The Weather 37 



ascending whirls of moisture-laden air and the formation of 

 the cloud within the central area may yield a heavy fall of rain. 

 Where the updraft is too strong, this occurs not as a rain, but 

 as a torrent, commonly called a cloudburst. When passing 

 over lakes or oceans or even large streams of water, such whirl- 

 winds may suck up water and scatter it in a whirl, called a 

 waterspout, over a considerable space ; but the larger part of 

 the water is generally due to rapid condensation. 



24. Thunderstorms. Only certain features of this variety 

 of storm need to be mentioned. The range of occurrence of 

 thunderstorms is wide both north and south of the equator. 

 They occur with greatest frequency and violence in the tropical 

 regions and diminish toward the higher latitudes. In the United 

 States they occur most frequently and with greatest severity 

 in the general region of the Mississippi valley and in Florida, 

 less often in the region north and east of the Mississippi, and with 

 still less frequency west of the Rocky Mountains. In several 

 sections of California thunderstorms are of rare occurrence. 



A thunderstorm resembles a tornado in several respects. 

 It is of local origin, develops generally on the southeastern or 

 southern side of a cyclone ; and travels in most instances towards 

 the northeast over a tract not more than two hundred miles 

 long and seldom more than twenty-five miles wide. The 

 rate of progress varies from fifteen to twenty-five miles an 

 hour. Like the tornado, a thunderstorm depends upon a 

 sufficient supply of hot, moisture-laden air started upward by 

 currents of cold air or by its own movement up the sides of 

 hills or mountains. In its upward movement (Fig. 12) it 

 sometimes acquires the rotary or spiral whirl, as shown in 

 clouds characteristic of the tornado. The surface air flows in- 

 ward toward the central updraft area, sometimes with great 

 velocity. When the surface air currents fail to supply moisture 

 and heat, the energy of the storm diminishes and is finally dis- 

 sipated. Thunderstorms are usually accompanied by heavy 

 rainfall and occasionally by hail. 



