CLIMATIC CONDITIONS OF THE UNITED STATES. 



333 



(4) Ratios of Normal Total Precipitation for Period of Average Frostless Season 

 Plus preceding 30 Days, to Total Evaporation for the Same Period, July 1887 

 to June 1888 {ir/E). (Table 11, Plate 59.) 



This form of the moisture ratio is based on the idea, already men- 

 tioned, that some of the precipitation occurring before the beginning 

 of the frostless season is effective to supply water for plant activities 

 in the early part of that season. The numerator (P) of the first form of 

 ratio has thus been increased, in each case, by adding to it the normal 

 total precipitation for the 30 days just preceding the beginning of the 

 average frostless season. This increased precipitation value has been 

 termed tt, to distinguish it from P and p, so that the form of ratio here 

 considered becomes -k/E. The values of tt and E, and those of tt/E, 

 are given, for our list of stations, in table 11, the ratio values occupying 

 the last column. 



The chart based on these ratios is shown as plate 59. The essentials 

 of this chart are so nearly like those shown by plates 57 and 58 that 

 no special comment is here needed. It may be noted, however, that the 

 full line separating the humid from the semihumid zone is here taken 

 as having the value 1.10 instead of 1.00, as in plates 57 and 58. 



(5) Ratios of Normal Total Annual Precipitation to Total Annual Evaporation, July 

 1887 TO June 1888 (Pa/Ea). (Table 15, Plate 60.) 



These annual ratios are derived by the method employed by Tran- 

 seau. We have termed the normal total annual precipitation P^, and 



139* 187* 125" IW* Ijf 119' n:* 115° Ulf 



Fig. 15.— Moisture eonation of the eastern United States, according to precipitation-evaporation 



ratio (annual), after Tran.seau. 



