CORRELATION OF DISTRIBUTIONAL FEATURES. 507 



to about 45°. Inasmuch as the coldest portion of the year is coincident 

 with the moist portion, this circumstance is of great importance, as 

 indicating that the highest temperatures of the coldest days of the 

 year are still well above frost, while the winter days outside the coldest 

 period present conditions which are favorable for photosynthesis and 

 growth. The normal daily mean temperature of the year ranges from 

 below 50° for the coldest stations to above 55° for the warmest. 



The whole series of moisture conditions, including evaporation and 

 the moisture ratios, is remarkable in this region for the extremely wide 

 amplitudes exhibited. The highest values for all of these conditions 

 are secured from Cape Disappointment, and the lowest values from 

 Roseburg, Oregon, a town located in the valley of the Umpqua River, 

 in the driest conditions that are to be found in the region. The normal 

 daily mean precipitation exhibits an amplitude which is about five- 

 sixths of that for the entire United States. The values and amplitude 

 of the number of days in the longest normal rainy period of the frostless 

 season are here very similar to these conditions for the Grassland, 

 while the greatest number of days in the longest normal dry period of 

 the frostless season (198) is greatly in excess of the smallest number of 

 days in this period (127) for the Desert region. The mean total pre- 

 cipitation for the year is not only higher for this vegetation than for 

 any other, but it also shows a greater amplitude than elsewhere. The 

 daily mean evaporation for tlie frostless season ranges from the lowest 

 value in the United States, 0.052 inch at Cape Flattery, Washington, 

 to a value of 0.143 inch at Roseburg. The values and amphtudes of 

 evaporation conditions in the Hygrophytic Forest are closely similar 

 to those in the Southeastern Mesophytic Evergreen Forest and in the 

 eastern section of the Northern Mesophytic Evergreen Forest. The 

 maximum values for the three vegetations are nearly the same, but the 

 very low values for the Hygrophytic Forest are not found in the two 

 latter regions (see fig. 24) . There is a shght overlapping of the evapora- 

 tion conditions with those of the western section of the Northern 

 Mesophytic Evergreen Forest, into which the Hygrophytic Forest 

 merges to the east and south, and with those of the Pacific Semi- 

 desert region, the conditions of which are approached in all of the 

 broad valleys of coastal Washington and Oregon which he in the lee of 

 the mountains. 



The values for all three forms of the moistm-e ratio are remarkable, 

 in the Hygrophytic Forest, for their great amplitude. It has already 

 been seen (fig. 25) that the highest values of the ratios for this region 

 far exceed the maximum values recorded for any of the other vegeta- 

 tions of the United States. The relatively low values of the South- 

 eastern Evergreen Forest and of the eastern section of the Northern 

 Evergreen Forest are completely overlapped in the Northwestern 

 Forest, and the moisture conditions of such locahties as Astoria and 



