THE THUNDERSTORM— EVANS AND McEACHRON 201 



of square miles. They may thus form in widely scattered locations 

 with comparatively little reference to local topographical conditions. 

 Their direction of travel will nearly always be determined by the 

 general air movement at cloud levels. 



In regions where thunderstorms are of the heat or mountain type, 

 it may be desirable to consider ridge-top versus valley-side or valley- 

 bottom locations for transmission lines. These storms apparently 

 often tend to follow certain valleys and rivers. In traveling along 

 such valleys, many more strokes may strike into the valleys than to 

 the sides and top of adjacent mountains because lightning discharge 

 centers are often of limited dimensions. This condition has fre- 

 quently been seen by one of the writers in thunderstorm studies ^ in 

 Colorado. Location of lines along ridge tops may then result in less 

 exposure to lightning. 



In regions where thunderstorms are of the cold-front or warm-front 

 types, the haphazard formation of such storms over wide areas and 

 the fact that their direction of travel is usually controlled by the 

 general air movement at cloud levels greatly decreases the lil^elihood 

 of preferred paths being followed. Storms carried from west to east 

 by the general air circulation will travel over mountain ridges oriented 

 in northerly and southerly directions. The lightning exposure of 

 lines located on the top of such ridges would be greater than for lines 

 located on the valley sides in the lee of the ridge or on valley bottoms. 



POWER-SYSTEM PROTECTION 



The degree of protection economically justifiable for a given power 

 system depends to a considerable degree on the frequency of occur- 

 rence and severity of thunderstorms in the region in which it is located. 

 Alexander's charts should be of value in determining the probable 

 thunderstorm frequency for any section of the United States. This 

 information should be supplemented by knowledge of local storm 

 severity in the region served by the system in order to evaluate the 

 potential hazard due to the storms to be expected there. 



POWER-SYSTEM OPERATION 



Despite many improvements in operation, lightning still represents 

 a serious source of interruption to many power systems. To insure 

 continuity of service, many systems connect additional generating 

 capacity to their lines when thunderstorms are known to be approach- 

 ing. Knowledge of the occurrence, direction of travel, and severity 

 of thunderstorms is therefore of importance to them. Weather 

 Bureau reports serve as prehminary warnings of the probable occur- 

 rence of thunderstorms. Short-time warnings of the appearance of 

 an actual storm over the system are much more important, however. 

 Some power plants have installed "howlers" which warn the operator 



