338 ENVIRONMENTAL CONDITIONS. 



the total annual evaporation for Russell's year, Ea, so that this form 

 of ratio becomes Pa/E^. The values for both terms and for the ratios 

 themselves are given in table 15, and the chart derived from the ratio 

 values is here presented as plate 60. 



This chart has the same essential characteristics as those of plates 

 57, 58, and 59, and it agrees, in general, with Transeau's similar chart, 

 which is here reproduced for comparison, as figure 15. It includes only 

 the eastern half of the country. Since he did not publish the station 

 data on which it is based, it is impossible to determine just wherein lie 

 the discrepancies between Transeau's calculations and our own. The 

 agreement between the two charts is close enough, however, for present 

 purposes. 



If the chart of plate 60 is compared with those of plates 57, 58, and 

 59, the main difference is seen to lie in the fact that the line separating 

 the humid from the semihumid zone, in the East, bends northward in 

 plate 60, to include in the humid zone all of Mississippi and Alabama 

 and parts of Tennessee and Missouri, which is not true for any other 

 moisture-ratio chart of our series. 



(6) Ratios of Normal Total Precipitation for the Three Summer Months, June to 

 August, to Total Evaporation for July and August 1887 and June 1888, Ps/Eg. 

 (Table 15, Plate 61.) 



As also in the case of the annual ratios just discussed, the duration 

 factor employed for these summer ratios is the same for all stations; 

 all represent the period of the months June, July, and August. The 

 two terms and the ratio are shown, for each station considered, in 

 table 15, the ratios occupying the last column. 



Plate 61 shows the chart based on these summer ratios. Here the 

 zonation is different from that of the preceding moisture-ratio charts 

 in several particulars. In the first place, the arid region (values below 

 0.20) is here extended west to the Pacific and includes nearly all of 

 Washington and Oregon and all of California, this difference from the 

 preceding charts of this feature being probably related to the charac- 

 teristic summer drought of California. The semiarid and semihumid 

 regions indicated in the northwest are very restricted. Looking at 

 this chart from any point of view, it is clear that the whole Pacific 

 coast and the Pacific Northwest are characterized as far more arid for 

 the summer months than for the period of the average frostless season 

 or for the year. 



In the East, the most pronounced difference between this chart and 

 the preceding ones lies in the fact that the great semihumid lobe pro- 

 jecting eastward from the Plains is here shown as extending farther to 

 the north than on the preceding charts. This suggests that New Eng- 

 land and the states bordering on the Great Lakes are more arid for the 

 period of the three summer months than for the other periods we have 

 considered. 



