60 



The Sea-water and its Physical and Chemical Properties 



Fig. 33. Back radiation from the interior of the sea towards the water surface. 



concerned. Lauscher (1944) has obtained the same result in another way and found 

 the value 0-9535 for the constant. Falkenberg (1928) has made similar calculations 

 and has found the somewhat lower value 0-937 for this constant. 



{d) The Colour of Sea-water 



The colour of sea-water in the scientific sense is taken to include all those colour 

 phenomena which arise because of the optical properties of sea-water and the sub- 

 stances dissolved and suspended in it. The colour of the sea can vary widely and may 

 assume any shade from a yellowish green to the deepest blue. To observe the colour 

 of the sea undisturbed by external reflections it is best to look through a tube which is 

 blackened inside, dipped in the water. The colour can be determined by comparison 

 with standard colours or by spectrophotometry. Kalle (1938) has designed a special 

 colour measurement tube in which the colour of the sea can be determined with a 

 comparator. In practice, the colour is for preference determined with standard 

 colours, using the Forel-Ule scale. Accurate colour determinations in the open sea are 

 by no means frequent and have been made almost only by oceanographic expeditions. 

 The largest part of the surface of the ocean is blue {Forel 1 and 2), particularly, the 

 regions within the tropics and subtropics, while the green colour is prevalent in coastal 

 areas and shallow seas, especially in adjacent seas and polar regions. In the 

 Atlantic Ocean (Schott, 1942) there is a certain symmetry in the distribution of colour. 

 From 15° to 35° N. and from 10° to 30° S. it is a deep blue. The purest and richest 

 colour is in the central parts of these areas, roughly from the Bermudas to near 

 Madeira and off the Brazilian coast till St. Helena. In the Benguela current, generally 

 in areas of upwelling, for example off the West African coast in the north and off the 

 south-west African coast in the south the sea-water has a more greenish colour. In the 

 Southern Hemisphere a tongue of greenish blue water runs from this coast of South 

 Africa far up to the north between 0° and 10° S. (up to St. Paul Island). 



The higher latitudes in both hemispheres are always discoloured. Greenish blue 

 predominates north of 40° N. and gradually changes to green. The waters of the English 

 Channel, the North Sea and the Baltic are of the same colour. In the Southern 



