Chapter 12 — THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



accounting for the differences in the amounts 

 received. 



Reflection 



Reflection is the process whereby a surface 

 turns back a portion of the incident radiation 

 into the medium through which the radiation 



came. 



Some insolation is reflected by a substance. 

 This means that the electromagnetic waves 

 simply bounce back to space. The earth reflects 

 an average of 36 percent of the insolation. The 

 percent of reflectivity of all wavelengths of a 

 surface is known as its ALBEDO. The earth's 

 average albedo is from 36 to 43 percent. In 

 calculating the albedo of the earth, the assumption 

 is made that the average cloudiness over the 

 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: From 40 to 

 80 percent, with an average of about 55 per- 

 cent. 



2. Snow surfaces: Over 80 percent for cold, 

 fresh snow; as low as 50 percent for old, 

 dirty snow. 



3. Land surfaces: From 5 percent for dark 

 forests to 30 percent for dry land. 



4. Water surfaces: 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; after the sun is more 

 than 25° above the horizon, the albedo is less 

 than 10 percent so that, in general, the albedo 

 of water is quite low. 



When the earth as a whole is considered, 

 cloud surfaces are most important to deter- 

 mining the earth's albedo. 



Absorption 



Insolation is absorbed by a substance and 

 is converted to heat. Some substances absorb 

 all radiant energy which reaches them; some 

 absorb none; and some absorb only that in 

 specific wavelengths. The latter is the case 

 with some of the constituents of the atmosphere. 

 The earth's surface absorbs an average of 

 51 percent of the insolation. 



Greenhouse Effect 



The atmosphere itself absorbs some of the 

 insolation, but on a selective basis. The shorter 

 wavelengths of insolation pass through the 

 atmosphere to the earth, but earth radiation 

 (longer wavelength infrared radiation) is 

 "trapped" by the atmosphere. Some of this 

 trapped radiation is reradiated to the earth. 

 This causes a higher earth temperature than 

 would occur from direct insolation alone. This 

 is the greenhouse effect, so named because the 

 process is the same as that taking place in 

 a greenhouse: Most of the short-wave solar 

 radiation passes through the glass roof and is 

 absorbed by objects inside. These objects 

 reradiate at their temperatures, which are 

 relatively low compared to the sun's temperature; 

 consequently they emit infrared radiation which 

 is absorbed by the glass. The glass, in turn, 

 radiates energy back into the greenhouse, as 

 well as outward, so that the temperature inside 

 the greenhouse remains warmer than that 

 outside. 



Dispersion 



Earlier in the chapter it was learned that 

 the 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 the earth 

 at varying angles, depending on the position 

 of the earth. When the sun's rays are not 

 perpendicular to the surface of the earth, they 

 become DISPERSED over a greater area, as 

 can be seen in figure 12-4. Therefore, if the 



Figure 12-4. — Differential insolation. 



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