WATER DENSITY AND ITS APPLICATIONS 



J. W. Chanslor 



Maritime Safety Division 



U. S. Naval Oceanographic Office 



The sea is a wilderness full of secrets, upon which man has sailed 

 since the earliest times. Even in these days of accelerated scientific 

 achievement, mysticism of the sea remains. The purpose of this 

 article is to touch upon one facet of the phenomena of nature — 

 density of sea water and its applications to mariners. 



Some of the complexities discussed include changes in draft, due 

 to changes in density, currents affecting a voyage, causes of harbor 

 siltation, and the explanation of certain related phenomena observed 

 at sea. 



The density of sea water depends upon three variables: salinity, 

 temperature, and pressure. At constant temperature and pressure, 

 density varies with salinity or, because of the relationship between 

 this and chlorinity, with the chlorinity. Density and specific gravity 

 are virtually identical numerically. A temperature of 32°F and 

 atmospheric pressure are considered standard for density determina- 

 tion. The effects of thermal expansion and compressibility are used 

 to determine the density at other temperatures and pressures. And, 

 the density at a particular pressure is quite important in its relation 

 to ocean currents. 



The greatest changes in density of sea water occur near the sur- 

 face where the water is subject to influences not present at depths. 

 At the surface density is decreased by precipitation, runoff from 

 land, melting of ice, or heating. When the surface water becomes 

 less dense, it tends to float on top of the more dense water below. 

 There is little tendency for the water to mix, so the condition 

 is one of stability. The density of surface water is increased by 

 evaporation, formation of sea ice, and cooling. If the surface 

 water becomes more dense than that below, it sinks to the level at 

 which other water has the same density. At that level it tends to 

 spread out to form a layer or to increase the thickness of the layer 

 below it. The less dense water rises to make room for it, and the 

 surface water moves in to replace that which has descended. Thus, 

 a convective circulation is established. It continues until the density 

 becomes uniform from the surface to the depth at which a greater 

 density occurs. If the surface water becomes sufficiently dense, it 

 sinks all the way to the bottom. If this occurs in an area where 

 horizontal flow is unobstructed, the water which has descended 

 spreads to other regions, creating a dense bottom layer. Since the 

 greatest increase in density occurs in polar regions, where the air 

 is cold and great quantities of ice form, the cold dense polar water 

 sinks to the bottom and then spreads to the lower latitudes. This 

 process has continued for a sufficient length of time that the entire 

 ocean floor is covered with this dense polar water, thus explaining 

 the layer of cold water at great depths in all the oceans. 



Changes in salt content of ocean waters, with position, depth, 

 and time, are brought about in the main by greater evaporation 

 taking place in subtropical latitudes and greater precipitation in the 

 polar seas. In the equatorial region surface water increases in 

 salinity because of evaporation but decreases in density because of 

 rise in temperature. 



As the density of sea water depends on temperature and salinity, 

 all processes that alter the temperature or the salinity influence the 

 density. The water of the greatest density is formed in high latitudes. 

 Generally, density is a physical property possessed by solids, 

 liquids, and gases. It is defined as mass per unit volume. Although the 

 quantities, temperature and pressure , have only small effects on 

 density in the laboratory, these factors must be given close atten- 

 tion in oceanography. In the oceans density is further complicated 

 by the effect of salinity. 



The fact that density is an inverse function of volume indicates 

 that an increase in volume will cause a corresponding decrease in 

 density, whereas a decrease in volume will cause the density to in- 

 crease. It logically follows that whatever factors cause the volume 

 of some quantity to undergo a change will also cause density to 

 undergo an inverse change, other factors being constant. The fol- 

 lowing explanations show how a change in pressure, temperature, 

 or salinity bring about a change in density. 



Pressure:— Though, as a general rule, liquids may be considered 

 incompressible, such an assumption cannot be made when consider- 



ing a unit volume of water at oceanic depths. Due to increase of 

 pressure with increasing depths, a small volume of water carried 

 from the surface down to some point beneath the surface will be 

 compressed, or stated differently, will be caused to occupy a smaller 

 volume. This decrease in volume causes an increase in density if the 

 mass of the sample remains constant without a change in tempera- 

 ture. Thus, the effect of pressure is to bring about a change in vol- 

 ume and thereby influence the density as described. 



Smoked glass slide 



Temperature:— Like pressure, the effect of changes in tempera- 

 ture changes the volume of a sample of water; however, unlike 

 pressure, the variation of volume with temperature is direct, causing 



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