HAIL. 147 



XV. HAIL. 



HAIL occurs at times when great differences of 

 temperature exist between neighboring masses of 

 very moist air. 



By permitting great differences of temperature 

 to occur, the destruction of the forest is, in many 

 cases, followed by an increase in the number and 

 severity of hail-storms. 



In order to understand the manner in which the 

 destruction of the forest may influence the occur- 

 rence of hail-storms, it will be necessary to study 

 some of the peculiarities of such storms and to re- 

 view what are now generally believed to be their 

 causes. 



Although hail may fall at any time of the year, 

 yet it occurs most frequently in summer towards 

 the close of a very warm day. 



The exact causes which produce hail are not 

 known. The conditions necessary for its occur- 

 rence appear to be the rapid mixture of very warm 

 and very cold moist air. 



A hail-storm is generally preceded by the appear- 



