TEMPERATURE OF THE AIR 229 



Primary distribution of heat. It is the rotation of the earth 

 on an inclined axis while it revolves about the sun (Fig. 202), which 

 makes the sun appear to move north and south during the year. 

 From Fig. 209 we see that when the sun sends his rays to the earth 

 from the direction W (perpendicular 23^ S. of the equator), 

 they are more oblique than at any other time in the northern 

 hemisphere, and less oblique than at any other time in the southern 

 hemisphere. At this time, therefore, the southern hemisphere is 

 receiving more heat than the northern, because of the lesser ob- 

 liquity of the sun's rays. At this time, too, the days are longer in 

 the southern hemisphere than in the northern, and this is a second 

 reason why the southern hemisphere is receiving more heat than 

 the northern at this season. 



After the time (winter solstice, December 22d) when the sun's 

 rays are vertical at 23J^ S, they become perpendicular to the sur- 

 face in latitudes farther and farther north, and on March 21st 

 they are vertical at the equator. Days and nights are then equal 

 everywhere, because all parallels are cut into two equal parts by 

 the circle of illumination (p. 212), and the sun's rays are equally 

 oblique in corresponding latitudes north and south of the equator. 

 Any latitude in one hemisphere is then receiving the same amount 

 of heat as the corresponding latitude in the other hemisphere. 



After March 21st, the sun appears to continue its journey north- 

 ward until June 21st, when its rays are vertical at the tropic of 

 Cancer, 23J/2 N. The days of the northern hemisphere are then 

 longest and the nights shortest, and the rays of the sun are then 

 less oblique in this hemisphere than at any other time. At this 

 time, therefore, the northern hemisphere is being heated more 

 rapidly than at any other. 



From June 21st to December 22d, the sun appears to move so 

 that his rays become vertical farther and farther south, and the 

 preceding changes are reversed. 



The latitudes where the sun's rays fall vertically range from the 

 tropic of Cancer to the tropic of Capricorn; and the sun's rays are, 

 on the average, least oblique between these limits. This is why the 

 low latitudes are, on the whole, warmer than the high latitudes. 



The amount of heat received in different latitudes is determined 



