Page 143 - SST Class 06
P. 143
The envelope of air is made up of a mixture of light and rises, creating lower pressure. Due to
various gases. The gases that form the mixture these differences, we have areas of low pressure
are nitrogen—78 percent, oxygen—21 percent and areas of high pressure. This difference in
and 1 per cent consists of argon, carbon dioxide, pressure makes the air move and when it moves
water vapour and other gases. Nitrogen required the air is called wind. Wind always moves from
for maintaining soil fertility, and plant and animal high-pressure areas to low-pressure areas in the
growth. Oxygen helps us to breathe and carbon same way that water moves from a higher level
dioxide helps plants to manufacture food. It also to a lower level. In this way, a balance is always
keeps the earth warm. Apart from these gases, maintained in the atmosphere.
there are dust particles and water vapour.
DO YOU KNOW?
The atmosphere also has another gas called
ozone, which forms a protective shield against It is feared that the global warming is being caused by
the use of CFCs (chlorofluorocarbons) used in
the harmful ultraviolet rays of the sun. If you
refrigerants, aerosols, spray cans, etc. Alternative
have been listening to the news or reading
chemicals ae now being used instead of CFCs.
newspapers, you would know that the thinning
ozone layer is causing great concern among
HUMANS AND THE ENVIRONMENT
scientists as exposure to the sun’s ultraviolet rays
The habitat is mainly formed of non-living
can cause health problems such as skin cancer. It
elements, i.e., air, land and water. This environ-
has been recently discovered that the hole in the
ment is called the physical environment. It is this
ozone layer over the southern hemisphere,
physical environment that creates the actual
especially above the continent of Australis, has
realm for living beings, i.e., the biological envi-
now stabilised. The Australians have become
ronment. The biological environment consists of
very particular about protecting their skin against
plants, animals, human beings and all other
the sun’s rays.
living beings. Both these environments are
interrelated and interdependent. If one changes,
DO YOU KNOW?
the other automatically undergoes changes as
The atmosphere protects us from the impact of
well.
millions of meteors that fall on the earth. Most of the
The interdependence of all the regions or realms
meteors get burnt out as they pass through the
of the earth is the most important fact of life. Life
atmosphere.
requires cooperation for coexistence. If the
ATMOSPHERIC PRESSURE balance of coexistence is disturbed the entire
The atmosphere is held in place by the earth’s cycle of life is affected. If nature and man can
exist in a balanced way the earth will remain a
force of gravity and its own weight. The weight of
beautiful planet but disruption of this balance by
air is felt closest to the surface of the earth or at
sea level. The entire mass of air exerts pressure human beings has resulted in problems. Our
on the surface of the earth. Air pressure changes greed and lack of concern for the earth has
due to two factors: height and temperature. The resulted in the pollution of the environment by
higher and warmer regions have lower pressure. reckless cutting of forests, pollution of rivers by
As one goes higher, the column of air gets industrial wastes, speeding up of soil erosion and
thinner. Thus, the air pressure drops. In warm destruction of marine life. Excessive carbon
regions when the air becomes hot it becomes emissions has led to an alarming rise in the level
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