THE SUN THE CAUSE OF THE SEASONS 287 



(6) Repeat the exercise with the north pole tilted 23^ away from 

 the light. We then find that but % 4 of the 40th parallel is lighted; 

 that the north pole is without light; and that the equator is again 

 exactly half lighted (Fig. 216). 



This increased length of day greatly increases the sun's 

 power of heating the earth's surface and the atmosphere in the 

 north latitudes during our summer months. 



1. Notitrithstanding the fact that the vertical rays of the sun 

 never fall farther north than the Tropic of Cancer, 23% north 

 latitude, it still is true that for the three months from May 5 to 

 August 5 the zone of the sun's greatest heating is about 4lnorth 

 latitude. 



2. During the 45 days from May 31 to July 16, the region about 

 the north pole actually receives mor.e heat than does an equal area 

 at any other portion of the earth's surface. 



3. And again, it can be shown that, at the time of the SUMMER 

 SOLSTICE, the region of the north pole is receiving 36 per cent, more 

 heat than an equal area at the equator is then receiving during the 

 2-hour day. 



If these are facts, why does not the north polar region become 

 warmer? 1. Because the north pole receives no heat what- 

 ever from the sun for six months each year. 2. Because 

 nearly all of the heat furnished by the sun during the summer 

 months is consumed in melting the ice and snow formed 

 during the long winter months. During the following 

 winter a fresh supply of ice and snow again accumulates in 

 the polar regions. 



Without a knowledge of these facts it is impossible to under- 

 stand the seasonal changes of the weather of the United States. 



The maximum temperature of a summer day in the United 

 States rarely occurs in the southern states; it most frequently 

 occurs in the region extending from Oklahoma and Illinois to 

 South Dakota and Eastern Montana. Can you explain why 

 this should be so? 



Indian corn (maize) requires high temperature both day and 

 night during about three months, or its growing season. Can 



