Page 23 - sst_class_07

Our Earth is called the ‘watery planet'. 71% of the earth’s surface is covered with water while only
29% is the land area. The water is found in oceans, seas, lakes, rivers, etc. The domains of water is
called the hydrosphere.

WatEr cyclE

Water continually changes its form. The heat of
the sun evaporates water which changes its form
and becomes water vapour. This cools down and
condenses to form clouds. This causes rain and
the water returns to the earth falling on land or
sea etc. Water that falls on land collects in rivers
which carry it back to the oceans. This never-

ending movement of water is called the water cycle or the hydrological cycle. The amount
of water available varies from place to place and from season to season. But the general
pattern is almost the same. However, there are certain human activities which effect the
water cycle. Cutting down of trees, polluting water bodies and wasting water can affect
the water cycle. Man must make efforts to conserve this precious resource.

The World Water Day is celebrated annually on March 22. It was established by the United Nations
in the year 1992. It focuses attention on the world’s water crisis as well as the solutions to address it.

ocEaN WatEr circulatioN

Like the circulation of air, the ocean water also circulates. The rays of the sun do not fall
equally all over the earth. They are maximum at the equator and minimum at poles. This
leads to large temperature slope between the equator and the poles.

As you have read above, the quantity of water on earth always remains the same. It
constantly changes form and circulates between oceans, atmosphere and land. The
movements that occur in oceans can be broadly categorised into three as given below :

• Waves • Tides • Currents

factors

The factors affecting the movements of ocean water are as follows :
• Waves are formed as a result of winds, earthquakes, volcanic eruptions and underwater
landslides.

• Tides are caused by the strong gravitational pull exerted by the sun and the moon on
the earth’s surface.
• Another factor affecting the movement of ocean water is temperature. Ocean currents
are formed because of the temperature differences between the ocean water present
at different latitudes.

Social Studies-7 23