282 



ENVIRONMENTAL CONDITIONS. 



east and south of the northwestern semihumid area, the plains region, 

 roughly defined as between the mountains and the Winnipeg-Corpus 

 Christi hne, and portions of the States bordering the Great Lakes; (4) 

 the arid (or dry) rain-province, occupying, roughly, the region west of 

 the Rocky and Big Horn Mountains that is not included in the west- 

 ern and northwestern portions of the other provinces. The general 

 forms of these rain-provinces are set forth in figure 2, which is taken 

 from plate 46. 



127 125 12;i 121 119 117 llo 113 111 109 107 lOP 103 101 09 97 95 93 91 89 87 86 83 81^ '8 '? 



13 71 69 07 



Fig. 2. Moisture zonation, according to precipitation indices for period of average frostless season. 

 Precipitation provinces: Humid, more than 140; semihumid, 100 to 140; semiarid, 60 to 100; arid, 

 less than 60. Numerical values are in the thousandth of an inch. (See also plate 46.) 



Plates 48 and 51 show somewhat marked departures from this 

 generalization, consisting mainly in the westward or southwestward 

 displacement of the arid province, so that the latter comes to include 

 the southern half or more of the Pacific coast and little or none of the 

 Great Basin. 



The other features of these two plates are somewhat different from 

 those of the other precipitation charts, but still agree with them in a 

 general way. The southeastern humid province appears on plate 49 

 as three localized areas, two of which are where they would be expected 

 (from a study of plates 46, 47, 49, 50, and 51) . The third area, however, 

 is differently placed, occupying portions of Minnesota, Iowa, Mis- 

 souri, Wisconsin, and Illinois. On plate 51 this southeastern humid, 

 or rainy, region is much more extensive northward and northwestward 



