FAQ-OceanProperties

1. What is seawater composed of?
2. What are “practical salinity units” or psu?

The Practical Salinity Scale describes the conductivity ratio of a seawater sample to a standard KCl solution. Since it is a ratio it is dimensionless (NASA Science Glossary)

3. The thermodynamic equation of state for seawater gives its density as a function of temperature, pressure, and psu. Would this formula give the same value as an independent, in situ density measurement?

No, the density calculated using the equation of state would not be the same value as an independent, in situ measurement, although the values could potentially be reasonably close. Using the standard, conventional method of measuring in situ density, a measured mass of a given sample of seawater is divided by its (known) volume. This measurement is typically performed in standard atmospheric conditions in a laboratory. The thermodynamic equation of state, however, takes into account the temperature of the water sample and the pressure of the water at the place in the water column where the sample was taken from. Hence, the equation of state density could be considered an “adjusted” density value.

4. How does the density of water depend on temperature, pressure, and salinity?

The density of water increases with decreasing temperature until about 4oC, which is its maximum density. When the temperature is decreased further and water becomes ice, the density decreases. Density increases with increasing pressure, and increases with increase in salinity.

5. Compare the densities of fresh water, typical seawater, and fresh ice.

The density of seawater is around 1.025 g/ml. The density of fresh water is about 1.0 g/ml. The density of ice is 0.92 g/ml. This explains why ice usually floats in a cup of water. It also explains why it is easier to float on top of the water at the Dead Sea. Your body is made up of mostly fresh water which is less dense than seawater. (http://www.windows.ucar.edu/tour/link=/earth/Water/density.html&edu=high)(http://www.elmhurst.edu/~chm/vchembook/122Adensityice.html).

6. Compare the densities at the freezing points for seawater and freshwater.
7. Compare the speeds of sound for air, water, the crust, mantle, and earth’s core.
8. Explain why there are sound channels in the atmosphere, ocean, and terrasphere.
9. What are the factors that cause attenuation of sound in the hydrosphere?

Attenuation of sound in the hydrosphere is due to scattering and absorption caused by the inhomogeneity of water due to fluctuations of density, salinity, temperature, and pressure. Attenuation is also caused by the conversion of energy into heat or chemical energy.

10.** How does attenuation depend on the frequency of the sound waves?**

Lakes are significantly more eutrophic (rich in nutrients) than the open ocean. This results in a higher amount of primary production in lakes than in the open ocean and consequently more light is absorbed near the surface in lakes. The open ocean often looks bluer or clearer than lakes because it is less rich in nutrients and primary production, so light tends to pass through the water column farther than in lakes.

11. What factors cause attenuation of light in the atmosphere and hydrosphere?
12. Compare the attenuation of light in lakes with the open ocean?
13. When looking across the hydrosphere-air interface objects generally are not where they appear to be. Write a paragraph explanation of this for your parents, with some simple rules for locating objects from either side.
14. Compare the heat capacities of water, standard seawater, and air.
15. What are some climate implications of the disparity in heat capacities of the ocean and atmosphere?

It takes a lot less energy to heat up the atmosphere than it does the ocean. It is expected that the ocean temperature will raise somewhat over the next 100 years. However, since it is easier to heat the air it is probable that the average air temperature will raise more. The opposite effect could happen if the ocean temperatures dropped.

16. The diffusivity and thermal conductivity of seawater are different. What are typical values and how might this disparity impact the stratification of the ocean?

page revision: 7, last edited: 08 Sep 2008 14:06