Scott: Lakes of Tippecanoe Basin 



9 



greatest of any two layers in the lake. But the effect of wind 

 is also greatest near the surface. On a windy day this upper 

 homothermous layer is gradually thickened. It is evident 

 that the addition of the lower cooler water to the homothermous 

 warmer water above reduces the difference in the temperature 

 of the two layers, hence their resistance to mixture. However, 

 the effect of the wind decreases as the depth increases. 



With the advance of the season and the consequent increase 

 in the rate of heat absorption, the effect of the wind in the lower 

 levels of the lake is overcome, thus cutting them off from the 

 wind-circulated region nearer the surface. The result is that 

 during the summer months there are established three strata 

 in all lakes in this latitude, except the very shallow ones. The 

 first is a warm stratum near the surface, about 5 or 6 meters 

 thick in our lakes; then a layer below this extending to 10 or 12 

 meters in which the temperature changes very rapidly, and 

 finally a cold layer reaching to the bottom. These layers have 

 been called by Professor Birge the epilimnion, thermocline, and 

 hypolimnion respectively. The thermocHne is called ^'Sprung- 

 schicht" by Richter, and German limnologists generally follow 

 his terminology. English writers use the awkward phrase ''dis- 

 continuity layer". 



Heat Budgets of Lakes 



The temperature of a lake indicates the amount of heat it 

 contains. To develop the summer condition just described, 

 much heat must be absorbed. However, not all the heat that 

 impinges on the lake is absorbed. Much of it is reflected. Wedder- 

 burn (1910) shows from the calculations of Knott that Loch Ness 

 reflects about three times as much heat as it absorbs. The 

 annual ''budget of heat", as Birge has called it, is calculated from 

 the difference in the winter and summer temperatures. Concern- 

 ing the annual budget in Indiana lakes, nothing is known because 

 we have no records of winter temperatures. The heat that is 

 absorbed after the lake passes 4°C., or the point of maximum 

 density, is due in large part to the influence of wind, which carries 

 water from the surface to the deeper levels of the lake, thus 

 storing the heat. This has been called "wind-distributed heat". 



This has been calculated for the Indiana lakes appearing in 

 Table III, which follows. It is absolutely true only for the date 

 on which the observations were made. But it has been shown 



