332 



ENVIRONMENTAL CONDITIONS. 



Table 18. — Data of precipitation and evaporation for the period of the average frostless season, 

 for the year of Russell's observation {July 1887 to June 1888), and corresponding ratios 

 of precipitation to evaporation, together iinth similar ratios derived by employing normal 

 data of precipitation instead of those for this single year. — Continued. 



The chart of plate 58 was prepared from these ratio values (p/E), the 

 isoclimatic lines representing increments of 0.20. This chart agrees 

 in its essentials with that of plate 57; the great eastern lobe of the 

 semihumid region (values between 0.60 and 1.00) is here again apparent 

 and the localized area of semiarid conditions, within this lobe, is more 

 pronounced than in the former case. The arid region (values below 

 0.20) is here indicated much as in plate 57, but the line for the value 

 0.60, in the middle of the country, here swings farther westward at its 

 northern end than in the other case. In general, it appears to make 

 no serious difference which of these two charts is studied, since all their 

 essential features are so nearly alike. 



Whenever agricultural climatology begins to receive attention in 

 this country, and when evaporation observations are made for the 

 period without frost, it will be desirable to prepare a chart similar to 

 that of plate 58 for every year. Finally, a normal ratio value for each 

 station may be actually obtained, after which the value for any par- 

 ticular station and season may be stated by comparison with the 

 normal for that station, just as is now done in the case of temperature 

 and rainfall. 



