NCERT Textbook: Air | Social Studies (SST) Class 7 PDF Download

 Page 1


Our earth is surrounded by a huge blanket of air
called atmosphere. All living beings on this earth
depend on the atmosphere for their survival. It
provides us the air we breathe and protects us from
the harmful effects of the sun’s rays. Without this
blanket of protection, we would be baked alive by the
heat of the sun during day and get frozen during
night. So it is this mass of air that has made the
temperature on the earth liveable.
COMPOSITION OF THE ATMOSPHERE
Do you know that the air we take in while breathing is
actually a mixture of many gases? Nitrogen and oxygen
are two gases which make up the bulk of the
atmosphere. Carbon
dioxide, helium,
ozone, argon and
hydrogen are found
in lesser quantities.
Apart from these
gases, tiny dust
particles are also
present in the air. The
pie chart gives you the
percentage of different
constituents of air (Fig. 4.1).
Nitrogen is the most
plentiful gas in the air. When
we inhale, we take some
amount of nitrogen into our
lungs and exhale it. But
plants need nitrogen for their
survival. They can not take
4
Air
Fig. 4.1: Constituents of Air
Do you know?
Carbon dioxide
released in the
atmosphere creates a
green house effect by
trapping the heat
radiated from the
earth. It is therefore
called a greenhouse
gas and without it the
earth would have been
too cold to live in.
However, when its
level in the
atmosphere increases
due to factory smoke
or car fumes, the heat
retained increases the
temperature of the
earth. This is called
global warming. This
rise in temperature
causes the snow in
coldest parts of the
world to melt. As a
result the sea level
rises, causing floods in
the coastal areas.
There may be drastic
changes in the climate
of a place leading to
extinction of some
plants and animals in
the long run.
Reprint 2024-25
Page 2


Our earth is surrounded by a huge blanket of air
called atmosphere. All living beings on this earth
depend on the atmosphere for their survival. It
provides us the air we breathe and protects us from
the harmful effects of the sun’s rays. Without this
blanket of protection, we would be baked alive by the
heat of the sun during day and get frozen during
night. So it is this mass of air that has made the
temperature on the earth liveable.
COMPOSITION OF THE ATMOSPHERE
Do you know that the air we take in while breathing is
actually a mixture of many gases? Nitrogen and oxygen
are two gases which make up the bulk of the
atmosphere. Carbon
dioxide, helium,
ozone, argon and
hydrogen are found
in lesser quantities.
Apart from these
gases, tiny dust
particles are also
present in the air. The
pie chart gives you the
percentage of different
constituents of air (Fig. 4.1).
Nitrogen is the most
plentiful gas in the air. When
we inhale, we take some
amount of nitrogen into our
lungs and exhale it. But
plants need nitrogen for their
survival. They can not take
4
Air
Fig. 4.1: Constituents of Air
Do you know?
Carbon dioxide
released in the
atmosphere creates a
green house effect by
trapping the heat
radiated from the
earth. It is therefore
called a greenhouse
gas and without it the
earth would have been
too cold to live in.
However, when its
level in the
atmosphere increases
due to factory smoke
or car fumes, the heat
retained increases the
temperature of the
earth. This is called
global warming. This
rise in temperature
causes the snow in
coldest parts of the
world to melt. As a
result the sea level
rises, causing floods in
the coastal areas.
There may be drastic
changes in the climate
of a place leading to
extinction of some
plants and animals in
the long run.
Reprint 2024-25
nitrogen directly from the air. Bacteria, that live in the
soil and roots of some plants, take nitrogen from the
air and change its form so that plants can use it.
Oxygen is the second most plentiful gas in the air.
Humans and animals take oxygen from the air as they
breathe. Green plants produce oxygen during
photosynthesis. In this way oxygen content in the air
remains constant. If we cut trees then this balance
gets disturbed.
Carbon dioxide is another important gas. Green
plants use carbon dioxide to make their food and
release oxygen. Humans or animals release carbon
dioxide. The amount of carbon dioxide released by
humans or animals seems to be equal to the amount
used by the plants which make a perfect balance.
However, the balance is upset by burning of fuels, such
as coal and oil. They add billions of tons of carbon
dioxide into the atmosphere each year. As a result, the
increased volume of carbon dioxide is affecting the
earth’s weather and climate.
Activity
Read and Ponder: Is global warming a serious issue in today’s world?
Do you know?
When air is heated, it
expands, becomes
lighter and goes up.
Cold air is denser and
heavy. That is why it
tends to sink down.
When hot air rises,
cold air from
surrounding area
rushes there to fill in
the gap. That is how
air circulation takes
place.
