AIR TEMPERATURES AND THE SEASONS 35 



We therefore know that as the seasons change there is 

 also a change in the apparent altitude of the sun and in the 

 length of day and night. It is to these changes that the 

 seasons are due. 



34. Effects of changes in the sun's altitude. Why should it 

 make any difference in the heat that we get whether the sun 

 is high up in the heavens or lower down near the horizon ? 

 We all know that it does make a difference, for we readily 

 feel that the heat which 

 we receive at noon in the 

 winter and at noon in the 

 summer are not the same. 

 We notice the same thing 

 when we compare the 

 morning or evening effect 

 of the sun with the effect 

 at noon (fig. 22). 



The reason for this dif- 

 ference in the effect of 

 the sun's rays is not diffi- 

 cult to understand if we 

 make use of a simple ex- 

 periment. Suppose in the 

 morning, when the sun is 



not very high, we hold a piece of paper with a large hole in 

 it squarely facing the sun and measure the spot of light which 

 passes through the hole and appears on the floor or ground. 

 At noon we do the same, making certain that the card is the 

 same distance from the spot and also squarely facing the sun. 

 It will be seen that the spot of light observed in the morning 

 experiment was larger; that is, in the morning, when the 

 rays of the sun were more aslant than at noon, the rays of 

 light and heat that came through the hole covered a larger 

 space on the surface of the earth than at noon, when the 

 rays were more nearly at right angles with the earth's 



FIG. 22. Heating effects of the sun's rays 



Diagram showing the difference in inclina- 

 tion of the earth's surface to the sun's rays. 

 A sunheam of the given size would cover 

 more area at B than at A 



