as areas of clear skies, where carbon dioxide and water vapor are at a minimum and so is absorption. These areas are called atmospheric windows and allow insolation to pass through the atmosphere relatively un-impeded.

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