AIR 21
Reprint 2024-25
Page 3


Our earth is surrounded by a huge blanket of air
called atmosphere. All living beings on this earth
depend on the atmosphere for their survival. It
provides us the air we breathe and protects us from
the harmful effects of the sun’s rays. Without this
blanket of protection, we would be baked alive by the
heat of the sun during day and get frozen during
night. So it is this mass of air that has made the
temperature on the earth liveable.
COMPOSITION OF THE ATMOSPHERE
Do you know that the air we take in while breathing is
actually a mixture of many gases? Nitrogen and oxygen
are two gases which make up the bulk of the
atmosphere. Carbon
dioxide, helium,
ozone, argon and
hydrogen are found
in lesser quantities.
Apart from these
gases, tiny dust
particles are also
present in the air. The
pie chart gives you the
percentage of different
constituents of air (Fig. 4.1).
Nitrogen is the most
plentiful gas in the air. When
we inhale, we take some
amount of nitrogen into our
lungs and exhale it. But
plants need nitrogen for their
survival. They can not take
4
Air
Fig. 4.1: Constituents of Air
Do you know?
Carbon dioxide
released in the
atmosphere creates a
green house effect by
trapping the heat
radiated from the
earth. It is therefore
called a greenhouse
gas and without it the
earth would have been
too cold to live in.
However, when its
level in the
atmosphere increases
due to factory smoke
or car fumes, the heat
retained increases the
temperature of the
earth. This is called
global warming. This
rise in temperature
causes the snow in
coldest parts of the
world to melt. As a
result the sea level
rises, causing floods in
the coastal areas.
There may be drastic
changes in the climate
of a place leading to
extinction of some
plants and animals in
the long run.
Reprint 2024-25
nitrogen directly from the air. Bacteria, that live in the
soil and roots of some plants, take nitrogen from the
air and change its form so that plants can use it.
Oxygen is the second most plentiful gas in the air.
Humans and animals take oxygen from the air as they
breathe. Green plants produce oxygen during
photosynthesis. In this way oxygen content in the air
remains constant. If we cut trees then this balance
gets disturbed.
Carbon dioxide is another important gas. Green
plants use carbon dioxide to make their food and
release oxygen. Humans or animals release carbon
dioxide. The amount of carbon dioxide released by
humans or animals seems to be equal to the amount
used by the plants which make a perfect balance.
However, the balance is upset by burning of fuels, such
as coal and oil. They add billions of tons of carbon
dioxide into the atmosphere each year. As a result, the
increased volume of carbon dioxide is affecting the
earth’s weather and climate.
Activity
Read and Ponder: Is global warming a serious issue in today’s world?
Do you know?
When air is heated, it
expands, becomes
lighter and goes up.
Cold air is denser and
heavy. That is why it
tends to sink down.
When hot air rises,
cold air from
surrounding area
rushes there to fill in
the gap. That is how
air circulation takes
place.
AIR 21
Reprint 2024-25
22 OUR ENVIRONMENT
STRUCTURE OF THE ATMOSPHERE
Our atmosphere is divided into five layers starting from
the earth’s surface. These are Troposphere,
Stratosphere, Mesosphere, Thermosphere and
Exosphere (Fig. 4.2).
Troposphere: This layer is the most important layer of
the atmosphere. Its average height is 13 km. The air
we breathe exists here.
Almost all the weather
phenomena like rainfall,
fog and hailstorm occur in
this layer.
Stratosphere: Above the
troposphere lies the
stratosphere. It extends up
to a height of 50 km. This
layer is almost free from
clouds and associated
weather phenomenon,
making conditions most
ideal for flying aeroplanes.
One important feature of
stratosphere is that it
contains a layer of ozone
gas. We have just learnt
how it protects us from
the harmful effect of the
sun rays.
Mesosphere: This is the
third layer of the
atmosphere. It lies above
the stratosphere. It extends
up to the height of 80 km.
Meteorites burn up in
this layer on entering from
the space.
Thermosphere: In
thermosphere temperature
rises very rapidly with
increasing height.
Ionosphere is a part of this
layer. It extends between
Fig. 4.2: Layers of the Atmosphere
Reprint 2024-25
Page 4


Our earth is surrounded by a huge blanket of air
called atmosphere. All living beings on this earth
depend on the atmosphere for their survival. It
provides us the air we breathe and protects us from
the harmful effects of the sun’s rays. Without this
blanket of protection, we would be baked alive by the
heat of the sun during day and get frozen during
night. So it is this mass of air that has made the
temperature on the earth liveable.
