236 PHYSIOGRAPHY 



sun each day. On the other hand, the temperature at the time of 

 the spring equinox is lower than that which the daily heating would 

 seem to produce, because the cold of the winter just past has not 

 been altogether overcome. In middle and high latitudes, snow and 

 ice cover land and water to some extent, and the water in the ground 

 is still frozen. This keeps the lower part of the air cool. 



The summer solstice is not the hottest part of the year in the 

 northern hemisphere, for the summer's heat has not altogether 

 overcome the effect of the preceding winter. The time of greatest 

 heat lags behind the time of greatest heating. In middle latitudes the 

 lag is about a month, but it is more over oceans than over lands, 

 because land is heated and cooled more readily than water. Simi- 

 larly the time of greatest cold does not come till after the time of 

 least heating. 



Seasons in other latitudes. The seasons in some other latitudes 

 are unlike our own. At the equator, for example, the sun's rays 

 are vertical twice each year, at the time of the equinoxes. Twice 

 a year, too, the sun's rays are vertical 23^ from the equator, once 

 to the north and once .to the south. The equator, therefore, has 

 two seasons, occurring at the time of our spring and autumn, which 

 are somewhat warmer than two other seasons occurring at the 

 time of our summer and winter. The variations in temperature 

 are much less than in our own latitude, for the length of day and 

 night never varies at the equator, and relatively little in the tropics. 

 The angle of the sun's rays, too, varies less than with us. At the 

 equator, therefore, there is a fourfold division of the year, but the 

 differences in temperature are less than in our latitudes. 



In high latitudes the conditions are still different. In latitude 

 60, for example, the differences in the seasons are similar to those 

 of the central part of the United States, except that they are greater. 

 This is because of the greater difference in the length of day and 

 night. (Fig. 213.) 



The change of seasons in latitude 75 N. may be taken to illus- 

 trate the conditions in latitudes above the polar circle. When 

 the sun's rays are vertical 15 south of the equator (D, Fig. 213), 

 the sun appears on the horizon at noon in latitudes 75 N., for this 

 latitude is 90 from the place where the sun's rays are vertical. 



