CHAPTER VI 



CLIMATIC FACTORS: RADIANT ENERGY 

 TEMPERATURE AND LIGHT 



The sun is the source of the earth's radiant energy (insolation). 

 This energy, radiating as waves, includes those wave lengths of 

 the visible spectrum that we term "light" and those that lie just 

 beyond the visible spectrum, called "heat" if slightly longer, or 

 "ultraviolet light" if shorter. The amount of insolation reaching 

 the earth is always reduced because of absorption by the atmos- 

 phere (6-8 percent), and as much as 40 percent may be reflected 

 by clouds. The remainder reaching soil or water on the earth may 

 be further varied by such factors as distance from the sun at dif- 

 ferent seasons, duration of radiation, and the angle of the rays with 

 the earth's surface. The last determines the amount of air through 

 which the rays pass, modifies the amount of reflection and absorp- 

 tion, and likewise controls the amount of energy falling on a unit 

 area simply by spreading or concentrating a given amount of 

 energy over more or less space. With these things in mind, insola- 

 tional variation with latitude and topography are more easily ex- 

 plained. 



Insolation varies only slightly at the Equator because the angle 

 of the sun's rays never exceeds 23 1 /2° from zenith, and the days 

 are uniformly twelve hours long. Twice a year, on March 2 1 and 

 September 21, called the equinoxes, the sun is at zenith at the 

 Equator at noon and its circle of illumination exactly reaches the 

 North and South poles simultaneously. After March 21, because 

 of the tilt of the earth's axis, the North Pole comes progressively 

 nearer to the sun until June 21, after which the shift is reversed 

 to bring the pole back to the equinox position by September 21. 

 The North Pole's movement away from the sun continues until 

 December 21, after which it starts its shift back to the June posi- 

 tion again. The shifting of the pole toward the sun causes the 

 circle of illumination to extend far beyond the pole and results in 

 continuous insolation at the pole during the June solstice, but, 



116 



