Physical Properties of Water. Heat Energy and Water Density Structure of Seawater Optical (Light) Properties Sound in Seawater . Density & Temperature. Taking heat energy away →.
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Taking heat energy away →
Heat energy causes water molecules to vibrate greatly and space themselves out; also few H-bonds that break and reform very rapidly.
Maximum density at 4ºC when water molecules vibrate less and pack together the tightest.
Density decreases at < 4ºC as more and more H-bonds form.
The dramatic drop in density occurs when the perfect crystal forms; complete H-bonding; ice. This requires removal of a lot of heat without a temperature change.
Perfect water crystal
Liquid water temperature responds in a constant manner as heat is added or removed; we call this sensible heat as we can make sense of heat change by measuring temperature change with a thermometer.
However, no temperature change is seen upon removing the 80cal/g from liquid water at 0ºC to get ice at 0ºC. This is called the latent heat of fusion (freezing). There is also a latent heat of evaporation of 585 cal/g to get water to go to gas, i.e. energy needed to break all H-bonds.
Temperature is the dominant factor in controlling density in the ocean, particularly when salinity does not vary dramatically.
Surface water (<200 m) absorbs the majority of solar radiation and converts it to heat, slowly raising surface water temperature.
A distinct layer of warmer lower density seawater forms at the surface zone when there is ample sunlight and calm conditions.
The transition depths between warm surface zone and cold denser deep water is called the thermocline (thermo = heat; cline = change).
Thermocline characteristics vary with latitude due to differences in solar radiation inputs with latitude.
Weak to no thermocline is seen at the poles due to cold surface water being mixed by sinking and mixing by storms.
When a thermocline does exists, we refer to the water as being stratified (layered).
Seawater can also be stratified due to density differences related to salinity. The depth of rapid salinity change is called a halocline (halo = salt).
The combination of temperature and salinity determines overall density; the depth of rapid density change is called a pycnocline (pycno = density).
Largely due to the temperature influence on density, pycnoclines are greatest (deepest) at the equator, and shallow to non-existent above 60º North of South latitude.
Winds generate waves to keep the thin surface zone mixed to the top of the thermocline. Deep water is a massive volume (80%) and very cold (2-5ºC)
Light is absorbed and scattered as it passes through water, so less light reaches greater depths.
Different wavelengths of light absorb differently in pure water. Blues and greens penetrate deepest and reds the least. Why the “deep blue sea”?
Particles and colored dissolved organic matter and further influence how quickly different colors of light are absorbed in natural waters.
Light greatly influences life!
Sound is an energy wave produced by rapid pressure change. Like light it is lost over distance in seawater due to spreading, scatter, and absorption, but sound can travel much farther than any light in seawater, particularly low frequency sound waves.
Some marine mammals use sound to “see”, a process called echolocation, much like Sound Navigation and Ranging (SONAR) technology used by submarines and ships.
Average speed of sound in seawater is 5-times faster than in air (1,500 m/s).
Increases in temperature or pressure will increase sound speed in seawater.
Sound traveling through water of different temperature or pressure will bend; refraction.
A depth of minimum sound velocity results from these properties.
Best depth for listening to others!