COMPOSITION OF THE ATMOSPHERE
Do you know that the air we take in while breathing is
actually a mixture of many gases? Nitrogen and oxygen
are two gases which make up the bulk of the
atmosphere. Carbon
dioxide, helium,
ozone, argon and
hydrogen are found
in lesser quantities.
Apart from these
gases, tiny dust
particles are also
present in the air. The
pie chart gives you the
percentage of different
constituents of air (Fig. 4.1).
Nitrogen is the most
plentiful gas in the air. When
we inhale, we take some
amount of nitrogen into our
lungs and exhale it. But
plants need nitrogen for their
survival. They can not take
4
Air
Fig. 4.1: Constituents of Air
Do you know?
Carbon dioxide
released in the
atmosphere creates a
green house effect by
trapping the heat
radiated from the
earth. It is therefore
called a greenhouse
gas and without it the
earth would have been
too cold to live in.
However, when its
level in the
atmosphere increases
due to factory smoke
or car fumes, the heat
retained increases the
temperature of the
earth. This is called
global warming. This
rise in temperature
causes the snow in
coldest parts of the
world to melt. As a
result the sea level
rises, causing floods in
the coastal areas.
There may be drastic
changes in the climate
of a place leading to
extinction of some
plants and animals in
the long run.
Reprint 2024-25
nitrogen directly from the air. Bacteria, that live in the
soil and roots of some plants, take nitrogen from the
air and change its form so that plants can use it.
Oxygen is the second most plentiful gas in the air.
Humans and animals take oxygen from the air as they
breathe. Green plants produce oxygen during
photosynthesis. In this way oxygen content in the air
remains constant. If we cut trees then this balance
gets disturbed.
Carbon dioxide is another important gas. Green
plants use carbon dioxide to make their food and
release oxygen. Humans or animals release carbon
dioxide. The amount of carbon dioxide released by
humans or animals seems to be equal to the amount
used by the plants which make a perfect balance.
However, the balance is upset by burning of fuels, such
as coal and oil. They add billions of tons of carbon
dioxide into the atmosphere each year. As a result, the
increased volume of carbon dioxide is affecting the
earth’s weather and climate.
Activity
Read and Ponder: Is global warming a serious issue in today’s world?
Do you know?
When air is heated, it
expands, becomes
lighter and goes up.
Cold air is denser and
heavy. That is why it
tends to sink down.
When hot air rises,
cold air from
surrounding area
rushes there to fill in
the gap. That is how
air circulation takes
place.
AIR 21
Reprint 2024-25
22 OUR ENVIRONMENT
STRUCTURE OF THE ATMOSPHERE
Our atmosphere is divided into five layers starting from
the earth’s surface. These are Troposphere,
Stratosphere, Mesosphere, Thermosphere and
Exosphere (Fig. 4.2).
Troposphere: This layer is the most important layer of
the atmosphere. Its average height is 13 km. The air
we breathe exists here.
Almost all the weather
phenomena like rainfall,
fog and hailstorm occur in
this layer.
Stratosphere: Above the
troposphere lies the
stratosphere. It extends up
to a height of 50 km. This
layer is almost free from
clouds and associated
weather phenomenon,
making conditions most
ideal for flying aeroplanes.
One important feature of
stratosphere is that it
contains a layer of ozone
gas. We have just learnt
how it protects us from
the harmful effect of the
sun rays.
Mesosphere: This is the
third layer of the
atmosphere. It lies above
the stratosphere. It extends
up to the height of 80 km.
Meteorites burn up in
this layer on entering from
the space.
Thermosphere: In
thermosphere temperature
rises very rapidly with
increasing height.
Ionosphere is a part of this
layer. It extends between
Fig. 4.2: Layers of the Atmosphere
Reprint 2024-25
AIR 23
Do you know?
You will be surprised
to know that the
earth receives only 1
in 2,000,000,000
parts of the sun’s
energy.
80-400 km. This layer helps in radio transmission. In
fact, radio waves transmitted from the earth are reflected
back to the earth by this layer.
Exosphere: The upper most layer of the atmosphere
is known as exosphere. This layer has very thin air.
Light gases like helium and hydrogen float into the
space from here.
WEATHER AND CLIMATE
“Is it going to rain today?” “Will it be bright and sunny
today?” How many times have we heard this from
anxious cricket fans speculating the fate of a One Day
match? If we imagine our body to be a radio and the
mind its speaker, weather is something that fiddles
with its control knobs. Weather is this hour-to-hour,
day to day condition of the atmosphere. A hot or humid
weather may make one irritable. A pleasant, breezy
weather may make one cheerful and even plan for an
outing. Weather can change dramatically from day to
day. However, the average weather condition of a place
for a longer period of time represents the climate of a
place. Now do you understand why we have daily
weather forecasts.
