PHYSICAL OCEANOGRAPHY 239 



ature, the density of sea - water with a sahnity of 35 per 

 thousand and at a temperature of 0° C. being 1.028 13. By 

 means of Knudsen's Tables the density is quickly found when 

 both salinity and temperature are known. The value of most 

 interest to us is the density at the potential temperature (see 

 above, p. 221) corresponding to the temperature in situ. It has 

 been found that this density always increases from the surface 

 downwards to the bottom, even when the compression is left 

 out of account. If this were not so, in order to attain equilibrium 

 the heavier overlying water and the lighter underlying water 

 would have to change places, and this is what actually 

 takes place in winter, when the density at the surface exceeds 

 that of the waters below. The layers will always arrange them- 

 selves in such a way that the lighter water is on the top and 

 the heavier water underneath. 



Salt water freezes at a lower temperature than fresh water ; Freezing- 

 thus sea-water with a salinity of 35 per thousand freezes at p°"^'" 



— 1.9° C, so that temperatures below zero are found in the sea, 



— I J° C, for instance, being a common temperature in the polar 

 currents. When the salinity exceeds 24.7 per thousand the 

 water becomes heavier on being cooled, until the freezing-point 

 (below zero) is reached. This implies an essential difference 

 between salt water and fresh water. In the deep water of lakes 

 temperatures below 4° C. are never found, while in the bottom- 

 water of the ocean considerably lower temperatures prevail, as, 

 for instance, — 1° C. or still lower recorded in the Norwegian 

 Sea, and about + 2° C. recorded in the Atlantic. Thus it is, as 

 a general rule, colder in the great depths of the ocean than it is 

 at the bottom of deep lakes. 



We shall now indicate in a general way the distribution of Distributioi 

 salinity. It must be remembered that the salinity is raised by of^^^™')- 

 evaporation, and lowered by dilution with fresh water either 

 from rainfall or from rivers. Where the evaporation outweighs 

 the supply of fresh water the salinity increases, as is the case, 

 for instance, in the Mediterranean and in the Red Sea, where 

 the air is dry and hot, and in the ocean north and south of the 

 equator, where the warm trade-winds blow, producing a strong 

 evaporation. In such places a high salinity will be found. 

 There is a steady inflow of Atlantic surface-water with a salinity Medi- 

 of about 36 per thousand into the Mediterranean Sea, where the t^rranean. 

 water removed by evaporation is far greater than the supply of 

 fresh water, so that the salinity rises to 38 per thousand, accom- 

 panied by an increase in density, which is accentuated by the 



