WEATHER AND CLIMATE 149 



What days of continuous sunlight would mean for us in our 

 latitudes, especially in summer with the sun high up in the 

 heavens at noon, is easily imagined. With the sun shining 

 down steadily, even though clouds might at times give some 

 relief, the heat would be intolerable and destructive. 



However, in the polar regions the elevation of the sun above 

 horizon cannot at any time exceed 23^. As a result of this 

 low altitude the heat intensity is small, the area covered by 

 a sunbeam of any cross-section being relatively very large. 

 In a similar manner in our latitude the increasing obliquity 

 of the sun's rays from about September 21 each year through 

 the three succeeding months results in decreased insolation 

 (solar heating) , and the coming on of winter. For a long time 

 after March 21 the increasing amount of heat received does 

 not become sufficient to maintain plant growth. It is to be 

 mentioned in this connection, too, that the more obliquely 

 heat and light fall upon any surface the larger is the portion 

 of both reflected. Such reflected heat is lost so far as warm- 

 ing the earth's surface is concerned. 



SUMMARY 



By the obliquity of the sun's rays is meant their slant, or departure 

 from being directly overhead. This obliquity is measured in degrees. 



The periods of continuous sunshine within the Polar Circles range 

 from twenty-four hours to six months according to nearness to the 

 Poles. But these periods of unbroken sunlight, and of entire lack of 

 darkness save for clouds and storms, does not result in temperatures 

 sufficiently high to banish the cold of either of the Frigid Zones. So 

 low down upon the horizon is the sun during the long polar day that 

 the intensity of the solar heat is small. Then, too, of this heat the 

 larger portion is reflected because of the great obliquity of the rays, 

 leaving but little to be absorbed to warm the earth. 



Exercises 



1. What causes the rise and fall of the mercury in (a) thermometers; (b) 

 barometers? 



