CLIMATIC CONDITIONS OF THE UNITED STATES. 327 



of the same table. These ratios are shown graphically on plate 57, 

 where the numbers on the lines represent hundredths. 



Inspection of table 11 and plate 57 shows that the moisture-ratio 

 values range from 0.04 (Independence, California, and Winnemucca, 

 Nevada) to 1.76 (Cape Hatteras, North Carolina) and to 3.84 (Tatoosh 

 Island, Washington). The highest values are in western Washington 

 and Oregon and the lowest are in the region of the Great Basin and in 

 the arid Southwest. The isoclimatic lines show that the country may 

 be divided into three main zones or regions: (1) an arid zone (ratio 

 values below 0.20), (2) a humid zone (ratio values above 1.00), and 

 (3) an intermediate zone (ratio values between 0.20 and 1.00). The 

 intermediate zone is conveniently subdivided into a semiarid zone 

 (ratio values between 0.20 and 0.60) and a semihumid zone (ratio 

 values between 0.60 and 1.00). There are thus four climatic zones 

 or provinces to be considered, as in the case of our precipitation and 

 evaporation charts. Their limits are denoted by full lines on plate 57. 



The arid province extends, roughly, from the Rocky and Big Horn 

 Mountains westward to the Cascades, the Sierra Nevadas, and the 

 Coast ranges. It includes a little of southeastern Washington. 



The semiarid province forms a belt lying west, east, and north of 

 the arid region, extending westward nearly to the coast of Washington 

 and northern Oregon, and to the coast of southern Oregon and Cali- 

 fornia, northward into Canada, and eastward to about the ninety- 

 ninth meridian of west longitude. 



The semihumid province is shown northwest and east of the semi- 

 arid area. It includes a narrow strip of western Washington and north- 

 western Oregon and extends eastward from the semiarid region to 

 about the ninety-third meridian at north and south, but is broadened 

 to include most of the country in its middle portion. This zone also 

 includes southern peninsular Florida. The western portion of the 

 humid region occupies western Washington and a little of northwestern 

 Oregon, while the eastern portion includes the Gulf coast east of the 

 Mississippi River (excepting extreme southern Florida) and the 

 Atlantic coast north of southern Florida (excepting the northern 

 part of the New Jersey coast and the coast about Boston) . It also 

 occupies the country north of middle New England and extends south- 

 ward from Canada to include northern Michigan, northern Wisconsin, 

 and eastern Minnesota. 



The most interesting special feature of this chart is the enormous 

 eastward enlargement of the semihumid region, which corresponds to a 

 similar eastern lobe evident on the evaporation chart for the period 

 of the average frostless season (plate 53). It should be remarked that 

 a localized portion of the semiarid region here appears to be located in 

 the middle of this enlargement, occupying southern Illinois, Indiana, 

 and Ohio, and northern Kentucky. A similar locahzed arid area is shown 



