358 



ENVIRONMENTAL CONDITIONS. 



these three charts, are probably continuous in Canada, to form a single 

 zone. In the eastern half of the country this zone occupies all of the 

 interior, as far west as about the hundredth meridian of west longitude. 

 The semiarid province embraces the Great Plains and a belt lying east 

 of the western semihumid zone. Finally, the arid province occupies 

 the region generally known as arid, extending approximately from 

 Arizona to eastern Washington. This zonation is in general agreement 

 with that shown by the precipitation-evaporation ratios. 



From these considerations it appears quite clear that the data of 

 relative humidity, as here employed, furnish an exceedingly valuable 

 and rational climatic zonation of the United States. This may be taken 



!»• 127* 155* IM* 



iw isr ii«° iir° lis" tif III- lOK' i<g* !<»• iW lof »sr »?• «• M* «f 



•r ts- «*• u" W Ji* w rt -.t nt if f 



-^ n? Tl? M? • llV lOaf lOr IPS' 'O^* I'll' Vf 97° SS- Kf 



9f if 87° 60° 



Kf ei° T!r -,r I? -.f 



Fig. 17. — Moisture zonation, according to indices of relative humidity for period of average frost- 

 less season. Humidity provinces: Humid, more than 75; semihumid, 65 to 75; semiarid, 60 

 to 65; arid, less than 50. Numerical values represent percentages. (See also Plate 65.) 



as satisfactory evidence that the theoretical objections to the concept 

 of relative humidity and to the usual manner of its employnaent are 

 not serious enough to prevent this method giving a clear zonation that 

 generally agrees with that obtained from the precipitation-evaporation 

 ratios as we have used the latter. It seems hardly probable, however, 

 that detailed studies of the annual march of climatic conditions as 

 related to plants may be as well served by these data as by those of 

 vapor-tension deficit, evaporation, and the precipitation-evaporation 



ratio. 



Considering the fact that data from 14 years of observations are 

 here employed, and that many more years are now available, so that 



