CLIMATIC CONDITIONS OF THE UNITED STATES. 281 



regions. The humid northwest occupies the western half of Washing- 

 ton and of Oregon, practically the full width of northern Cahfornia, 

 and the whole of the Sierra Nevada Mountains. The arid and semi- 

 arid central region occupies the country south and east of the zone 

 just described, and extends eastward as far as a line drawn from the 

 western end of Lake Superior to the Rio Grande at a point about 100 

 miles above its mouth. In this general description we of course neg- 

 lect the restricted mountain areas of Idaho, Wyoming, Colorado, etc. 

 The humid east occupies the region lying east of the line just men- 

 tioned. It should be added that the northern half of the southern 

 peninsula of Michigan is also to be classed as semiarid on this basis, 

 since its normal annual precipitation is less than 30 inches; this is a 

 restricted area. 



The arid precipitation zone is here shown as having values below 

 10, and it occupies the Great Basin and extends southward into Mexico 

 from Arizona and southern California. The humid region, as above 

 defined, is here considered as divided into two provinces by the line 

 for value 50, thus indicating a humid and a semihumid, or a rainy and 

 semirainy province. These four precipitation provinces do not require 

 special discussion; their general characteristics are very similar to 

 those pointed out for plate 46. 



The general north and south trend of the isohyetal zones is here 

 seen to be modified by the Gulf of Mexico and by the southern Atlantic 

 so that the Hues of the eastern portion of the humid region trend north- 

 eastward, or even eastward, instead of southward. The mountainous 

 regions, of course, have higher precipitation indices than lowland 

 regions of the same latitude. In general, the zones tend toward an 

 arrangement parallel to the two coast-lines, which is readily explain- 

 able on meteorological grounds and which is the reason for the north- 

 south trend noted in all charts representing moisture relations. 



(10) Conclusions fbom Study of Precipitation Conditions. 



The charts of precipitation conditions (plates 46 to 52) exhibit 

 features that are markedly unlike those of the temperature charts 

 (plates 34 to 45), as was of course to be expected. The precipitation 

 zones have a strong general tendency to extend in a north-south 

 direction, while those of temperature generally extend from west to 

 east. We have found it convenient to consider the following four pre- 

 cipitation provinces in the United States: (1) the humid (or rainy) rain- 

 province, occupying a small area of the extreme Northwest and a larger 

 area of the southeastern Gulf and Atlantic coasts; (2) the semihumid 

 (or semirainy) rain-province, occupying a rather narrow strip of country 

 southeast of the northwestern humid area and nearly all of the country 

 east of a line drawn from Corpus Christi, Texas, to Winnipeg, Manitoba ; 

 (3) the semiarid (or semidry) rain-province, occupjdng a narrow strip 



