Nowhere in nature can absolutely
clear water be found. Even rainwater contains dissolved minerals. For example,
30kg of rainwater contains about 1g of solid substances, whereas 1kg of
seawater contains about 35g different salts. Salinity is a very important
characteristic and it directly influences the viability of the organisms
living in the water. The
total amount of dissolved salts in the oceans is 48 quadrillion tonnes.
Most of it is NaCl (77.8%), to which the saline taste of seawater is due.
Magnesium chloride accounts for 10.9% and it gives the water a specific
bitterish taste. Then come sulfates (10.8%), carbonates (.5%), etc.
The overall amount of salts may vary in different regions of the Ocean
but the composition and the percentile content remain the same. The average
ocean salinity is 35% and increases starting at the Equator and reaching
1520º latitude, then it decreases towards the poles. This phenomenon
can be explained with the distribution of water vapors and also with the
saline waters transported from the tropical zones by warm currents. Salinity
in the northern part of the Indian Ocean changes according to the season.
Seas have greater salinity than oceans. The Dead Sea has the greatest salinity
of all (256%), and the smallest is that of the Baltic Sea (57%).
The presence of more than
44 chemical elements has been established so far. Besides the popular oxygen,
hydrogen, chlorine, potassium, magnesium, sulfur, calcium, and iodine,
seawater also contains Al, Cu, Zn, Tn, Pb, Au, Ag, U, Mn, Hg, etc.
There are many gases dissolved
in seawater, the largest amount being contributed by oxygen, nitrogen,
and carbon dioxide. Water dissolves oxygen better than nitrogen. That is
why the ratio N:O which is 4:1 on dry land, changes to 2:1 under water.
It is interesting to note
that 1liter of air weighs .013N and 1liter of water weighs 10N, that is
770 times heavier than air. The relative weight of seawater depends on
the density and temperature of the water. Density itself, though insignificantly,
depends on temperature. That is why at 20ºC the density of the water
is lower by .2% than it is at 4ºC. Pure distilled water has a relative
weight of 1 at a temperature of 4ºC, that is, 1 cm3
of water weighs 1g. Seawater is heavier than fresh water by 2.53% because
of the greater amount of salts dissolved in it; its relative weight is
1.025. It may be concluded that a diver weighs less in seawater than in
fresh water. Relative weight is important for determining buoyancy.
Just like any other liquid,
water practically does not shrink. That is why its density almost does
not change at different depths. At a pressure of 500at water shrinks by
1/47,000,000 of its volume. If it did not shrink at all, however,
the sea level would rise by 30m. Water resistance is greatest in the surface
layer. Therefore, less effort is needed for swimming in that layer.
The relative transparency
of seawater is determined by the average depth, at which a white disc of
a 30cm diameter is no longer seen. Greatest transparency has the Sargatian
Sea (66.5m), second greatest transparency have the Syrian coasts of the
Mediterranean Sea. Least transparent is the North Sea (the British Channel)
Temperature, Heat Capacity and Heat
The heat capacity of seawater
is 3134 times greater than that of air. Water has insignificant heat conductivity.
That is why distribution of heat to greater depths is very slow and is
mainly achieved through convection.
The highest temperature of
water is registered to occur between 3 and 4 p.m., and the lowest a couple
of hours after sunrise. There are three temperature layers of seawater:
surface layer (epilymnion), intermediate layer (metalymnion), and deepest
layer (hypolymnion). The thickness of the former two layers varies with
the weather, season, and currents. The temperature of the surface layer
is almost constant, being between 19 and 25ºC in the summer. As the
deepest layer begins, temperature drops by a few more degrees and it remains
constant thereafter (79ºC). That is the temperature of sea depths
and it does not depend on the season.
Water motion constitutes
sea currents and waves. The reason for the formation of currents might
be the different density of water, constant winds, etc. Ocean currents
are usually caused by constant winds, whereas local ones are mainly due
to the character of coastlines. According to he direction of their flow,
currents can be classified as vertical or horizontal. There are three main
types of waves: wind waves, standing waves, and seismic waves.
is the main reason for the formation of waves. The process of wave formation
can be divide into different stages. When the speed of wind is less than
1m/s, air motion does not affect the surface of the water. If wind intensifies,
these rows of waves become irregular and peaks appear, which are due to
the different pressure at the front and at the back of the wave. At a greater
speed of wind large waves are formed, running in parallel rows. Th largest
waves reaching hundreds of meters continue even when the wind has ceased.
They create the so-called dead drift.
Waves change their form when
they reach shallow regions. When the depth becomes equal to the height
of the wave, the water particles no longer move in a circle: their orbit
becomes elliptical. The length of the waves decreases and the height increases.
The front slope of the wave becomes vertical, the top is inclined forward,
then it falls and eventually destroys the wave. This phenomenon is called
a surf. Its force may reach up to 38 tonnes/m2.
are formed when the level of the water rises at one coast and in the same
time drops at the other. A sudden decrease of atmospheric pressure at one
of the coasts, appearance of strong wind or heavy rain can all be the causes
for standing waves. The fluctuation of the sea level may reach 80cm, which
is dangerous for vessels at the harbors.
are formed because of underwater earthquakes. A vessel that is nest the
site of the earthquake experiences a hydraulic blow which is why old maps
frequently contain non-existent reefs. Seismic waves are often present
in the Hawaii region where they have the special name zunami. Such waves
are formed in the Pacific Ocean, the Mediterranean, the Caribbean Sea,
and the Malaya Archipelago as well. Sometimes, these waves reach the height
of 35m and are dangerous not only for the ships but also for the native
population because of their destructive power.
The change of water density,
resulting from changes of temperature, salinity, and pressure has no practical
importance to diving. Even though, water is a dense medium and creates
significant difficulties for a divers movements. He or she cannot walk
or turn as fast as in the air. While working under water, divers must choose
positions and movements that create least resistance, e.g. walking sidewards
being slightly bent forward. The use of tools is also hindered. For
example, the use of a hammer is much more difficult under water than it
is in the air. As a result, divers quickly get tired. That is why the work
that is to be performed under water should be organized so as to facilitate
the diver by minimizing unnecessary movements and providing possible help
from the surface. Rapid currents additionally impede the accomplishment
of underwater work. Mire, too, can create considerable difficulties for
divers. Even the execution of simplest types of work becomes complicated
and requires dexterity, resistance, and fitness. That is why rigorous physical
preparation is crucial.