32 PHYSICAL PROPERTIES OF SEA WATER 



The color of sea water has been examined by observing the color that 

 the water appears to have when seen against the white, submerged surface 

 of the Secchi disk (see p. 27). This color is recorded according to a 

 specially prepared color scale, the "Forell scale." The method is a rough 

 one, and the scale is not adapted to the extreme colors in coastal waters. 



Kalle has critically reviewed earlier theories as to the causes of the 

 color of the sea water and arrives at conclusions that appear to be con- 

 sistent with all available observations. The blue color is explained, in 

 agreement with earlier theories, as a result of a scattering against the 

 water molecules themselves, or against suspended, minute particles 

 smaller than the shortest wave lengths. The blue color of the water is 

 therefore comparable to the blue color of the sky. The transition from 

 blue to green cannot be explained, however, as a result of scattering, and 

 Kalle concludes that it is due to the presence of the water-soluble ''yellow 

 substances." He points out that the combination of the yellow color 

 and the ''natural" blue of the water leads to a scale of green colors as 

 observed at sea. Fluorescence may contribute to the coloring but 

 appears to be of minor importance. 



Suspended larger particles can give color to the sea water if they are 

 present in large quantities. In this case the color is not determined by 

 the optical properties of the water or by dissolved matter, but by the 

 colors of the suspended inorganic or organic particles, and the water is 

 appropriately called "discolored." Discoloring can be observed when 

 large quantities of finely suspended mineral particles are carried into the 

 sea after heavy rainfall or when very large populations, several million 

 cells per liter, of certain species of diatoms or dinoflagellates are present 

 very near the surface. Thus, the "red water" (often more brown than 

 red) which is quite frequently observed in many areas and after which 

 the Red Sea and the Vermilion Sea (the Gulf of California) have been 

 named, is due to an abundance of certain dinoflagellates. Discoloring, 

 however, is a phenomenon of the typical coastal waters, the green colors 

 being frequent in waters near the coast or at sea in high latitudes and the 

 blues characteristic of the open ocean in middle and lower latitudes. 



Sea Ice 

 Freezing and Melting of Ice. If sea water of uniform salinity 

 higher than 25.0 °/oo is subjected to cooling at the surface, the density 

 of the very surface layer increases, giving rise to convection movements 

 that continue until the surface water is cooled to the freezing point, when 

 ice begins to form. At first elongated crystals of pure ice are produced. 

 Since the salinity of the very surface water increases correspondingly, 

 the convective movements are maintained. As the freezing continues, 

 the ice crystals grow into a matrix in which a certain amount of sea water 

 becomes mechanically trapped, and the more rapid the freezing the greater 



