LOCAL STORMS 



243 



stood. One of the principal theories is illustrated in Fig. 167. 

 Suppose that a snowflake falls from H in the upper part of the 

 cloud, and is coated with water in the warmer portion near K, 

 and is then carried aloft in the uprush of air and receives 



10 10 



FIG. 171. Average annual number of thunderstorms in the 10 years 1904- 



1913. 

 (W. H. Alexander, in Monthly Weather Review.) 



FIG. 172. The layers of hail stones. Figs. 1, 2, and 3 show the formations 

 of hailstones having five, seven and nine layers, respectively, outside the 

 central nucleus. The stippled dark portions represent snow. 



another coating of snow. Falling again, the process is repeated 

 until the stone becomes too heavy and falls to the ground. 



Hailstones may be split with a sharp knife, showing the 

 layers (Fig. 172). As many as 20 to 25 layers have sometimes 



