68 



temperature gradients that might be associated with currents 

 in the bay. The initial assumption which can be made is that 

 the density of water near the surface of the sea generally 

 is more dependent upon the temperature than upon the salinity. 

 Since vertical profiles show that the salinities in these «* 

 coastal waters are nearly constant with depth and with dis- 

 tance from shore , examination of temperature conditions alone 

 may permit first approximations as to the directions of the 

 currents. Under such a theme of utilization, the term 

 "lighter**, as used with density slopes, may be replaced by 

 "warmer**, and "denser** by "colder**. Bearing these conditions 

 in mind and for an understanding of the following discussion, 

 one has then the simple rules In the northern hemishpere the 

 warmer water lies on the right hand side of an observer looking 

 in the direction of the current , and the colder water lies on 

 the left hand . 



The Effect of a Temperature Gradient 



A change in water temperature per unit distance is called 

 a temperature gradient . The term may be used to describe a 

 change in temperature with depth (vertical temperature gradient), 

 or it may refer to a change per unit distance along a level 

 surface (horizontal gradient). The following discussion deals 

 entirely with horizontal temperature gradients which are most 

 conveniently represented by charts showing the topography of 

 selected isothermal surfaces. 



The gradients associated with slopes of isothermal sur- 

 faces are directly related to currents only under four 



