CLASS-VII GEOGRAPHY
CHAPTER 5. WATER
Understanding
water cycle:
The
sun’s heat causes evaporation of water vapour. When the water vapour cools
down, it condenses and forms clouds. From there it may fall on the land or sea
in the form of rain, snow or sleet. The process by which water continually
changes its form and circulates between oceans, atmosphere and land is known as
the water cycle
•
Terrarium:
It is an artificial enclosure for keeping small house plants.
•
The major sources of fresh water are the
rivers, ponds, springs and glaciers. The ocean bodies and the seas contain
salty water.
•
March 22 is celebrated as World Water
Day.
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Salinity is the amount of salt in grams
present in 1000 grams of water. The average salinity of the oceans is 35 parts
per thousand.
Distribution
of water bodies
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Oceans : 97.3
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Ice-caps : 02.0
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Ground water : 0.68
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Fresh water lakes : 0.009
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Inland seas and salt lakes : 0.009
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Atmosphere : 0.0019
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Rivers : 0.0001
OCEAN
CIRCULATION
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Unlike the calm waters of ponds and lakes,
ocean water keeps moving continuously. It is never still. The movements that
occur in oceans can be broadly categorized as: waves, tides and currents.
Waves
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When the water on the surface of the ocean
rises and falls alternately, they are called waves.
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During a storm, the winds blowing at very
high speed form huge waves.
•
These may cause tremendous destruction.
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Tsunami An earthquake, a volcanic eruption
or underwater landslides can shift large amounts of ocean water. As a result a
huge tidal wave called tsunami, that may be as high as 15m., is formed.
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The largest tsunami ever measured was 150m.
high.
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These waves travel at a speed of more than 700
km. per hour.
•
The tsunami of 2004 caused wide spread
damage in the coastal areas of India. The Indira point in the Andaman and
Nicobar islands got submerged after the tsunami.
Tides
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The rhythmic rise and fall of ocean water
twice in a day is called a tide.
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It is high tide when water covers much of
the shore by rising to its highest level.
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It is low tide when water falls to its
lowest level and recedes from the shore.
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The strong gravitational pull exerted by
the sun and the moon on the earth’s surface causes the tides. The water of the
earth closer to the moon gets pulled under the influence of the moon’s
gravitational force and causes high tide.
•
During the full moon and new moon days, the
sun, the moon and the earth are in the same line and the tides are highest.
These tides are called spring tides.
•
But when the moon is in its first and last
quarter, the ocean waters get drawn in diagonally opposite directions by the
gravitational pull of sun and earth resulting in low tides. These tides are
called neap tides.
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High tides help in navigation. They raise
the water level close to the shores. This helps the ships to arrive at the
harbour more easily.
•
The high tides also help in fishing. Many
more fish come closer to the shore during the high tide. This enables fishermen
to get a plentiful catch.
•
The rise and fall of water due to tides is
being used to generate electricity in some places.
OCEAN
CURRENTS
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Ocean currents are streams of water flowing
constantly on the ocean surface in definite directions.
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The ocean currents may be warm or cold.
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Generally, the warm ocean currents
originate near the equator and move towards the poles.
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The cold currents carry water from polar or
higher latitudes to tropical or lower latitudes.
•
The Labrador Ocean current is cold current
while the Gulf Stream is a warm current.
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The ocean current influence
the temperature conditions of the area.
•
Warm currents bring about warm temperature
over land surface.
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The areas where the warm and cold currents
meet provide the best fishing grounds of the world. Seas around Japan and the
eastern coast of North America are such examples.
•
The areas where a warm and cold current
meet also experience foggy weather making it difficult for navigation.
………The
end………
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