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The complete dependence of the density of seawater is described by the Equation of State. Because the ideal gas equation cannot be used to give an accurate description of seawater density, the Equation of State was derived by careful laboratory measurements. Water samples from many parts of the world were collected for this purpose, to cover the range of salinities experienced in the world ocean.
Formally, the Equation of State can be written as
ρ = ρ ( T, S, p )
(density ρ is a function of temperature T, salinity S and pressure p). The actual equation is a rather complicated polynomial expression, which need not concern us here. Let us get a feeling for typical values of seawater density.
With few exceptions (such as estuaries, which have very low salinity, and evaporative basins, where salinity can climb to values of 40 and more), oceanic salinity is in the range 33 - 37. The calculator below lets you explore density values of seawater at 10 degrees Celsius for the salinity range 35 - 36. Enter a salinity value and then click on the button "calculate the density" to find the corresponding density. Can you find out by how much the salinity has to be changed to produce the same density change that results from a 1°C change in temperature? You may need your notes from an earlier question to work this out.
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© 2000 M. Tomczak