Underwater
Underwater, sometimes shortened as U/W, is a term describing the realm below the surface of
water where the water exists in a natural feature such as an
ocean,
sea,
lake,
pond, or
river. Three quarters of the planet earth is covered by water. A majority of the planet's solid surface is abyssal plain, at depths between 4000 and 5500 m below the surface of the oceans. The solid surface location on the planet closest to the center of the orb is the
Challenger Deep, located in the
Mariana Trench at a depth of 10,924 m under the sea.
Although a number of
human activities are conducted underwater—such as
scuba diving for
work or
recreation, or even underwater warfare with
submarines—this very extensive environment
Encyclopedia
Underwater, sometimes shortened as
U/W, is a term describing the realm below the surface of
water where the water exists in a natural feature such as an
ocean,
sea,
lake,
pond, or
river. Three quarters of the planet earth is covered by water. A majority of the planet's solid surface is abyssal plain, at depths between 4000 and 5500 m below the surface of the oceans. The solid surface location on the planet closest to the center of the orb is the
Challenger Deep, located in the
Mariana Trench at a depth of 10,924 m under the sea.
Although a number of
human activities are conducted underwater—such as
scuba diving for
work or
recreation, or even underwater warfare with
submarines—this very extensive environment on planet
earth is hostile to humans in many ways and therefore little explored.
An immediate obstacle to human activity under water is the fact that human
lungs cannot naturally function in this environment. Unlike the
gills of
fish, human
lungs are adapted to the exchange of gases at atmospheric
pressure, not
liquids. Aside from simply having insufficient musculature to rapidly move water in and out of the lungs, a more significant problem for all air breathing animals, such as mammals and
birds, is that water contains so little dissolved
oxygen compared with atmospheric air. Air is around 21% O
2; water typically is less than 0.001% dissolved oxygen.
The density of water also causes problems that increase dramatically with depth. The
air pressure at the planet surface is 101 kilopascals . A comparable water pressure occurs at a depth of only 10 m Thus, at about 10 m below the surface the water exerts twice the pressure on the body as air at surface level.
For solid objects like our bones and muscles, this added pressure is not much of a problem; but it is a problem for any air-filled spaces like the
mouth,
ears, paranasal sinuses and lungs. This is because the air in those spaces reduces in volume when under pressure and so does not provide those spaces with support from the higher outside pressure. Even at a depth of 8 feet underwater, an inability to equalize air pressure in the middle ear with outside water pressure can cause pain, and the tympanic membrane can rupture at depths under 10 ft . The danger of pressure damage is greatest in shallow water because the rate of pressure change is greatest at the surface of the water. For example the pressure increase between the surface and 10 m is 100% , but the pressure increase from 30 m to 40 m is only 25% .
Any object immersed in water is provided with a
buoyant force that counters the force of
gravity, appearing to make the object less heavy. If the overall density of the object exceeds the density of water, the object sinks. If the overall density is less than the density of water, the object rises until it floats on the surface.
With increasing depth underwater,
sunlight is absorbed, and the amount of visible
light diminishes. Because absorption is greater for long
wavelengths than for short wavelengths , the
colour spectrum is rapidly altered with increasing depth. White objects at the surface appear bluish underwater, and red objects appear dark, even black. Although light penetration will be less if water is
turbid, in the very clear water of the open ocean less than 25% of the surface light reaches a depth of 10 m . At 100 m the light present from the sun is about 0.5% of that at the surface.
The
euphotic depth is the depth at which light intensity falls to 1% of the value at the surface. This depth is dependent upon water clarity, being only a few meters underwater in a turbid estuary, but may reach 200 meters in the open ocean. At the euphotic depth, plants have no net energy gain from photosynthesis and thus cannot grow.
At depths greater than a few hundred meters, the sun has little effect on water temperature, because the sun's energy has been absorbed by water at the surface. In the great depths of the ocean the water temperature is very cold. In fact, 75% of the water in the world ocean has a temperature between 0 °C and 2 °C.
Water conducts heat around twenty five times more efficiently than air. Hypothermia, a potentially fatal condition, occurs when the human body's core temperature falls below 35 °C. Insulating the body's warmth from water is the main purpose of
diving suits and exposure suits when used in water temperatures below 25 °C.
Sound is transmitted about four times faster in water than it is in air . The human brain can determine the direction of sound in air by detecting small differences in the time it takes for sound waves in air to reach each of the two ears. For these reasons divers find it difficult to determine the direction of sound underwater.
See also
- Timeline of underwater technology
References
- Dueker, C. W. 1970, Medical aspects of sport diving. A.S. Barnes and Co., New York. 232 pp.
- – Oceanography notes at San Jose State University.
