CLIMATIC CONDITIONS OF THE UNITED STATES. 



323 



which these profiles pass are indicated at proper distances along the 

 horizontal axis. As ordinates, extending upward at the points so 

 marked, are represented the mean weekly losses from the porous-cup 

 atmometer, as given in table 17, these numerical data being placed 

 upon the profiles. Below each profile are conventionally indicated the 

 vegetational types traversed and their approximate boundary lines, 

 these boundaries having been obtained from the generalized vegetation 

 chart (plate 2). We need not digress here to discuss the relations 

 brought out between vegetation and summer evaporation intensity 

 as here indicated ; these matters will receive attention in their own place. 

 The northern west-east profile (lowest in figure 13) shows little 

 variation in the intensity of evaporation throughout its extent. The 

 southern west-east profile, on the other hand, shows a very great 

 variation in evaporation values, the nmximum being at Dalhart and 

 the minimum at Pisgah Forest. The north-south profiles bring out 

 the relatively high summer evaporation values obtained at Dickinson, 

 at Easton, and at Gainesville and Oxford. 



(4) Conclusions from the Study of Evaporation Conditions. 



Ml the evaporation charts agree in their main features, especially 

 the charts derived from Russell's observations. The four evaporation 

 provinces are represented in figure 14, based on plate 53, and may be 



Fig. 14. — Moisture zonation, according to evaporation indices (1887-88) for period of average frostlesa 

 season. Evaporation provinces: Humid, less than 120; semihumid, 120 to 160; semiarid, 160 to 

 240; arid, more than 240. Numerical values are in thousandths of an inch. (See also Plate 63.) 



