The Yearly Cycle 35 



is more real stagnation during the winter period; for, 

 after the formation of a protecting layer of ice, this 

 shuts out the disturbing influence of wind and sun and 

 all the waters are at rest. The surface temperature 

 bears no further relation to air temperature but remains 

 constantly at 0° C. 



After the melting of the ice in late winter the surface 

 waters begin to grow warmer; so, they grow heavier, 

 and tend to mingle with the underlying waters. When 

 all the water in the lake is approaching maximum 

 density strong winds heaping the waters upon a lee 

 shore, may put the entire body of the lake into complete 

 circulation. How long this circulation lasts will depend 

 on the weather. It will continue (with fluctuating 

 vigor) until the waters are warm enough so that their 

 thermal stratification and consequent resistance to 

 mixture are great enough to overcome the disturbing 

 influence of the wind. Thereafter, the surface may be 

 stirred by storms at any time, but the deeper waters of 

 the lake will have passed into their summer rest. 



On the approach of autumn the cooling of surface 

 waters starts convection currents, which mix at first the 

 upper waters only, but which stir ever more deeply as 

 the temperature descends. When nearly 4°C.,_ with 

 the aid of winds, the entire mass of water is again put 

 in circulation. The temperature is made uniform 

 throughout, and what is more important biologically, 

 the contents of the lake, in both dissolved and suspended 

 matters, are thoroughly mixed. Nothing is thereafter 

 needed other than a httle further cooling of the surface 

 waters to bring about the inverse stratification of the 

 winter period. 



Vernal and autumnal circulation periods differ in 

 this, that convection currents have a smaller share, and 

 winds may have a larger share in the former. For the 

 surface waters are quickly warmed from 0° C. to 4° C, 



