238 PHYSIOGRAPHY 



of 75 N. is lighted, and all points on that parallel have alternating 

 light and darkness in the course of twenty-four hours. 



In latitude 75 N., therefore, there are four natural divisions of 

 the year, one (summer) when daylight is continuous, one (winter) 

 when darkness is continuous, one (spring) when there is alternating 

 day and night, with the days lengthening, and one (autumn) when 

 there is alternating day and night, with the nights lengthening. 

 According to this subdivision of the year, summer is the time dur- 

 ing which the vertical sun appears to move from 15 N. to 23J/2 N., 

 and back again to 15 N. Autumn is the time during which it 

 appears to pass from the position where its rays are vertical 15 

 N. to the position where its rays are vertical 15 S. Winter is 

 the time when it appears to pass from 15 S. to 23}^ S., and back 

 again to 15 S., and spring the time when it is passing from 15 S. 

 to 15 N. 



It will be noted that the lengths of the seasons defined in this 

 way are not the same. In latitude 75 the summer would be as 

 long as the winter, and the spring as long as the autumn; but the 

 spring and autumn would be nearly twice as long as the summer 

 and winter, for during each of the former the sun appears to move 

 through 30, and during each of the latter, 17. Not only this, but 

 the lengths of the several seasons vary with the latitude. In 

 latitude 85 the summer and winter would be longer than in lati- 

 tude 75, and the springs and autumns shorter. 



There is a common idea that in polar regions there is a day of 

 six months and a night of six months each year, but this notion is 

 not correct. There is a six-month day and a six-month night at the 

 poles only. This can be worked out from Fig. 213. 



Effect of Altitude on Temperature 



High altitudes are colder than low ones in the same region. 

 The average decrease of temperature is about 1 F. for 330 feet 

 (1 C. for 594 feet) of rise, for the altitudes where observations are 

 common. One mile of rise in the air means about the same de- 

 crease of temperature as a journey of 1,000 miles toward the poles. 

 High altitudes are colder than low because the air is thin- 

 ner. Since it is thinner, it absorbs less heat from the direct rays 



