28 WILLIAM H. ALEXANDER Vol. XXII, No. 1 



while it is safe to sa}^ that the thunderstorm, in one form or 

 another, does occur at some time or other in all parts of the 

 earth, yet from what we know of the meteorological conditions 

 ordinarily prevailing over the various portions of the earth, we 

 are very sure that it is very rare over large areas and may 

 never occur in some regions. In the United States^ for exam- 

 ple, we find two centers of maximum thunderstorm activity, one 

 over Tampa, Florida, and the other over Santa Fe, New Mexico. 

 In the ten-year period, 1904-1913, 944 thunderstorms were 

 recorded at Tampa and 710 at Santa Fe. Tampa is near sea- 

 level and Santa Fe is about 7,000 feet above the sea. 



7. Classification of Thunderstorms. 



One is impressed with the very great variety and many 

 variations met with in the study of these storms. This is true 

 whether one is considering the attendant circumstances, the 

 varying degree of intensity exhibited by them, the frequency of 

 occurrence, the resulting effects, the distribution through the 

 day, the year, or over the earth's surface, or whether one is 

 considering the factors operating to produce and maintain 

 these storms. Variety everywhere! 



At one time, and not so long ago, it was thought that all 

 thunderstorms were local phenomena and were therefore not 

 subject to any general law. In an important sense the thunder- 

 storm is a local phenomenon but the forces operating to pro- 

 duce many of them are far from local. It is now known that 

 a majority of these storms travel in a definite direction and 

 are therefore moving under a general law. In general, with 

 respect to the producing causes or conditions out of which 

 they grow, thunderstorms may be divided into (1) local or 

 "heat" thunderstorms, and (2) the cyclonic thunderstorms, or 

 "thundersqualls." Or, if we wish to be a little more exact or 

 "scientific," w^e may follow Professor Humphreys and make 

 five classes, namely, (1) the "heat" or local, (2) the cyclonic, 

 (3) the tornadic, (4) the anti-cylconic, and (5) the "border," 

 thunderstorm. The significance of this classification will be 

 pointed out later in connection with the illustrated portion of 

 this lecture but it seems appropriate at this time to refer to 

 Durand-Greville's famous theory of "the squall zone" in con- 

 nection w4th cyclonic thunderstorms. He holds that "cyclonic 

 thunderstorms" — and that means all except the "local" or 



