296 ENVIRONMENTAL CONDITIONS. 



eastern portion. The western portion is much larger in this case, how- 

 ever, occupying the Pacific Slope for practically the full length of the 

 western coastline and widening at the north to include about the 

 western half of Washington. The eastern humid region has an entirely 

 different form from that shown on the precipitation charts. Here it 

 does not occupy the southeastern part of the country, but embraces 

 the northern margin from about the one-hundredth meridian eastward. 

 It also occupies portions of the Atlantic coast as far south as Cape Fear. 

 It appears that the line for value 120 passes into Canada from Wash- 

 ington and reenters the United States in North Dakota, so that these 

 two portions of the humid province are probably to be regarded as a 

 single one. It should be noted, furthermore, that the Atlantic coastal 

 portion from Massachusetts, or New Jersey, southward appears to be 

 separated from the northeastern portion, and that a small area of humid 

 conditions is shown about Corpus Christi and Brownsville, Texas. 

 These features will appear more prominently on the charts of pre- 

 cipitation-evaporation ratios and on those of relative humidity, to 

 be considered below. 



The arid province (values above 240) occupies much the same 

 region as in the case of the precipitation charts, but it does not here 

 extend west of the Sierra Nevada Mountains. Of course, the western 

 mountains are largely humid, but our charts do not generally present 

 such details. The semiarid province (values between 160 and 240) 

 occupies a belt outside of the area of the arid province, and this belt 

 is extended eastward in the middle of the country nearly to the Appa- 

 lachian Mountains. This eastern lobe of the semiarid province will 

 also be pronounced on the charts of moisture ratios and relative 

 humidities. The line separating the semiarid from the eastern semi- 

 humid area (value 160) does not here bend eastward at its northern 

 end as it does on the precipitation charts; on the contrary, it here 

 bends westward and apparently joins the corresponding line which 

 enters Canada from western Montana. 



Annual evaporation intensities. (Table 15, plate 54.) — Russell's 

 table gives the yearly totals for his series of stations, in inches of 

 depth from some hypothetical pan of water, and he also presents a 

 chart to represent these annual data. The data are reproduced in the 

 third column of table 15 and they are shown graphically by the chart 

 of plate 54, which is not exactly the same as Russell's chart, a number 

 of obvious errors in the latter having been corrected here. The total 

 range for the country is from 18.1 (Tatoosh Island, Washington) to 

 101.2 (Fort Grant, Arizona), and the isoatmic lines are placed at 

 intervals of 10 inches, with full lines for the values 30, 50, and 80. 



This chart has a pronounced general resemblance to the one repre- 

 senting evaporation for the period of the average frostless season 

 (plate 53), but it differs quantitatively therefrom in several important 



