15. Density of Sea Water
Try Again!

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/cm³) 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/cm³ 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.

Used with Permission of Matthias Tomczak ©Copyright 1997t 1997

You should be able to determine the effect of temperature and salinity on density from the information displayed in this graph.

How does the density of sea water vary with:

• increasing temperature?
• increasing salinity?
• decreasing salinity?
  
  

Send to Don Reed
Department of Geology
San Jose State University

#9 Which of the following parameters affects sea water density?

a) Temperature.
b) Salinity.
c) Pressure.
d) Water depth.
e) All of the above.