THE THUNDERSTORM— EVANS AND McEACHRON 203 



cold-front, over-running-cold-front, and warm-front — are explained. 

 Humphreys' theory explaining how thunderstorms are maintained as 

 they travel over the country is reviewed, and observational evidence 

 is advanced which supports it. Alexander's charts, which indicate 

 the monthly and yearly frequency of occurrence of thunderstorm 

 days over the United States for a 20-year period, are given. Their 

 value in lightning protection planning is pointed out. Conditions 

 under which the occurrence, direction of travel, and probable sever- 

 ity of storms can be predicted are enumerated. The value of appli- 

 cation of thunderstorm knowledge to the location of transmission 

 lines, to the determination of economically justifiable power-system 

 protection, to maintenance of continuity of service, and to the loca- 

 tion of explosive and oil-storage depots is pointed out. 



