WATER DENSITY AND ITS APPLICATIONS 



volume increases with increasing temperature. The net result of an 

 increase in temperature causes a decrease in the density if other fac- 

 tors remain constant. 



Salinity:— Salinity differs from the other factors in that its 

 variation essentially involves a change in mass of a given volume. 

 Increasing the salinity of a sample of water is much the same as 

 adding mass to volume which increases the density. 



DENSITY AT VARIOUS TEMPERATURES 



For some practical uses of density, it is more important to know 

 the density at the temperature apt to be encountered than at the 

 standard temperature. Figure 1 provides for converting density at 



TEMPERATURE IN DEGREES FAHRENHEIT 

 25 30 3S 40 45 50 55 60 65 70 75 



B5 90 96 



Figure 1. Sea water density at various temperatures. 



expands so that the surface of the sea is slightly elevated in the 

 center of the warm spot. Immediately, the water begins to flow 

 downward in all directions away from the center. Cooler and denser 

 water then presses up against the warmer and lighter water, 

 buoying it up. Circulation therefore continues until there is an even 

 distribution of density and pressure. 



The rotation of the earth causes all these movements to be de- 

 flected to the right (in the Northern Hemisphere). This results in 

 a whorl of clockwise water movements spreading out from the warm 

 center. Under ordinary circumstances, this eddy of warm water can 

 be expected to spread until the density differences are equalized. 

 The process is retarded, however, by the tendency of the water par- 

 ticles to be deflected more and more to the right, so that at the edge 

 of the eddy the movement follows very nearly a circular path. 



The sinking of cold or saline water produces a somewhat similar 

 current pattern, but in the opposite direction. If the surface water 

 in a particular area is cooled sufficiently to make it denser than the 

 water underneath, it will sink until it reaches a level of its own den- 

 sity. As it does, the sea surface is depressed, and warm surface 

 water flows in from all sides to take its place. 



All these factors are interrelated — the difference in density 

 between the warm water above and the colder water below, the 

 slope of the thermocline (as influenced by latitude), and the velocity 

 of the current produced. The relationship can be expressed by a 

 mathematical formula, so that by measuring the vertical density 

 structure of ocean waters, it is possible to determine the direction 

 and velocity of currents. Current systems are commonly studied 

 in this way, for actual measurements of the currents are very in- 

 accurate unless the vessel is equipped for anchoring in deep water, 

 and even then measurements are difficult and laborious. The prin- 

 ciple of determining ocean currents from the density structure of 

 the sea, which is generally known as the Bjerknes Circulation Theory, 

 is one of the most important contributions to modern oceanography. 



While the mathematical relations of currents with the density 

 structure of the water are always about the same, the casual rela- 

 tionships are often inextricably mixed. The wind may set up a 

 current, and the transport of water that results will cause the den- 

 sity surfaces to slope. Contrariwise, density slopes that result from 

 heating or cooling processes will cause a current to be formed. Often 

 both processes are involved, and some of the most powerful ocean 

 currents are those in which the prevailing winds and the density 

 distribution work together. 



In examining the currents it is worthwhile to compare them 

 with the wind systems of the world which are similar in many re- 

 spects and are, to a large degree, responsible for the general pattern 

 of the currents. This is particularly true of the west wind drift in 

 the Antarctic and of the North and South Equatorial Currents which 

 lie in the trade wind belts. The latter currents form part of the 

 great eddies centering in the mid-latitudes, clockwise in the Nor- 

 thern Hemisphere, and counterclockwise in the Southern, corre- 

 sponding to the prevailing wind direction. As they are primarily 



59°F. (15°C.) as published in U. S. Coast and Geodetic Survey Pub- 

 lication Nos. 31-2 and 31-4 (Density of Sea Water, Atlantic Coast, 

 North and South America, and Density of Sea Water, Pacific Coast, 

 North and South America and Pacific Islands). As it is intended 

 for use with true density rather than observed hydrometer readings, 

 no glass correction was included. 



To convert a density at 59°F. to density at another temperature, 

 enter the graph horizontally from the left with the known density 

 and downward from the top with the desired temperature; the posi- 

 tion of the point of intersection of the curves gives the density at 

 the desired temperature. Interpolate between curves when necessary. 



Example: If certain water has a density of 1.0274 at 59°F. 

 (15°C.) what would its density be at a temperature of 79°F.? En- 

 tering the graph from the side and top with these values, it is found 

 that the point of intersection lies about 4/10 of the way between 

 curves 1.0240 and 1.0250, so that the density at 79° F. is 1.0244. 



DENSITY AND OCEAN CURRENTS 



When the surface water in a particular area is heated until it is 

 warmer than the water around and underneath it, the heated water 



Figure 2. Diagram of typical circulation, wind driven eddy 

 and subpolar convergence. 



70 



