14 THE LIFE OF DEVILS LAKE 



The amount of seepage water reaching the lake is likewise an 

 unknown factor. The porous mantle of glacial till overlying the 

 impervious Pierre clay undoubtedly furnishes a certain supply in 

 this way J especially in spring when the melting snow surcharges the 

 ground with moisture, as may be seen in the little pockets in the 

 fields which are too wet for plowing. The slight slope of most of 

 the drainage basin, however, coupled with the fact that during 

 the summer the rainfall is seldom more than enough for the grow- 

 ing crops, must keep this supply at a minimum. Conversely the 

 loss thru seepage must be a negligible factor owing to the impervious 

 clay underlying the lake. 



The daily amount of evaporation at the lake shore determined 

 by readings on a Livingstone porous cup atmometer from Aug. 4 

 to Sept. 5, 1914, shows, as would be expected, a great range of vari- 

 ation. These readings, however, have a comparative value only, and 

 are of no significance in themselves. 



Factors determining evaporation are three — temperature, wind 

 and humidity. Like other interior regions at this latitude and alti- 

 tude the climate of the Devils Lake region is marked by great ex- 

 tremes of temperature, maxima and minima for 24 years from 1897 

 to 1920 at Devils Lake being 41.1°C on July 28, "^1917 and 42.2° 

 on January 12, 1916. 



Daily extremes are considerable. The average daily range of 

 temperature at Devils Lake city compiled monthly for 19 years is 

 11.98°C. The greatest average variation occurs in August, but the 

 maximum was in January (30.84°). 



Maximum and minimum temperatures at the lake shore were 

 taken from June 13 to Sept. 5, 1914, with a few omissions. These 

 showed a maximum daily range of 23.3° as compared with one of 

 22.8° at Devils Lake city. The former, however, occurred in June, 

 and the latter in August, so there is evidently no very close cor- 

 respondence in the temperature range of the two localities. 



The annual and monthly mean maxima and minima from 1900 

 to 1920 are shown in table 1. The annual average number of 

 sunny days* is 125 while at the summer solstice the number of 

 hoiirs of sunshine is sixteen. 



Twilight lasts till about 11 :00 p. m. and the first flush of day 

 appears about 3 :00 a. m. This great amount of sunshine ensures 

 rapid growth and early maturity to the crops of grain for which 

 the northwest is famous, and doubtless determines in large measure 

 the development of organisms in Devils Lake in early summer. 

 Midday in summer is rarely oppressive and the nights are almost in- 

 variably delightful, days of excessive heat being rare, and usually 

 enduring for but brief intervals. Devils Lake usually freezes be- 



•Ovar 70% sunshine. 



