CLIMATE 49 



ducted experiments showing that evaporation from a water sur- 

 face on the western plains of the United States may amount to 

 from 50 to 80 inches, and in some localities to even 100 inches 

 a year, while the rainfall (diminishing in inverse ratio) over 

 this area is from 30 to 12 inches and less. (25) In a series 

 of experiments to determine the relation of wind to evaporation 

 they arrived at the following conclusions : evaporation with the 

 wind at 5 miles an hour was 2.2 times greater than in a calm ; 

 "at 10 miles, 3.8; at 15 miles, 4.9; at 20 miles, 5.7; at 25 miles, 

 6.1, and at 30 miles, the wind would evaporate 6.3 times as much 

 water as a calm atmosphere of the same temperature and hu- 

 midity'." During these experiments the temperature of the air 

 was 84 degrees and the relative humidity 50 per cent. 



In four years in Montana from 1908 to 1911 the maximum 

 wind velocity was 60 miles per hour. This was in March. Sum- 

 mer winds of 46 miles per hour were reported during the same 

 period. These figures are for the plains country. West of the 

 Continental Divide the winds are much more moderate. Winds 

 which rise to considerable velocity are of short duration. In 

 this section of the State the only places where winds seem to be 

 effective are on high and exposed ridges and peaks, and on the 

 more arid southern and western exposures at low altitudes. 



Temperature records from many places in Montana, rep- 

 resenting observations of from 10 to 30 years, afford a reason- 

 able basis for judgment of the climatic conditions of the region 

 in relation to the vegetation. The lowest temperature thus far 

 recorded in ilontana is -65 degrees, at Allies City, and the higli- 

 est 117 degrees at Glendive. Glendive and Miles City are about 

 75 miles apart, and, while the figures just cited as to tempera- 

 tures represent the greatest extremes for these places and do 

 not at all indicate the usual conditions, they do indicate that 

 that section of the state is subject to seasonal extremes and 

 that the annual range of temperature may be considerable. As 

 a matter of fact it may range from 125 to 175 degrees. Other 

 sections of the State on the plains may likewise be subject to 

 extremes and the annual temperatures range from 120 to 150 

 degrees. The lowest mean annual temperature in Montana is 

 recorded from Bowen in the Big Hole, averaging 34.1 degrees, 

 and the highest recorded mean annual temperature is at Billings, 



