12 



METEOROLOGY AND CLIMATOLOGY: 



layers characterized by the breaking down of mole- 

 cules into positively and negatively charged fragments 

 called ions. 



The final zone, the exosphere (beyond 220 miles), is 

 an almost airless area that grades into interplanetary 

 space. This outer zone does not stop abruptly; rather, 

 the boundary is a "spray zone," where atmospheric 

 particles spray out into space and then return im- 

 mediately, assume temporary orbits before returning, 

 or escape into space. 



AURORAS 



In the vicinity of the magnetic north and south 

 poles, and 50 to 600 miles above the earth's surface, 

 are the northern and southern lights, the auroras. Both 

 lights are produced as a result of the following series 

 of events: Explosive activities in the sun's atmosphere 

 cause electrified particles to escape in all directions 

 from the sun and into space. Some of these particles 

 enter the earth's magnetic field and are deflected 

 toward the magnetic poles. When the particles strike 

 upper air molecules and atoms, the energy transferred 

 to the latter causes them to glow, the different air 

 components glowing in different colors. This glowing 

 is observed as an aurora from the surface of the earth. 



SKY COLORS 



The color of the sky at any particular time and 

 place is the result of visible sunlight being scattered 

 by atmospheric particles. The actual color of the sky 

 depends on the amount of atmosphere traversed by 

 the light. As sunlight enters the atmosphere, the first 

 wavelengths of light to be scattered are the violet 

 waves. Therefore, when the sun is directly overhead, 

 the black of space found within 20 miles of the earth 

 gradually changes to a definite violet at about 12 

 miles altitude. Other colors are due to sunlight 

 traveling farther. When the sun is directly overhead, 

 the last wavelengths to be scattered are blue, which is 

 the reason the sky is blue during most of the day. 

 However, when sunlight strikes a place upon the 

 earth at an angle, at dawn or sunset, the light passes 

 through a greater distance of air, a distance sufficient 

 for the red wavelengths to be scattered by air mole- 

 cules. Therefore, sky colors have a direct relationship 

 to the density of air particles, the differential scatter- 

 ing of the sun's visible wavelengths, and the distance 

 traveled by sunlight through the atmosphere. 



Rainbows and related phenomena such as halos 

 about the sun and moon (the rings are upon clouds) 

 are produced by much the same process. However, 

 these latter phenomena are due to light's being scat- 

 tered by rain droplets, ice crystals, and other cloud 

 particles. 



ATMOSPHERIC TEMPERATURE 



The primary source of air temperature is solar 

 radiation; therefore, surface temperatures increase 

 during the day and decrease at night. Solar radiation 

 is most effective when solar rays are from directly 

 overhead, when the air is cleanest and driest, and 

 when radiation acts upon the land rather than upon 

 the water. 



Solar rays that are direct, or perpendicular, to a 

 part of the earth's surface are most effective in in- 

 creasing air temperature because such radiation is 

 most concentrated per unit area of the surface. This 

 is true because any given amount of solar radiation 

 that strikes the earth at an angle, no matter how 

 slight, is spread over more of the earth's surface than 

 is the same amount of radiation when the rays are 

 perpendicular (Figure 2.2). In addition, indirect rays 

 must pass through more atmosphere, hence be scat- 

 tered by more air particles and be less effective in pro- 

 ducing heat. Therefore, anything less than direct 

 solar rays results in some dissipation of the full effect 

 of sunlight striking the earth. 



The full importance of perpendicular solar rays and 

 their more efficient production of heat have already 

 been mentioned in connection with the earth's sea- 

 sons (Chapter 1, p. 7). If many people were asked 



Figure 2.2 Angle of solar radiation and amount of solar energy strik- 

 ing the eortfi's surface. Maximum energy is received from directly over- 

 head rays that strike the earth at a 90 angle. 



