502 CORRELATION OF DISTRIBUTIONAL FEATURES, 



and the other five conditions shown in figure 29 run through about 

 half the gamut for the country. Although the mean total precipita- 

 tion is both low in amount and narrow in its amplitude for this region, 

 the normal daily mean precipitation ranges through a greater ampli- 

 tude of values and reaches maximum values which are slightly more 

 than half as great as the maximum values for the Northwestern Hygro- 

 phitic Evergreen Forest. This difference points to the importance 

 of the seasonal distribution of rainfall in the Grassland region and to 

 the heavier precipitation of the frostless season. The evaporation 

 index for the frostless season ranges from a daily mean of 0.117 inch, at 

 Moorhead, Minnesota, to 0.275 inch, at Cheyenne, Wyoming, which is 

 nearly half the amplitude for the United States. Numerous isoclimatic 

 lines cross the central portion of the Grassland, as this would indicate. 

 A comparison of the position of the Grassland (plate 2) with the 

 location of the isoclimatic lines showing the values of the moisture 

 ratio tt/E for the frostless season (plate 59) indicates a correspondence 

 as close as that exhibited for the Desert. The isoclimatic hne for the 

 value 0.40 lies slightly to the east of the western edge of the Grassland 

 in Montana, South Dakota, and Nebraska, although very close to it 

 in Texas. The line for the value 0.30 would lie much nearer this 

 boundary along the northwestern edge of the area. The eastern edge 

 follows closely the line for the value 0.60 in Texas and Oklahoma, but 

 crosses rapidly in Kansas and Nebraska to the lines for higher values, 

 and reaches its maximum of 0.117 in western Minnesota. A corre- 

 spondence almost as close is shown between the Grassland and the 

 area comprising normal mean relative humidities between values of 

 50 per cent and 65 per cent for the frostless season (plate 65). Along 



TCMPCRATURC 



DaVS in NORMAt Fro«tix«« Scason (F. 5.) I 



Hot Davs. F. S. C 



Colo Dats. F. 8. I 



Physiological Summation, F. S. IZ 



Normal Daily Mcan. colocst 14 days or Year ■ 

 Normal Daily Mean, Yean I 



Precipitation 

 Normal Daily Mean. F. S. C 



Days in longest Normal Rainy Period. F. S. ■ 

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

 Mean Total. Year C 



Evaporation 

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



Moisture Ratios 

 Normal P/C. F. S. 

 Normal rr/E, F. 5. 

 Normal P/E, Year C 



Humidity 

 Normal Mean, F. S. L. 



Sunshine 

 Normal Daily Duration, F. S. I 



Moisture-Temperature Indices 

 Normal PfZ x T. F. S., Physiological Method I 



FiQ. 29. Cb'matic extremes for Grassland. 



