CLIMATIC CONDITIONS OF THE UNITED STATES. 225 



Charts were made for each of these four series of seasonal efficiency 

 indices, but they all agree in the general delineation of the climatic 

 zones and only the one for summations above 39° F. is here presented, 

 in plate 38\ This chart differs from the corresponding one of Living- 

 ston in a few details, but the two are practically identical. 



The increments of seasonal temperature efficiency indices shown on 

 the chart of plate 38 are each 1,000 in the East, and the numbers placed 

 upon the isoclimatic lines denote thousands. These values may be 

 reduced to the corresponding ones based on the centigrade thermometer 

 scale by the use of the familiar factor 5/9, the starting-point for our 

 summations being 39° F., or 3.9° C. 



The lines of this chart are seen to have a generally west-east direction 

 east of the Rocky Mountains, several of them being southwardly 

 displaced by the Appalachians. The western mountains produce a 

 very great southward displacement, and another considerable dis- 

 placement of some lines, in the same direction, appears due to the 

 immediate vicinity of the Pacific Ocean. It is interesting to note that 

 the area having an index of 7,000 or less extends southward on the 

 California coast nearly to the parallel of latitude 33° north, while the 

 same area on the Atlantic coast extends southward only to about 35° 

 north latitude. 



Most of the country appears to be characterized by these seasonal 

 indices of temperature efficiency having values between 3,000 and 

 10,000. The region where these indices are less than 3,000 seems to 

 occupy northern New England, northeastern Michigan, northern 

 Minnesota and North Dakota, western Montana, central Wyoming, 

 and the Rocky Mountain system. The region having indices above 

 10,000 appears to occupy the vallej^s of the Gila and lower Colorado 

 Rivers, a narrow strip of the Gulf coast of Texas, and the southern 

 half of the peninsula of Florida. A closed area mth indices between 

 4,000 and 6,000 is shown on this chart as occupying a region extending 

 from the Columbia River to Great Salt Lake. 



(D) SUMMATION OF EXPONENTIAL INDICES OF TEMPERATURE EFFICIENCY 

 FOR PERIOD OF AVERAGE FROSTLESS SEASON. (TABLE 7, PLATE 39.) 



In applying the exponential method of deriving temperature effi- 

 ciency indices from normal daily mean temperature indices as given in 

 Bulletin R, we have followed Livingston and Livingston. The efficiency 

 index corresponding to each normal daily mean within the period 

 of the average frostless season, for each station considered, was 

 first obtained from table 5, and then all these indices were summed 

 to give the seasonal index in each case. The results of these summa- 

 tions, which are the data given on the chart of Livingston and Living- 

 ston's figure 2, are presented in the second column of table 7. 



^ The chart derived from these summations of normal daily means above 32° F. has been 

 presented by Livingston. See Livingston, 1913a. 



