180 



GENERAL SCIENCE 



several air currents alternately above and below the freez- 

 ing point of water. 



The distribution of rainfall over the surface of the earth 

 (Figure 150) shows that there is more rainfall in the tropical 

 belts and in regions where the trade winds and prevailing 

 westerlies ascend the mountain slopes. The desert regions 

 are the lowlands which are crossed by the trade winds, or 

 interior regions which are crossed only by winds that have 

 previously lost most of their moisture. Such a region is 

 found just east of the Rocky Mountains. The heaviest 

 rainfall in the world occurs on the land bordering the 

 Bay of Bengal and the southwest coast of India, where 

 the average annual rainfall is about 30 feet. 



Rainfall is measured in an instrument called a rain 



gauge. Figure 151 shows the 

 rain gauge used by the United 

 States Weather Bureau. An 

 inch of rainfall means that enough 

 rain has fallen to cover the area 

 considered to a depth of one 

 inch of water. Snow and hail 

 are melted before the measure- 

 ments are taken. 



Perform the experiment to de- 

 termine how many inches of snow 

 are equivalent to one inch of 

 rainfall. 



Thunderstorms are frequent 

 in the United States. They 

 commonly occur on the warmest summer days, and dur- 

 ing the warmest parts of those days. The first indica- 

 tion of a thunderstorm is usually the appearance of a 

 heavy cumulus cloud in the west. The original cloud is 



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k 4 



FIG. 151 . Rain Gauge. 



