10 PHYSICAL PROPERTIES OF SEA WATER 



Bothnia, it may be below 5 °/oo. On the other hand, in isolated seas in 

 intermediate latitudes where evaporation is excessive, such as the Red 

 Sea, salinities may reach 40 °/oo or more. As the range in the open oceans 

 is rather small, it is sometimes convenient to use a salinity of 35.00 °/oo 

 as an average for all oceans. 



Temperature. In oceanography the temperature of the water is 

 stated in degrees centigrade. In the oceans the temperature ranges from 

 about —2° to +30°C. The lower limit is determined by the formation 

 of ice, and the upper limit is determined by processes of radiation and 

 exchange of heat with the atmosphere (p. 74). In landlocked areas the 

 surface temperature may be higher, but in the open ocean it rarely 

 exceeds 30°C. 



Pressure. Pressure is measured in units of the c.g.s. system, in 

 which the pressure unit is 1 dyne/cm^. One million dynes/cm^ was 

 designated as 1 bar by V. Bjerknes. The corresponding practical unit 

 used in physical oceanography is 1 decibar, which equals }{o bar. The 

 pressure exerted by 1 m of sea water very nearly equals 1 decibar; that is, 

 the hydrostatic pressure in the sea increases by 1 decibar for approxi- 

 mately every meter of depth. Therefore the depth in meters and the 

 pressure in decibars are expressed by nearly the same numerical value. This 

 rule is sufficiently accurate when considering the effect of pressure on the 

 physical properties of the water, but details of the pressure distribution 

 must be computed from the density distribution (p. 99). 



Owing to the character of the distribution of temperature and salinity 

 in the oceans, some relationships exist between these quantities and the 

 pressure. The temperature of the deep and bottom water of the oceans 

 is always low, varying between 4° and — 1°C, and high pressures are 

 therefore associated with low temperatures. Similarly, the salinity of 

 deep and bottom water varies within narrow limits, 34.5 °/oo — 35.0 °/oo, 

 and high pressures are therefore associated with salinities between these 

 limits. Exceptions are found in isolated seas in intermediate latitudes 

 such as the Mediterranean and Red Seas, where water of high temperature 

 and high salinity is found at great depths — that is, under great pressures. 



Density of Sea Water 

 The density of any substance is defined as the mass per unit volume. 

 Thus, in the c.g.s. system, density is stated in grams per cubic centimeter. 

 The specific gravity is defined as the ratio of the density to that of 

 distilled water at a given temperature and under atmospheric pressure. 

 In the c.g.s. system the density of distilled water at 4°C is equal to 

 unity. In oceanography specific gravities are now always referred to 

 distilled water at 4°C and are therefore numerically identical with 

 densities. The term density is generally used, although, strictly speaking, 

 specific gravity is always considered. 



