The density of sea water (mass per volume) is one of the
most important processes that controls the motion of the ocean.

Small density differences, caused, for example, by
differences in surface heating or cooling, can produce very strong currents. The
determination of seawater density, and its variation, is therefore one of the most
important tasks in oceanography.

How do you think the density of sea water changes
with water depth?

The density of sea water depends on:

- temperature T,
- salinity S and
- pressure P, which increases with water depth.

Oceanographers discuss the density of sea water somewhat
differently than a chemist or physicist.

The values of sea water density, labeled on the
diagonal lines in the graph below, do not include the density of fresh water (1 gram per
cubic centimeter g/cm^{3}) or the decimal places. These oceanographic measurements
are called "sigma-t" and derived from the following equation:

**"sigma-t"= [(Sea
water density
- 1) x 1000] **

Since seawater density is always slightly more than 1 g/cm^{3}
and measured to the nearest .00001 of a gram, oceanographers do not want to worry about
the decimal places or writing the 1 everytime.

The graph below shows values of seawater
density,
in the form of sigma-t, shown by the diagonal
lines for any given combination of seawater temperature and
salinity.