200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



dictable. The thunderstorms will be carried along with the general 

 ah" movement. Their movement will then be the same as the du'ec- 

 tion of cloud travel directly overhead (observations of cloud move- 

 ments at a considerable angle from the vertical may result in large 

 errors due to perspective). 



In the case of heat thunderstorms, which occur most frequently at 

 times when the general air movements are minor, it is more difficult to 

 predict their direction of travel. Direction of cloud travel overhead 

 is helpful but not certain for local topographical conditions and air 

 systems generated by the storm itself may easily cause the storm to 

 take an erratic course. 



SEVERITY 



The severity of thunderstorms depends upon the extent and violence 

 of their convection systems. This in turn depends upon the magni- 

 tude of the temperature differences between the warm moisture-laden 

 air and the air through which it is carried by convection, upon the 

 moisture content of the warm air, and upon the velocity of the general 

 air movement. The intensity of cold-front, overrunning-cold-front, 

 and warm-front storms can thus be predicted qualitatively from the 

 magnitude of the atmospheric instability indicated by Weather 

 Bureau measurements. The intensity of heat thunderstorms can be 

 qualitatively predicted from the humidity, the temperature, and the 

 mildness of the general air movement. The occurrence of high humid- 

 ity, high temperature, and very little air movement is favorable to a 

 severe storm. 



APPLICATIONS OF THUNDERSTORM KNOWLEDGE 



TRANSMISSION-LINE LOCATION 



Wliile the number of routes by which a transmission line can be run 

 between two locations is limited, it is possible that under certain 

 conditions substantial decreases in exposure to lightning might result 

 from changes of a few miles in line location. This is particularly true 

 in regions where thunderstorms follow preferred paths. Whether 

 they do so in a given locality depends to a large extent on the chief 

 cause of storms there. If thunderstorms are usually of the heat type, 

 or the mountain type, they may well follow such paths, for their 

 formation depends on local favorable meteorological and topographical 

 conditions which tend to remain more or less the same. In the light 

 air movements prevailing when storms of this type arc formed, local 

 land and air conditions have a much greater influence on their direc- 

 tion of travel. If, on the other hand, thunderstorms that visit an 

 area are usually of the cold-front or warm-front type, preferred paths 

 will be much less evident. Their formation depends on the inter- 

 action of adjacent cold- and warm-air masses often covering hundreds 



