CLIMATIC CONDITIONS OF THE UNITED STATES. 343 



Our charts represent the country as divided into climatic provinces, 

 which may be conveniently considered once more as (1) humid, (2) 

 semihumid, (3) semiarid, and (4) arid. The following general descrip- 

 tion of these areas will serve to summarize the descriptions of the 

 separate charts. Reference may be made to figure 16, which is de- 

 rived from plate 57. 



Three humid areas are apparent, one in the Pacific Northwest, a 

 second in the region of the Great Lakes and northern New England, 

 and a third in the Southeast along the Gulf of Mexico and the Atlantic 

 as far north as northern New Jersey or southern New England. The 

 last two humid provinces are nearly continuous, and may be repre- 

 sented as such by charts drawn from certain forms of moisture ratio 

 or from data of certain years. Our plate 60 shows them as continuous 

 by a narrow belt that embraces about the southern half of Delaware 

 and the eastern half of New Jersey. 



The two semihumid areas are irregular in shape. The eastern one 

 occupies all of the country not included in the humid area and east of 

 about the one-hundredth meridian of west longitude. The western 

 semihumid area occupies a rather narrow strip of country east of the 

 western humid province and extending southward along the Pacific, 

 south of that province, to about the middle of the California coast. 

 These two areas are almost surely joined at the north, in Canada, 

 though this point is not actually demonstrated by any of our data. 



The semiarid area appears to be made up of an eastern and a western 

 portion, joined together at the north. The eastern portion extends, 

 approximately, from the one-hundredth meridian westward to the 

 Rocky Mountains and to the San Francisco Mountains of the Arizona 

 Plateau. Its eastern boundary appears to cross the Rio Grande some- 

 what southeast of El Paso. In its northern part this eastern semiarid 

 region apparently broadens westward and extends through the lower 

 passes of the mountains to Washington, where it joins the western 

 portion of the same region. This western portion lies east of the 

 western semihumid area as above described and extends southward 

 to the Mexican boundary, along the coast of southern California. 

 From Washington to Lower California it is a rather narrow belt. 



The arid area occupies the intermontane region of the West, extend- 

 ing from the Rocky Mountains to the Sierras and from southern Cali- 

 fornia and Arizona to Idaho and southwestern Washington. It is thus 

 surrounded, on three sides within the United States, by the belt of the 

 semiarid area. 



Of course, it is understood that all the higher mountain ranges are 

 to be considered as belonging to the humid area, at least in their upper 

 portions, and that such high mountain masses are surely each bordered 

 by a zone of semihumid conditions when they he in an arid area. No 

 attempt is here made to consider the innumerable small areas of 



