48 Gen. A. W.Grcely — Rainfall Types of the United States. 



inaxiinnin to the single minimum through uninterruptedly 

 diminishing quantities, and thence rise with unbroken increases 

 to the maximum. The composite types are those in which the 

 graphic expression would be shown by two inflections, from a 

 primary maximum through the minimum to a secondary max- 

 imum and secondary minimum. 



In general terms it may be said that each simple type of rain- 

 fall in the United States appertains to a s-ingle body of water for 

 its resulting precipitation ; thus the Pacific type comes directly 

 from the Pacific ocean, the Mexican type from the gulf of Cali- 

 fornia, the Tennessee type from the gulf of Mexico, and the 

 Atlantic type from the Atlantic ocean. In the Missouri type, 

 however, two sources are evident — primarih^ the gulf of Mexico, 

 and secondarily, and to a much larger degree than has been 

 usually advanced, Hudson bay and the chain ofgreat American 

 lakes. 



In treating the fluctuations of rainfall throughout the year it 

 is evident that the unequal lengths of the different months affect 

 somewhat the accuracy of direct inter-comparisons of normal 

 monthly rainfalls. There fell under ni}^ observation lately a 

 curve showing such inter-comparisons which proved misleading, 

 as it showed a decrease of rain froni January to February and 

 an increase from February to March, when in reality, as shown 

 by the average amount daily for each month, the rainfall became 

 more copious from January to February and from February to 

 March. 



In this discussion the rule has been followed of obtaining 

 the normal daily rainfall by dividing the normal yearly rain- 

 fall by 365.25. In like manner the average daily rainfall of 

 February has been found by using 28.25 as a divisor, and the 

 longest months by using 31. In this paper, for the sake of 

 brevity and in order to avoid repetition, it is to be explained 

 that the term " normal daily rainfall " is applied to the mean 

 determined from the annual precipitation, and that the terms 

 '' January rainfall, March rainfall," etc, unless otherwise explic- 

 itly stated, mean the average daily amount determined for the 

 month in question by the methods above indicated. 



The best defined tj^pe of rainfall within the limits of the United 

 States is that which dominates the Pacific coast region ; hence 

 the specific name " Pacific " herein applied. In general terms 

 it may be said to dominate British Columbia, Washington, 