Temperature
The temperature you feel everyday
is the temperataure of the
atmosphere. The degree of hotness
and coldness of the air is known
as temperature.
The temperature of the
atmosphere changes not only
between day and night but also
from season to season. Summers
are hotter than winters.
An important factor that
influences the distribution of
temperature is insolation.
Insolation is the incoming solar
energy intercepted by the earth.
The amount of insolation
decreases from the equator
towards the poles. Therefore, the
Let’s do
For ten days note
down weather report
from a local
newspaper and
observe the changes
occurring in the
weather.
Wind Vane: Shows the
direction of the wind
Rain Gauge: Measures
the amount of rainfall
Thermometer:
Measures
the temperature
Barometre: Measures
atmospheric pressure
Fig. 4.3: Weather Instruments
Reprint 2024-25
Page 5


Our earth is surrounded by a huge blanket of air
called atmosphere. All living beings on this earth
depend on the atmosphere for their survival. It
provides us the air we breathe and protects us from
the harmful effects of the sun’s rays. Without this
blanket of protection, we would be baked alive by the
heat of the sun during day and get frozen during
night. So it is this mass of air that has made the
temperature on the earth liveable.
COMPOSITION OF THE ATMOSPHERE
Do you know that the air we take in while breathing is
actually a mixture of many gases? Nitrogen and oxygen
are two gases which make up the bulk of the
atmosphere. Carbon
dioxide, helium,
ozone, argon and
hydrogen are found
in lesser quantities.
Apart from these
gases, tiny dust
particles are also
present in the air. The
pie chart gives you the
percentage of different
constituents of air (Fig. 4.1).
Nitrogen is the most
plentiful gas in the air. When
we inhale, we take some
amount of nitrogen into our
lungs and exhale it. But
plants need nitrogen for their
survival. They can not take
4
Air
Fig. 4.1: Constituents of Air
Do you know?
Carbon dioxide
released in the
atmosphere creates a
green house effect by
trapping the heat
radiated from the
earth. It is therefore
called a greenhouse
gas and without it the
earth would have been
too cold to live in.
However, when its
level in the
atmosphere increases
due to factory smoke
or car fumes, the heat
retained increases the
temperature of the
earth. This is called
global warming. This
rise in temperature
causes the snow in
coldest parts of the
world to melt. As a
result the sea level
rises, causing floods in
the coastal areas.
There may be drastic
changes in the climate
of a place leading to
extinction of some
plants and animals in
the long run.
Reprint 2024-25
nitrogen directly from the air. Bacteria, that live in the
soil and roots of some plants, take nitrogen from the
air and change its form so that plants can use it.
Oxygen is the second most plentiful gas in the air.
Humans and animals take oxygen from the air as they
breathe. Green plants produce oxygen during
photosynthesis. In this way oxygen content in the air
remains constant. If we cut trees then this balance
gets disturbed.
Carbon dioxide is another important gas. Green
plants use carbon dioxide to make their food and
release oxygen. Humans or animals release carbon
dioxide. The amount of carbon dioxide released by
humans or animals seems to be equal to the amount
used by the plants which make a perfect balance.
However, the balance is upset by burning of fuels, such
as coal and oil. They add billions of tons of carbon
dioxide into the atmosphere each year. As a result, the
increased volume of carbon dioxide is affecting the
earth’s weather and climate.
Activity
Read and Ponder: Is global warming a serious issue in today’s world?
Do you know?
When air is heated, it
expands, becomes
lighter and goes up.
Cold air is denser and
heavy. That is why it
tends to sink down.
When hot air rises,
cold air from
surrounding area
rushes there to fill in
the gap. That is how
air circulation takes
place.
AIR 21
Reprint 2024-25
22 OUR ENVIRONMENT
STRUCTURE OF THE ATMOSPHERE
Our atmosphere is divided into five layers starting from
the earth’s surface. These are Troposphere,
Stratosphere, Mesosphere, Thermosphere and
Exosphere (Fig. 4.2).
Troposphere: This layer is the most important layer of
the atmosphere. Its average height is 13 km. The air
we breathe exists here.
Almost all the weather
phenomena like rainfall,
fog and hailstorm occur in
this layer.
Stratosphere: Above the
troposphere lies the
stratosphere. It extends up
to a height of 50 km. This
layer is almost free from
clouds and associated
weather phenomenon,
making conditions most
ideal for flying aeroplanes.
