HEAT 



93 



long, 2 kilometers wide, and have a mean 

 depth of 30 meters and a maximum of 50 

 meters or more, the annual heat budget of 

 the water, rather than of the lake as a 

 whole, is between 30,000 and 40,000 gram- 

 calories per square centimeter of surface. 

 Different types of soils also have charac- 

 teristic heat budgets, and supposedly rivers 

 do, too, although knowledge concerning the 

 heat budgets of rivers is remarkably scanty. 



THERMAL STRATIFICATION 



One important relationship between fresh 

 water and temperature is outstanding. As 



Deeper lakes, if covered with winter ice 

 for some weeks and not exposed to unu 

 sually strong or direct wind action, show 

 an annual cycle that is closely associated 

 with the four seasons of the year. The se- 

 quence is: (1) Under the ice in winter, the 

 lake is stratified inversely with the colder, 

 lighter water at the surface floating on the 

 denser water, which has a uniform tempera- 

 ture of about 4° C. (2) There is an over- 

 turning and a circulation of water through- 

 out the entire lake in the spring that results 

 from the surface water becoming warmed 

 to 4° C. when it has the same density as 



Jan. 



Feb. Mar. Apr. May June July Aug- Sept. Oct. 



Nov. 



Dec. 



Fig. 9. Mean annual energy receipt at Lake Mendota. Ordinates are in thousands. (Data 



from Juday.) 



the temperature approaches 4° C. from 

 either direction, the density, but not the 

 viscosity, of the water increases. With fur- 

 ther cooling or warming, the density falls. 

 This point of maximum density of fresh 

 water at 4° C. has an importance in the 

 temperature relations of a lake that is some- 

 what comparable, when broad implications 

 are considered, with the fact that the freez- 

 ing point of such water lies where it does 

 on the absolute temperature scale. 



the deeper water. It is then easy for the 

 spring winds to produce the spring over- 

 turn, (3) With rising temperature, the sur- 

 face water is soon warmed above the point 

 of maximum density and floats upon the 

 colder, denser water below (Fig. 10). The 

 spring warming takes place mainly in the 

 water near the upper surface, since from 

 two-thirds to nine-tenths of the incident 

 radiation is cut off, either by surface reflec- 

 tion or by absorption by the first meter of 



