T. Terada: 



25 



In actual cases, these three influences are generally super- 

 posed, the resultant effect varying largely according to the degree 

 of relative importance of each factor. For example, in the case of 

 a deep depression of small extent, the first factor plays no greater 

 part than slightly shifting the area with tlie heaviest precipitation 

 toward E, while the second and the third factors may be important 

 not only for determining the precipitation, l)ut also the subsequent 

 course of progression. It will Ije especiall}^ interesting to in- 

 vestigate the relation with respect to the cases of tliunderstorms 

 of the cyclonic type, i.e. those with circular depression in the 

 centres. 



The mathematical discussion of these different influences will 

 not l^e easy, till we have at hand a more or less complete theory of 

 cyclones in general. In the following, however, an attempt is made 

 to illustrate the essential influences of the above mentioned factors 

 for very simple ideal cases, and to found the starting point for analy- 

 sing the actual complicated phenomena into their essential ele- 

 ments. It must be emphasized that tlie whole is nothing more 

 than qualitati^'e considerations expressed in mathematical forms. 



(1) Planetary thermal influences : Referring to the annexed 

 figure consider a centre of cyclone at and draw two concentric 



isobars with the radii r and r + dr, in the 

 inner region, i.e. the region where the 

 ascending current proper to the cyclone 

 is taking place. Suppose now all the 

 isotherms are artificially maintained 

 parallel to the .r-axis. Assume tlie angle 

 of deflection of the wind </', i.e. the angle 

 between the barometric gradient and the 

 wind, everywhere constant — an assump- 

 tion apparently contradicting the idea of 

 varying latitude, but since here we are 

 only considering the effect of the tlierinal gradient, the small 

 difference of </' is evidently not essential. Then we may put 



Fig. 44. 



