learn about water, water cycles, tsunamis, fresh and saline water, high and low tides and many more valuable knowledge.

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About 71% of the surface of the Earth is covered
with water. No wonder then that our Earth is often
referred to as a watery planet. All the water
bodies such as oceans, seas, rivers, and lakes,
etc; together make up the hydrosphere.

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The water cycle or the
hydrological cycle is the
continuous change of water
from one physical state to
another and its circulation
from the surface of the Earth
to the atmosphere and back.

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Ocean water evaporates due to Sun’s
heat and reaches the atmosphere as
water vapor. In the upper layers of air,
cooling and condensation take place
and the water vapor changes into
water droplets. These grow in size and
eventually fall down as precipitation.
Some of the water falling on land,
seeps down to Earth, while most of it
flow into rivers which again join the
oceans. Evaporation starts all over
again and the cycle continues.

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water contains very little dissolved
salt is called fresh water. Although
¾ of the Earth is covered by water,
very little of it is fresh. 97% water
on Earth are oceans and seas. This
is saline water and is not suitable
for human consumption. Rest 3% is
fresh water, out of it 2% is in the
form of ice caps and glaciers,
which cannot be reached by us.
Thus, only 1% of the earth’s fresh
water is easily available and fit for
hum to drink. It is found as waters
in rivers & lakes as groundwater,
water vapor in the atmosphere.

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Water containing a large amount of
dissolved salt in it is called saline water.
Example, the oceans and seas. Sodium
chloride or common salt, magnesium
chloride, magnesium sulphate and
calcium sulphate are the main ones. The
amount of salt in the ocean water is
called salinity. The salinity of ocean water
is on an average of 35 per thousand. This
means that in 1000 grams of ocean water
there are 35 grams of dissolved salts.

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1. Rate of evaporation
2. Amount of precipitation
3. Amount of fresh water
that is added to the
ocean.

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The temperature of ocean
water is not the same
everywhere and at all times
of the year. Oceans are the
warmest near the equator
and becomes cooler towards
the poles.

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• Oceans are the prime source of humidity and
precipitation.
• They moderate the climate of the coastal regions.
• Marine life found in the ocean waters provides a
source of livelihood to many people.
• Salts dissolved in the ocean water and mineral
deposits on the ocean floor are economically
important to us.
• Oceanic waves and tides can be harnessed to
generate electricity.
• Oceans serve as means of transport.

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The waters of the ocean are constantly in motion.
They move horizontally as well as vertically. The
movements of the ocean waters are of 2 types:
waves and currents.
• Differences in temperature
• Differences in salinity
• Prevailing winds
• Variation in the density of sea water

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• Waves are the alternate up and down
movement of the water particles
caused by wind.
• Particles of the water are pushed
down by the wind and they come up
again.
• The upper part of the wave is called
crest, the lower part is called trough.
The vertical distance between the
crest and trough is called wave
height and the horizontal distance
between 2 crests or 2 troughs is
called wave length.
• A light breeze produces gentle
waves, while violent and stormy
winds create huge waves which often
cause widespread destruction.

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• The term tsunami comes from the
Japanese word tsu meaning harbor
and nami meaning wave.
• Traditionally, tsunami is referred to
as a huge tidal wave
• They are caused by earthquakes,
both on land and underwater,
volcanic eruptions on the ocean floor
and underwater explosions and
landslides.
• They cause massive destruction.

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• Tides are the periodic rise and fall of
water level in the sea which occur at
regular intervals, twice a day.
• When the level of sea water rises, its
high tide, when the level fall its low tide.
• Tides are caused by the gravitational
pull of the moon and sun.
• The moon and sun, exert their combines
gravitational pull in the same direction,
resulting in extra high tides. These are
known as spring tides.
• They nullify each other’s gravitational
pull resulting in a very low tides known
as neap tides.

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• Ocean currents are streams of ocean water
continuously flowing in a definite directions.
• They are caused by the planetary winds on the surface
of oceans and differences in temperature, salinity and
density of waters.
• Ocean current move clock-wise in the Northern
Hemisphere, anti-clockwise in the Southern
Hemisphere.
• They move at speeds of 3 to 10 kmph.
• Slow moving masses of water in the ocean are called
drifts.
• The ocean currents that flow from the equatorial region
to the polar regions are warm currents, e.g, Gulf stream,
North Atlantic Drift and kuro shio.
• Ocean currents that flow from the polar region to the
equatorial region are cold currents, e.g, Benguela
current, Peru current and oya shio.

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 Warm ocean currents raise the
temperature of the coasts while cold
ocean currents lower the temperature.
 Ocean currents influence navigation.
Ships sailing in the direction of the
current will sail faster and save time
and fuel.
 Wind that blows over warm currents
are moist and bring rainfall to the
coastal regions. Wind that blows over
cold currents are dry and hence they
don’t cause any rain.