130 FIRST YEAR SCIENCE 



64. The Warming of the Atmosphere. The sun trans- 

 mits both light and heat to the surface of the earth 

 through the atmosphere. On the top of a high mountain 

 the temperature is found to be colder than on the lower 

 levels. The amount of sun radiation, technically called 

 insolation, that falls upon a given surface on the mountain 

 is about the same as that which falls upon an equal sur- 

 face in the valley. If the heating effect is less, it must 

 be due to something besides the number of heat rays in- 

 tercepted. 



In the spring when gardeners wish to hurry the growth 

 of their plants, they cover them with boxes, the tops of 



which are made of glass (Fig. 

 63). It is found that the tem- 

 perature within the boxes is 

 higher than that outside. The 

 heat rays coming from the sun 



are in some way affected by the reflection of the ground so 

 that they are not able to get out through the glass as 

 readily as they get in. 



Now the atmosphere affects the heat rays reflected from 

 the earth in the same way that the glass does, and keeps 

 them from flying back into space and leaving the surface 

 'cold. Where the atmosphere is thin, as on the moun- 

 tains, this effect is not so great, and therefore the surface 

 is colder and often covered with snow. When there are 

 clouds in the air, they help to hold in the heat. That is 

 why in the fall, when it is getting cold enough for frosts, 

 the farmers say that the frosts are likely to come on clear 

 nights but not on cloudy ones. 



For the same reason plants are covered by pieces of 

 paper and smoky fires are built around cranberry bogs to 

 cover them with an artificial cloud of smoke on nights 

 when there is likely to be a frost. Thus the atmosphere 



