CLIMATIC CONDITIONS OF THE UNITED STATES. 279 



ing the Pacific. The droughtiness of the intermountain area is here 

 relatively exaggerated, on account of the choice of 0.10 inch as our 

 constant in deriving these indices; the stations here characterized as 

 having no normally rainy periods are not to be considered as really 

 all alike in this particular. For the detailed study of the arid regions 

 there seems to be no doubt that a lower value than 0.10 inch will be 

 required. The precipitation provinces of the country, as indicated 

 by the full lines of plate 51, are similar to those shown on plates 46, 

 47, and 49. 



(8) Length of Longest Normally Dry Period in Period of Average Frostless Season. 



(Table 14, Plate 51.) 



These indices for droughtiness, corresponding to those for raininess 

 just discussed, are given in the third column of table 14, and the 

 method by which they were obtained has already been described. 

 Plate 51 shows the chart constructed from these values. The isoclimatic 

 lines are shown for intervals of 25, with full lines for the values 150, 

 50, and 25. The total range of this index, for the entire country, is 

 from zero (Cape Hatteras, North Carolina) to 299 (Los Angeles, 

 California). The meridian of 101° west longitude again forms an 

 important demarcation, separating the more humid east from the 

 more arid west, and some other features are Uke those of the other 

 precipitation charts. In the details of the Southwest this chart has 

 peculiarities somewhat like those of plate 48, and will be referred to 

 below. 



(9) Normal Annual Precipitation. (Plate 52.) 



Since total annual precipitation is surely of considerable value in a 

 general estimate of the aridity or humidity of a region, we include in 

 our series a reproduction of the chart of this feature published by Gan- 

 nett.^ This chart has been drawn, as its author states, with consider- 

 able reference to topography as well as to the records of about 4,000 

 stations, and it appears to us to be the most useful annual rainfall 

 chart of the United States thus far published. Data for normal annual 

 precipitation for 167 stations are given in the second column of table 

 15, for use in other connections, but these are not the data from which 

 this chart was constructed. The data given in that table are taken 

 from Bulletin R of the U. S. Weather Bureau. Table 15 also includes 

 other data, which will be considered below. 



In Gannett's chart, as here reproduced (plate 52), the isohyetal 

 lines for 10, 30, and 50 inches are dotted and the others are full. All 

 are drawn for increments of 10 inches. By the emphasized line 

 for a precipitation of 30 inches the country is divided into 3 main 



'Gannett, Henry, Distribution of rainfall, U. S. Geol. Survey, Water Supply Paper No. 

 234, reprinted from report of National Conservation Commission, 1909, Washington, 1909. 

 The chart referred to is Plate I, and is published in color. The isohyetal lines of our chart (plate 

 52) have been copied from Gannett's plate I by means of a pantograph. 



