46 The Sea-water and its Physical and Chemical Properties 



Table 13. Elevation of boiling point and lowering of vapour pressure in sea-water 



A comparison of the temperature of the freezing point / a, that of maximum density 

 d and their corresponding densities at different sahnities is of some interest. Figure 23 

 shows the change in these temperatures with increasing sahnity. The temperature of 



10 



30 



40 



20 

 S, %„ 



Fig. 23. Dependence of freezing temperature to on the salinity S. 



maximum density decreases with increasing salinity more rapidly than the temperature 

 of the freezing point. At a salinity of 24-695%o (ctq = 19-839) both temperatures are 

 the same and 



{) = fa = -1-332°C and a^ = a,^ = 19-852. 



Reference might be made here to an oceanographic use of this (Helland-Hansen, 

 1911-12). Suppose a surface layer of a sea area is homo-haline with a salinity less than 

 24-695%o and that its surface is subject to strong cooling in winter. This cooling will 

 increase the density of the surface water, and as a consequence a vertical convection 

 must occur and will continue until the whole homo-haline surface layer reaches the 

 temperature of maximum density. It will then cease. The surface only will now be 

 cooled further by radiation until it reaches the freezing point and ice begins to form. 

 This will increase the salinity, and convection will again be set up and will be maintained 

 by the double effect of the increase in salinity and the decrease of temperature. These 

 conditions may occur, for example, in the Baltic. The homo-haline surface layer with 

 5 = 10%o during the winter cools and the vertical convection continues until the 



