CLIMATIC CONDITIONS OF THE UNITED STATES. 339 



(7) Ratios of Total Precipitation for 15 Weeks in the Summer of 1908 to Total Evapora- 

 tion FOR the Same Period and Year, the Evaporation Data obtained with the 



Cylindrical Porous-cup Atmometer ( -^ 1908/-^, 1908 ) . (Table 17, Plate 62.) 



These ratios were obtained to accompany the evaporation and rain- 

 fall data of 1908. The numerators and denominators and the ratio 

 values are presented in table 17. The chart of plate 62 represents 

 these ratio values, as well as the small number of stations will permit. 



It is to be noted at once that the values obtained by dividing inches 

 of precipitation by cubic centimeters of evaporation have an entirely 

 different order of magnitude from those heretofore considered, being 

 very much smaller. In order to render these numbers more readily 

 comparable with those used on the preceding ratio charts, the values 

 from the last column of table 17 have all been multiplied by 10,000 

 before employing them for the chart of plate 62. On this chart the 

 humid zone is considered as having ratio values above 75, the semi- 

 humid zone is characterized by values between 25 and 75, the semi- 

 arid zone has values between 10 and 25, and the values representing 

 the arid zone are all below 10. By this convention the chart before us 

 appears to agree in a rather satisfactory manner with the other mois- 

 ture-ratio charts. Like the chart for the three summer months (plate 

 61), the arid zone is here shown as including the Great Basin, the 

 Pacific coast, and most of the Colorado Desert. In the present case, 

 however, the arid zone is extended northward, so that no semiarid, 

 semihumid, or humid conditions are encountered in the extreme 

 western part of the United States. The eastern margin of this zone 

 lies farther west (except at the south, where it is farther east) than 

 the corresponding margin on plate 61, In short, the arid zone of plate 

 62 may be approximately obtained from plate 61 if we conceive that 

 this zone on the earlier chart is simply extended northward, curtailed 

 along most of its eastern margin, and extended eastward at its southern 

 end. The line for the value 25 here corresponds with that for 60 on 

 plate 61, indicating that the eastern margin of the semiarid zone here 

 lies much farther west than in the other case. The semihumid zone, 

 as shown on plate 62, appears as nearly cut into two portions by the 

 great lobe of the humid zone that extends northward from eastern 

 Texas, but its eastern projection is still as clear as on the other ratio 

 charts, only this is displaced northward. This great eastern lobe of 

 the semihumid zone here occupies the whole Atlantic region from 

 southern Maryland nearly to the Canadian boundary, being thus 

 also more extensive eastward than in the case of plate 61 . The locahzed 

 area of semiarid conditions is once more apparent in the region of the 

 lower Great Lakes, being displaced northeastward from its position 

 on the other ratio charts. Lack of stations in Canada brings it about 

 that no humid zone can here be depicted north of the Great Lakes 

 region; it appears simply to be displaced northward from its position 



