CLIMATIC CONDITIONS OF THE UNITED STATES. 259 



The daily normals of precipitation for the period of the average 

 frostless season are shown graphically by the chart of plate 46, where 

 the isoclimatic lines represent increments of 20 thousandths of an inch. 

 The values are seen to be high for the southeast (the maximum being 

 170, for Cape Hatteras, North Carolina). The lowest values occur in 

 the arid region as this term is commonly understood (the minimum 

 being 9, for Reno, Nevada). On this chart the lines for index values 

 60, 100, and 140 are represented as broader than the others, since these 

 lines are useful in delimiting the main precipitation zones or provinces 

 of the country. The line for value 60 extends northward from about 

 Cape Mendocino, roughly parallelling the Pacific coast and passing 

 into Canada near the western margin of the Rocky Mountains system. 

 This same line reenters the United States near the eastern margin of 

 the same mountain system, passes southward and somewhat eastward, 

 so as to lie just east of the Rocky Mountains proper, and enters Mexico 

 near the mouth of the Rio Grande. This line includes, within the 

 loop thus formed, all of the region commonly called arid, and some- 

 what more. Northwest from the area thus demarked as arid the 

 values increase, and the lines for values 100 and 140 lie just within the 

 United States, in the neighborhood of Tatoosh Island, Washington 

 (which station has the value 199). 



East of the arid region as above defined the values of these daily 

 normals increase slowly, and the line for value 100 passes from north 

 to south through the eastern plains or western prairies, approximating 

 a line drawn from Corpus Christi, Texas, to Winnipeg, Manitoba. 

 At its northern end, however, this line bends eastward, apparently 

 passing somewhere north of the upper Great Lakes, and then bends 

 southward so as to reenter the United States north of Port Huron, 

 Michigan. It crosses Michigan from east to west, bends southwest- 

 ward to Grand Haven, Michigan, and reaches Cincinnati, Ohio. From 

 this point it passes northward to Detroit and then follows the valley 

 of the lower Great Lakes and St. Lawrence River, to pass again into 

 Canada near the northern end of Lake Champlain. It apparently 

 again bends southward and touches the Atlantic coast once more near 

 Nantucket, Massachusetts. Between the arid region and the north- 

 south portion of this line, and north of the portion about the Great 

 Lakes, lies a region that may be called semiarid, the values lying 

 between 60 and 100. 



The line for value 140 delimits what may be called the southeastern 

 rainy region, which here includes southeastern Louisiana, southern 

 Mississippi, Alabama, and Georgia, all of Florida, and the Atlantic 

 coastal region north as far as the entrance to Chesapeake Bay. Key 

 West, Florida, lies outside of this rainy zone. 



The region lying between the line for value 100 and that for value 

 140, including most of the eastern half of the country, may be con- 



