Vertical Changes 139 



circumstances are somewhat different. For a fufler treatment of the 

 subject reference should be made to a textbook of oceanography such 

 as Sverdrup et al. ( 1942 ) . In the open ocean the seasonal changes are 

 superimposed upon the permanent thermal structure as relatively 

 minor fluctuations (Fig. 5.4), but in coastal regions the water may be 



Winter 

 stagnation 



ice cover 



->- Spring overturn ■ 



Summer 



20 1- 



FiG. 5.5 

 showing 



stratification 



Fall overturn 



Diagrammatic vertical section of a deep lake of the temperate region, 

 the seasonal cycle of temperature (by permission from Limnology by 

 Welch, 1952, McGraw-Hill Book Co.) 



sufficiently shallow for the seasonal effects to reach the bottom (Fig. 

 5.6). In this type of situation the thorough stirring of the water 

 during the winter produces a uniform temperature from top to bottom 

 with a minimum value in February for the North Temperate Zone. As 

 spring comes on, the larger supply of solar heat, often combined with 

 lowered salinity due to increased run-off, causes the surface water to 

 become lighter and tends to stabilize the upper layers. This effect is 

 self-accelerating. By August the relatively thin stratum at the top has 

 attained its maximum temperature. Below this layer a sharp ther- 



