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Topic: Properties of the Ocean

Theme: Measurement

Key Concepts:


  • Bathymetry is the measurement of depth in the ocean in order to delineate (trace the outline of) the submarine (under the sea) topography.
    • From the sinking of the Titanic in 1912 until the launch in 1978 of SEASAT satellite, bathymetric measurements were primarily made using echo sounders (sonar).
    • Now, satellites can observe seafloor topography by very precise radar measurement of seasurface elevation.
  • Surface measurements, usually made from ships and buoys, are extremely important for ocean science, but are very limited in space and time.
  • Earth orbiting satellites have revolutionized our ability to collect ocean data.


  • Pressure, force per unit area, increases with depth as the weight of overlying water accumulates.
  • The deepest part of the ocean is 11 kilometers (6.8 miles) deep. At that depth, pressure is almost 1,000 atmosphere (1,000 kilograms per square-centimeter or 14,700 pounds per square inch).


  • Accurate measures of salinity cannot be obtained by simply evaporating the water and weighing the residue. Some salt when heated (to cause evaporation) decompose and for gases and solid compounds not originally present in the water sample. Because of seawater's constancy of composition, we need to measure the concentration of only one major constituent--chlorine ions--to find the salinity of a water sample.
  • Water samples are collected from different depths using sampling bottles such as the Nansen and the Niskin.
  • Today's scientists use an electronic device called salinometer that measures the electrical conductivity of seawater.
  • A refractometer, an optical device that compares the degree to which light is bent by a sample of seawater to the degree of bending for water of known salinity, is a quick way to measure salinity in the field. It is not as accurate as a salinometer.


  • Temperature is measured in degrees. One degree of Celsius = 1.8 degrees Fahrenheit.
  • Celsius degrees are more important in science because they are based on two of pure water's most significant properties: its freezing point (0 degrees Celsius) and its boiling point (100 degrees Celsius).
  • Most substances become denser (weigh more per unit of volume) as they get colder.
  • Satellites measure sea surface temperature by monitoring thermal infrared wavelengths over the ocean.


  • Density is the expression of the relative heaviness of a substance.
  • Density is defined as the mass per unit volume, usually expressed in grams per cubic centimeter (g/cm3). The density of "pure" water is 1 g/cm3.
  • Most substances become denser (weigh more per unit of volume) as they get colder. Seawater's density increases with increasing salinity. Ocean water tends to form into stable layers, with the heaviest water at the bottom--a form of density stratification. The density zones are:
    • Surface zone or mixed layer (2% of ocean water)
    • Pycnocline--a combination of the thermocline & halocline (18% of ocean water)
    • Deep Zone (80% of ocean water)