342 



ENVIRONMENTAL CONDITIONS. 



on the other plates. The separation of this eastern humid region into 

 a northern and a southern portion, indicated on all the ratio charts, is 

 here much more pronounced than in any other case. The northern 

 portion is here indicated as occupying eastern Maine, while the south- 

 ern portion extends from eastern Texas to southern Maryland, reach- 

 ing farther inland than in any of the other cases, especially at its 

 southeastern extremity, where its great northern lobe reaches southern 

 Minnesota. This lobe appears here like an exaggeration of the similar 

 but smaller lobe shown on the chart of plate 60. It is thus seen that 

 plate 61 differs from the other ratio charts only in relatively minor 

 details, the main essentials being about the same. 



(8) Conclusions from the Study of the Precipitation-Evaporation Ratios. 



The results of our study of the various forms of Transeau ratios 

 above described lead clearly to the conclusion, which is in full agree- 

 ment with that reached by Transeau from his first use of this ratio. 



12!° 127' m° 123° 12f llSf IIT* US' llif 111° 10»" 107° 105° 103° 101° Of W° aS' 113° 'Jl° "J' W xf sf 



3° -f 60° irf «5° 



113" 117° 115° IIJ" Ilf 103° 107° 105° lOSf lOf ttf' 87' 



01" Sf 8*" 87° M" 83° «1° 



FiQ. 16. — Moisture zonation, according to precipitation-evaporation ratio (period of average 

 frostless season). Moisture provinces: Humid, more than 100; semihumid, 60 to 100; semi- 

 arid, 20 to 60; arid, less than 20. (See also Plate 57.) 



that we have here a climatic index by means of which the zonation of 

 the country with regard to moisture conditions may be clearly shown. 

 As will receive emphasis later, the cUmatic zonation thus brought 

 out is closely paralleled by certain prominent features of the zonation 

 of vegetation types, and there is little room to doubt that this division 

 of the country into moisture-ratio provinces will be of very great value 

 in the study of climatology with reference to agriculture and forestry. 



