TEMPERATURE OF THE AIR 235 



days are long (Fig. 207, northern hemisphere) and so have greater 

 heating power. In summer, therefore, the surface is heated more 

 hours a day than during the winter, and the average amount of 

 heat per hour is greater while the sun shines. These are the chief 

 reasons why summer is warmer than winter. 



Change of seasons. We have already seen (1) that the sun's 

 rays are vertical at the equator at the time of the equinoxes 

 (March 21st and September 22d), and that the days and nights 

 are then equal everywhere; (2) that the northern hemisphere is 

 being heated most by the sun at the time of the summer solstice, 

 and least at the time of the winter solstice; (3) that the days are 

 longer than the nights in the northern hemisphere from March 

 21st to September 22d; (4) that the sun's rays are less oblique in 

 either hemisphere during the half of the year when the days are 

 longer than the nights, and (5) that the relative lengths of day 

 and night and the angle of the sun's rays are reversed in each 

 hemisphere every half-year. These points are illustrated by Figs. 

 207 and 208. 



Times of greatest heat and cold. Since the northern hemi- 

 sphere is being heated most at the time of the summer solstice and 

 least at the time of the winter solstice, it would seem at first thought 

 that these dates, respectively, should be the times of greatest and 

 least heat; but this is not the case. It follows that the tempera- 

 ture of any given latitude is not altogether dependent on the amount 

 of heat it is receiving from the sun. Again, since corresponding 

 latitudes in the two hemispheres are being heated equally at the 

 equinoxes, it would seem, at first, that these latitudes in the two 

 hemispheres should have the same temperature at these times; 

 but this, again, is not the case. In our own latitude, for example, 

 March 21st is much colder than September 22d. 



The reason why a place in our latitude is warmer at the time of 

 the autumnal than at the time of the vernal equinox is because 

 the warmth of the summer just past has not all been lost. The 

 soil, the surface rocks, the lakes, etc., have all been warmed during 

 the summer, and they cool slowly. At this time, therefore, the 

 northern hemisphere has a temperature higher than that which it 

 would have if it depended entirely on the heat received from the 