One important feature of
stratosphere is that it
contains a layer of ozone
gas. We have just learnt
how it protects us from
the harmful effect of the
sun rays.
Mesosphere: This is the
third layer of the
atmosphere. It lies above
the stratosphere. It extends
up to the height of 80 km.
Meteorites burn up in
this layer on entering from
the space.
Thermosphere: In
thermosphere temperature
rises very rapidly with
increasing height.
Ionosphere is a part of this
layer. It extends between
Fig. 4.2: Layers of the Atmosphere
Reprint 2024-25
AIR 23
Do you know?
You will be surprised
to know that the
earth receives only 1
in 2,000,000,000
parts of the sun’s
energy.
80-400 km. This layer helps in radio transmission. In
fact, radio waves transmitted from the earth are reflected
back to the earth by this layer.
Exosphere: The upper most layer of the atmosphere
is known as exosphere. This layer has very thin air.
Light gases like helium and hydrogen float into the
space from here.
WEATHER AND CLIMATE
“Is it going to rain today?” “Will it be bright and sunny
today?” How many times have we heard this from
anxious cricket fans speculating the fate of a One Day
match? If we imagine our body to be a radio and the
mind its speaker, weather is something that fiddles
with its control knobs. Weather is this hour-to-hour,
day to day condition of the atmosphere. A hot or humid
weather may make one irritable. A pleasant, breezy
weather may make one cheerful and even plan for an
outing. Weather can change dramatically from day to
day. However, the average weather condition of a place
for a longer period of time represents the climate of a
place. Now do you understand why we have daily
weather forecasts.
Temperature
The temperature you feel everyday
is the temperataure of the
atmosphere. The degree of hotness
and coldness of the air is known
as temperature.
The temperature of the
atmosphere changes not only
between day and night but also
from season to season. Summers
are hotter than winters.
An important factor that
influences the distribution of
temperature is insolation.
Insolation is the incoming solar
energy intercepted by the earth.
The amount of insolation
decreases from the equator
towards the poles. Therefore, the
Let’s do
For ten days note
down weather report
from a local
newspaper and
observe the changes
occurring in the
weather.
Wind Vane: Shows the
direction of the wind
Rain Gauge: Measures
the amount of rainfall
Thermometer:
Measures
the temperature
Barometre: Measures
atmospheric pressure
Fig. 4.3: Weather Instruments
Reprint 2024-25
24 OUR ENVIRONMENT
temperature decreases in the same manner. Now do
you understand why poles are covered with snow? If
the earth’s temperature rises too high, it would become
too warm for some crops to grow. Temperature in cities
is much higher than that of villages. The concrete and
metals in buildings and the asaphalt of roads get
heated up during the day. This heat is released during
the night.
Also, the crowded high rise buildings of the cities
trap the warm air and thus raise the temperature of
the cities.
Air Pressure
You will be surprised to know that air above us presses
us with a great force on our bodies. However, we don’t
even feel it. This is because the air presses us from all
directions and our body exerts a counter pressure.
Air pressure is defined as the pressure exerted by the
weight of air on the earth’s surface. As we go up the
layers of atmosphere, the pressure falls rapidly. The air
pressure is highest at sea level and decreases with height.
Horizontally the distribution of air pressure is influenced
by temperature of air at a given place. In areas where
temperature is high the air gets heated and rises. This
creates a low-pressure area. Low pressure is associated
with cloudy skies and wet weather.
In areas having lower temperature, the air is cold. It
is therefore heavy. Heavy air sinks and creates a high
pressure area. High pressure is associated with clear and
sunny skies.
The air always moves from high pressure areas to low
pressure areas.
Wind
The movement of air from high pressure area to low
pressure areas is called wind. You can see wind at work
as it blows dry leaves down the pavement or uproots
trees during a storm. Sometimes when the wind blows
gently you can even see it blowing away smoke or fine
dust. At times wind can be so strong that it is difficult
to walk against it. You must have experienced it is not
easy to hold an umbrella on a windy day. Think of
some other examples when strong winds have created
Do you know?
On the moon there is
no air and hence no
air pressure.
Astronauts have to
wear special
protective space suits
filled with air when
they go to the moon. If
they did not wear
these space suits, the
counter pressure
exerted by the body of
the astronauts would
make the blood vessels
burst. The astronauts
would bleed.
Do you know?
The standard unit of
measuring
temperature is degree
Celsius. It was
invented by Anders
Celsius. On the
Celsius scale the
water freezes at 0°C
and boils at 100°C.
Do you know?
A wind is named after
the direction from
which it blows, e.g.
the wind blowing from
the west is called
westerly.
Reprint 2024-25