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DRAFT

"How do we measure the density of sea water?

In oceanographic terms, density is the weight of the water relative to
that of purely fresh water. Since fresh water weighs about 1000
kilograms per cubic meter and seawater weighs about 1.026 times
that, we say that the typical seawater density is 1026 kg/m3.
Even though many people will say that density is a unitless
quantity, many oceanographers will assign the units of kilograms per
cubic meters.
"Sigma-t" is simply density minus 1000 (without the effect of
pressure). Since the variations in seawater "density" varies only
in the last few significant figures, we subtract 1000 for
convenience in order to carry less numbers around. Hence, typical
sigma-t is 26.0. Note that density (and sigma-t) varies with
temperature and salinity but the typical values we find on the shelf
range between 23.0 and 28.0. Note that the units on sigma-t are now
"kg/m^{3}-1000".

There is another density measure sometimes denoted "sigma_0" (where
the last digit, a zero, is used to represent the greek letter theta). This is
important in deep water (I think) where a water parcel's^{1} temperature
varies due to the affect of pressure. When it is taken to a
reference pressure, the density is different. This altered density
which takes into account adiabatic heating/cooling^{2} with
changes in pressure is called the "potential density". I never
worry about this quantity in shelf environments.

We normally calculate "sigma-t" given observations of temperature and
salinity. Since it is a "derived quantity" , it is not always
posted on data servers. In these cases, code is available in MATLAB
and FORTRAN on request.

Jim Manning

jmanning@whsun1.wh.whoi.edu

**References**

See, for example, Pond and Pickard discussion in "Intro. to
Dynamical Oceanography" pages 6-11.

For a Matlab routine to compute sigma-t, see
http://globec.whoi.edu/globec-dir/sigmat-calc-matlab.html.

**Footnotes**

^{1}A water parcel is "a little finite chunk of water."

^{2}If you move a parcel of water from one pressure (depth) to
another it will undergo a slight change in temperature. This is why
is it is important to understand what pressure is used when
calculating the water parcel's density from temperature and salinity.

*Contributed by: Jim Manning, Feb. 2001*

Last modified: March 2, 2001