244 WEATHER AND CLIMATE 



During the winter the water gives up its heat more slowly 

 than the atmosphere. As it gradually yields the heat it 

 absorbed during the summer, the air above it is warmed, 

 and currents of this warmed air modify the . temperature 

 of the adjoining land. For these reasons a large body of 

 water slows up the approach of warm weather in spring 

 and of frosty weather in autumn. 



In middle latitudes where the prevailing winds are westerly, 

 these effects are naturally much more marked and de- 

 pendable on the east shore of a body of water than on the 

 west shore. In many places on the east shores of large 

 lakes, delicate fruits can be raised because the steadying 

 effect of these bodies of water prevents early " warm spells " 

 alternating with frosts in spring, and delays the autumn 

 frosts until the fruits have ripened. The tempering effects 

 of warm ocean currents, combined with prevailing westerly 

 winds, account for the mildness of climate even in high lati- 

 tudes along the west coasts of North America and Europe, 

 which are the east shores respectively of the Pacific and 

 Atlantic oceans. 



SUMMARY 



The earth's atmosphere acts both as a blanket and as a 

 sunshield to the earth's surface. In addition to this, it is 

 the circulatory medium of the earth, without which there 

 could be no life. 



Winds and all movements of air are caused by unequal 

 heating and consequently unequal atmospheric pressure at 

 different places on the earth's surface. The prevailing 

 directions of winds are also affected by the rotation of the 

 earth. Certain winds common to all planets are called 

 planetary winds; when modified by certain peculiarities of 



