210 WEATHER AND CLIMATE 



Now the atmosphere does for the earth what the glass 

 does for the cold frame. The rays of the sun pass through 

 the transparent atmosphere and warm the earth. When 

 the earth reflects the sun's raysior gives up the heat it has 

 absorbed, the atmosphere keeps this heat from immediately 

 passing off into space and leaving the surface cold. Where 

 the atmosphere is thin as on mountains, not so much of 

 heat is retained and therefore their surfaces are cold and 

 often covered with snow. Thus the atmosphere acts as a 

 blanket and keeps in the heat from the sun, as blankets on 

 a bed keep in the heat of the body. 



Clouds help to hold in the heat. Farmers know that 

 early frosts are likely to come on clear nights, but not on 

 cloudy ones. On nights when there is likely to be frost, 

 plants are covered with pieces of paper, smoky fires are 

 built around cranberry bogs, and orchards are smudged, 

 in order to blanket in the heat. 



The atmosphere also acts as a sun-shield and protects the 

 surface of the earth from the consuming heat of the sun. If 

 there were no atmosphere, the earth's surface would become 

 intensely hot during the day, when the sun shines directly 

 upon it, and intensely cold at night; so that life could not 

 possibly exist. t It has been estimated that if there were 

 no atmosphere, the mean temperature of the earth's surface 

 during the day would be 350 F., and during the night 123 

 F. On the moon, where there is no atmosphere, there can 

 be no life as we know it. 



If a column of air is heated it becomes lighter and the 

 atmospheric pressure at that point is lessened. The cooler 

 air flows in below and forces the heated air to rise. Thus 

 with the unequal heating of different places on the earth's 

 surface, there is a constant tendency of air to move from 



