CLIMATIC CONDITIONS OF THE UNITED STATES. 355 



Mexico. Instead of completely surrounding the Rocky mountains, 

 as in the other two charts, this zone extends eastward only to these 

 mountains, but there is here a small local area of values of 50 or below, 

 indicated by the data of Santa Fe, New Mexico, and Pueblo, Colorado. 

 The eastern hmit of the semiarid region (70) is here placed about the 

 99th meridian and this line has nearly a north-south direction. The 

 region here called semihumid, in the east, is somewhat restricted in 

 this case, but shows a very large eastern lobe, reaching from Nebraska 

 to Maine. The localized area of semiarid conditions within this lobe 

 is here very large, extending from Minnesota and Illinois to Maine. 

 It will be remembered that the area in question is not shown at all on 

 the humidity chart for the period of the average frostless season 

 (plate 65) and is represented as much smaller on that for the year 

 (plate 66). The eastern humid region is here exceptionally wide in 

 the southeast and is narrowed at the north, somewhat as on the chart 

 for the moisture ratio for the year (plate 60). 



This chart of relative-humidity values for the three sunmier months 

 of 1908 was prepared for comparison with the chart of evaporation for 

 a similar period of the same year (plate 56). The stations for the 1908 

 series were not numerous enough to give a satisfactory chart, but these 

 two charts agree, in a general way, in showing the great eastern exten- 

 sion of the region of semihumid and semiarid summer conditions, from 

 Oklahoma to northern Michigan and New England. For details, the 

 two charts can not be compared. 



(5) Generalizations from the Three Charts of Relative Humidity Values. 



(Plates 65 to 67, and Fig. 17.) 



The main points brought out by our study of relative humidity values 

 maybe briefly summarized as follows, reference being made to figure 17, 

 which is derived from plate 65. The country may be divided, on the 

 basis of relative humidity — ^as on that of other moisture features — into 

 four humidity provinces, which may be termed arid, semiarid, semi- 

 humid, and humid. 



The humid relative humidity province occupies : (1) western Wash- 

 ington and Oregon and a variable portion of the California coast 

 region (depending on the form of index employed) ; (2) the whole of the 

 Gulf coast region and that of the Atlantic coast as far north as Long 

 Island or Massachusetts; (3) northern New England and New York 

 and a variable portion of Michigan, Wisconsin, and Minnesota. It 

 appears probable that portions (1) and (3), as just defined, are con- 

 tinuous through Canada, so as to form a single northern humid region. 

 The southeastern portion (2) appears to be quite cut off from the 

 northern one, but only by a narrow neck. The semihumid province 

 lies just interior to the humid one, being very narrow in the Northwest 

 and West. The eastern and western portions of this zone, as shown on 



