506 CORRELATION OF DISTRIBUTIONAL FEATURES. 



the rate of evaporation for these areas. The influence which it exerts 

 in this respect is therefore embraced in the moisture ratios, and addi- 

 tional evidence is supplied to the view that the moisture ratios are of 

 particular importance in controlling the bounds of the Transition 

 region. 



While the moisture-temperature index shows little difference when 

 the Grassland and Transition areas are contrasted, it exhibits a very 

 wide amplitude for the Deciduous Forest, indicating that the interplay 

 of moisture and temperature differences causes a wide diversity of 

 conditions in the Deciduous Forest region. 



Northwestern Hygrophytic Evergreen Forest (fig. 32). — There are 

 many respects in which this region possesses the most marked set of 

 climatic conditions of any of the vegetational areas of the United 

 States. Although the Hygrophytic Forest merges into the western 

 section of the Northern Mesophytic Forest, both southward along the 

 California coast and in isolated areas on the western slopes of the 

 Rocky Mountains, nevertheless the climatic characteristics of the most 

 pronouncedly hygrophytic region, as indicated on plate 2, cause it to 

 stand out in sharp contrast with the Mesophytic Forest. 



The length of the frostless season in the Hygrophytic Forest varies 

 from 103 days at McKenzie Bridge, Oregon, to 316 days at Cape 

 Disappointment, the greatest ampHtude to be found in any vegetation 

 in the country. There are no hot days and no cold days, in our sense 

 of these terms, in any part of the region. The physiological summation 

 of temperature for the frostless season is both low in its values and 

 small in amplitude, resembling closely the eastern section of the 

 Northern Mesophytic Evergreen Forest. The normal daily mean 

 temperature of the coldest 14 days of the year ranges from about 35° 



TlMrCIIATUIIC 



DAf« IN Normal Fkostu** 8u(on (F. S.) 



Hot Day*, F. S. 



Cold Dat*. F. S. 



Phvsiolooical Summation, F. 8. 



NoKMAL Daily Mean, colocst 14 OAT* or Vcar 



Normal Daily Mcan, Ycan 



PlICCiriTATION 



Normal Daily Mcan, F. S. 

 Days in lonout Normal Rainy Pcrioo, F. S. 

 Days in lonqut Normal Drt Pirioo, F. S. 

 Mcan Total, Year 



cvaroration 

 Daily Mian, IBBT-S, F. •. 



MoioTURK Ratio* 

 Normal p/c, F. 8. C 



Normal it/C, F. S. C 



Normal P/t, ViM C 



Humidity 

 Normal Mean, F. t. C 



Sunshine 







6 — 



Normal Dailt Duration, F. S. i — ■-F" 



Moisture-Temperature Ind'iced 

 Normal P/t « T. F. 8., PHYSioLoaicAi Mctmoo I 



Fig. 32. Climatic extremes for Hygrophytic Forest. 



