520 CORRELATION OF DISTRIBUTIONAL FEATURES. 



however, of the fact that humid conditions exist in the extreme North- 

 west, as shown by the return of the isoclimatic Une of 0.60 for the 

 moisture ratio, running from the Upper Columbia River to San 

 Francisco Bay (see plate 59) . 



Transition Zone (fig. 37). — The dimensions of nearly all of the leading 

 climatic conditions exhibit a very wide amplitude in this zone, which 

 embraces the northern Great Plains, portions of the humid Pacific 

 Northwest, and mountain areas throughout the Western States. The 

 length of frostless season and the number of cold days in the frostless 

 season both range through nearly their entire amplitude for the United 

 States. The normal daily mean precipitation and the mean total 

 precipitation of the year likewise range through very wide amplitudes. 

 The ampHtude in the number of hot days is not great, and the extremes 

 for this condition are low, ranging from to 105. The physiological 

 temperature summation exhibits a narrow range, as might be expected 

 from the similarity of its derivation to that of the remainder summa- 

 tion above 32°, the use of which in outlining these zones has been 

 mentioned. Evaporation and humidity both show a much wider 

 amplitude in this zone than in any of the vegetational areas that have 

 been discussed, exceeding greatly the amplitude for the western section 

 of the Northern Mesophytic Evergreen Forest, with which this zone 

 has some distributional features in common. The moisture ratios all 

 show wide amphtude for the Transition Zone, P/E and ir/E ranging 

 through about half the total amplitude and P/E for the year through a 

 still greater amplitude of conditions. The Northwestern Hygrophytic 

 Evergreen Forest is the only one of our vegetational areas that exceeds 

 the Transition Zone in the amplitude of the second of these ratios. 

 The entire gamut of sunshine conditions for the United States is to be 



TcM»eit*Tuiic 

 DATS IN Normal FROtTtxw SUSON (F. S.> 

 Hot Days. F. S. ■ 

 Cold Days, F. S. 

 Physiological Summation. F. S. 

 Normal Daily Mean, coldcst 14 oats of Ycar 

 Normal Daily Mcan, Ycar 



Precipitation 

 Normal Daily Mean, F. S. 



Days in longest Normal Rainy Period, F. S. C 

 Days in longest Normal Dry Period, F. S. I 



Mean Total, Year I 



Evaporation 



Daily Mean. 1887-8, F. S. I I 



Moisture Ratios 



Normal P/C, F. S. 1^3 



Normal n/C, F. S. CU 



Normal P/E, Year (HB 



Humidity 

 Normal Mean, F. S. I 



Sunshine 



Normal Daily Duration, F. S. HI 



MoisTuRE-TeMPERATURE Indices -^ 

 Normal P/E x T. F. S.. Pmysiolocical Method 1 -1 



Fig. 37. Climatic extremes for the Transition Zone. 



