A LIMNOLOGICAL RECONNAISSANCE 15 



The Distribution of Heat. Each square centimeter of the 

 surface of lake Okoboji absorbs 20,933 cal. out of the heat 

 delivered to it by the sun after the water has reached 4° ; 

 and this heat is distributed through the water of the lake 

 in a manner shown by the temperature curve in Fig 2. The 

 length of the warming period is not known from observa- 

 tion, but probably this lake, like lake Mendota, reaches the 

 temperature of 4° about April 15 and continues to gain heat 

 in the average year until August 15, although very little is 

 gained after August 1. The sun should deliver about as 

 much heat at Okoboji as at lake Mendota, since their lati- 

 tude is practically the same and Okoboji is about 5.8° far- 

 ther west. We may therefore assume that Okoboji lake, 

 like Mendota, receives about 60,000 cal. per sq. cm. of sur- 

 face during the period when the summer heat-income is 

 gained, or about 54,000 cal. to August 1. If the lake, there- 

 fore, gains about 21,000 cal., it lays up about one-third of 

 its total receipts of heat and about 40% of its receipts to 

 Aug-ust 1. The remainder is partly reflected; it is partly 

 used in evaporation, and much of it is returned to the air 

 during the night and whenever the temperature of the air 

 falls. 



It is possible to follow the distribution of the heat re- 

 ceived by the surface through the water of the lake, as is 

 shown bj^ the following table : 



