Page 220 - NS-2 Textbook

OCEANOGRAPHY 215

contain silt or mud near shore Of the mouths of rivers. 200 meters). Below the twilight zone is the area of total
The colors change depending on whether the day is and eternal darkness called the dark ZOl1e. TIus is a very
cloudy or sunny. Actually, the water itself has no color. thick layer in which no plants grow and animal life con-
What we see as its color is caused by the reflection of the sists of carnivores and deh'itus (particles of plant or ani-
sky or scattering of light in the water. Some ocean bodies mal matter) feeders. This area has no light at all except
have been given theil' names because they are colored at that which is created by an object or animal itself.
times by plant or aninlallife in them, or by colored silt
flowing into them. TI1e Red Sea, for instance, is so named
because of the red phytoplankton in the water. The Yel-
WAVES
low Sea is so named because of the yellow clay silt car-
ried into it by the rivers of northern China. Waves in a liquid are caused by any energy source that
We know that the main source of energy for life is the dishtrbs the water surface. The energy transnutted by
Stm. Its radiant energy reaches us after traveling about ocean waves can be very great. Blocks of stone weighing
eight minutes and some 93 million miles through the void more than 1,300 tons have been moved by ·waves.
of space. Stmlight consists of a range or spectrum of dif- Any dishrrbance, even a raindrop in a puddle, will
ferent wavelengths of energy. These include infrared, vis- create ripples of tiny waves. The tsunami 'waves caused
ible, ultraviolet, and x-rays. The different colors of the by an exploding undersea volcano or an earthquake can
visible spectrum can be seen by using a prism, or they can travel all the way across the ocean. Wind, ho\,vever, is the
be seen in a rainbow. The atmosphere serves as a giant fil- most common cause of ordinary sea waves. Sailors often
ter, keeping out most of the dangerous ultraviolet (above call ,vind-driven waves I1sea," or the state of the sea. A
violet) rays. Much infrared light is absorbed by the water swell is a long, smooth wave coming from a distant storm
vapor and carbon dioxide in the atmosphere. This atmos- center. Swells may indicate an approaching storm, and
pheric blartket acts like a big greenhouse, keeping in the they are common in advance of hurricanes.
warmth that helps to sustain life on Earth. As the wind begins to blow over a smooth ocean sur-
Some of the visible light striking the surface of the face, a certain amotult of wind energy is imparted by fric-
ocean is reflected back, but some goes down into the tion and pressure on the underlying sea smface, causing
water. As it descends, it changes in quality and quantity. waves to be formed. Wave height depends on three main
The water acts as a filter also, gradually scattering vari- factors: wind speed, duration of the wind, and the length
ous wavelengths of light, starting at the red end of the of the fetch (the distance the wind blows over the water).
spectrum. Therefore, the deeper one goes into the wate,; The longer the fetch and the stronger the wind, the
the greater the amount of blue light. The color of the wa- higher and longer the wave will be. At about 13 knots of
tery world below about 90 feet (30 meters) is a dark zone wind, whitecaps will begin to form. Sea waves 12 to 15
of blues, violets, grays, and blacks, and nothing else. The feet high are not tulcommon during a strong sea. Waves
depth to which light penetrates varies according to the 25 to 30 feet high or more form during severe storms or
position of the Stul and the turbidity (suspended materi- hurricanes.
als) in the water. Waves in excess of 50 feet in height are very tulusual,
The oceans can be divided into three environments although a few are occasionally reported. Years ago the
on the basis of light. The topmost is the lighted ZOlle, Navy tartker USSRamapo reported a 114-footwave. What
which ranges in depth from a maximum of about 330 feet may have been seen and measured by eye in that inci-
(100 meters) in the open, clear sea to about 3 feet (1 dent, howeve1~ could have been the spray associated
meter) in muddy estuaries. Next is the twilight ZOlle, with a large, unstable wave. Another huge \i\Tave that
which is very dark violet, with only the slightest light capsized a fishing vessel -was inunortalized in the sum-
penetration. No effective plant production takes place mer 2000 movie Tile Perfect 5t01'111. One of the major diffi-
here; this layer ranges from about 260 to 655 feet (80 to culties in estinlating wave height is the lack of reference
points. There is also another factor: the perception of the
observer. For exanlple, a small frigate operating with an
aircraft carrier -will frequently report larger ,v-aves than
those reported by observers on the carrier.
The storm area of the sea over \i\Thich wind blows to
Violet Blue create waves 1nay extend over more than 2,000 square
Prism
miles on the open ocean. The larger the wave, the more eas-
ily the wind can add more energy to its crest. There is a
Light limit to a wave's growth, however. At the edge of the
source
fetch-that is, where the '\-vind effect on the -waves ceases-
The visible spectrum of sunlight. the waves gradually change into smooth swells.