NCERT Textbook Chapter 8 - Composition and Structure of Atmosphere, Class 11, Geography UPSC Notes | EduRev

 Page 1


CLIMATE
This unit deals with
• Atmosphere — compositions and structure; elements of weather
and climate
• Insolation — angle of incidence and distribution; heat budget
of the earth — heating and cooling of atmosphere (conduction,
convection, terrestrial radiation, advection); temperature — factors
controlling temperature; distribution of temperature — horizontal
and vertical; inversion of temperature
• Pressure — pressure belts; winds-planetary seasonal and local,
air masses and fronts; tropical and extra tropical cyclones
• Precipitation — evaporation; condensation — dew, frost, fog,
mist and cloud; rainfall — types and world distributon
• World climates — classification (Koeppen), greenhouse effect,
global warming and climatic changes
UNIT
IV
© NCERT
not to be republished
Page 2


CLIMATE
This unit deals with
• Atmosphere — compositions and structure; elements of weather
and climate
• Insolation — angle of incidence and distribution; heat budget
of the earth — heating and cooling of atmosphere (conduction,
convection, terrestrial radiation, advection); temperature — factors
controlling temperature; distribution of temperature — horizontal
and vertical; inversion of temperature
• Pressure — pressure belts; winds-planetary seasonal and local,
air masses and fronts; tropical and extra tropical cyclones
• Precipitation — evaporation; condensation — dew, frost, fog,
mist and cloud; rainfall — types and world distributon
• World climates — classification (Koeppen), greenhouse effect,
global warming and climatic changes
UNIT
IV
© NCERT
not to be republished
COMPOSITION AND STRUCTURE
OF ATMOSPHERE
C
an a person live without air? We eat
food two - three times a day and drink
water more frequently but breathe
every few seconds. Air is essential to the
survival of all  organisms. Some organisms like
humans may survive for some time without
food and water but can’t survive even a few
minutes without breathing air. That shows the
reason why we should understand the
atmosphere in greater detail. Atmosphere is a
mixture of different gases and it envelopes the
earth all round. It contains life-giving gases like
oxygen for humans and animals and carbon
dioxide for plants. The air is an integral part of
the earth’s mass and 99 per cent of the total
mass of the atmosphere is confined to the
height of 32 km from the earth’s surface. The
air is colourless and odourless and can be felt
only when it blows as wind.
Can you imagine what will happen
to us in the absence of ozone in the
atmosphere?
COMPOSITION OF THE ATMOSPHERE
The atmosphere is composed of gases, water
vapour and dust particles. Table 8.1 shows
details of various gases in the air, particularly
in the lower atmosphere.  The proportion of
gases changes in the higher layers of the
atmosphere in such a way that oxygen will be
almost in negligible quantity at the height of
120 km.  Similarly, carbon dioxide and water
vapour are found only up to 90 km from the
surface of the earth.
Table 8.1 : Permanent Gases of the Atmosphere
Constituent Formula Percentage by Volume
Nitrogen N
2
78.08
Oxygen O
2
20.95
Argon Ar 0.93
Carbon dioxide CO
2
0.036
Neon Ne 0.002
Helium He 0.0005
Krypto Kr 0.001
Xenon Xe 0.00009
Hydrogen H
2
0.00005
Gases
Carbon dioxide is meteorologically a very
important gas as it is transparent to the
incoming solar radiation but opaque to the
outgoing terrestrial radiation. It absorbs a part
of terrestrial radiation and reflects back some
part of it towards the earth’s surface. It is
largely responsible for the green house effect.
The volume of other gases is constant but the
volume of carbon dioxide has been rising in
the past few decades mainly because of the
burning of fossil fuels. This has also increased
the temperature of the air. Ozone is another
important component of the atmosphere found
between 10 and 50 km above the earth’s
surface and acts as a filter and absorbs the
ultra-violet rays radiating from the sun and
prevents them from reaching the surface of the
earth.
Water Vapour
Water vapour is also a variable gas in the
atmosphere, which decreases with altitude. In
the warm and wet tropics, it may account for
CHAPTER
© NCERT
not to be republished
Page 3


CLIMATE
This unit deals with
• Atmosphere — compositions and structure; elements of weather
and climate
• Insolation — angle of incidence and distribution; heat budget
of the earth — heating and cooling of atmosphere (conduction,
convection, terrestrial radiation, advection); temperature — factors
controlling temperature; distribution of temperature — horizontal
and vertical; inversion of temperature
• Pressure — pressure belts; winds-planetary seasonal and local,
air masses and fronts; tropical and extra tropical cyclones
• Precipitation — evaporation; condensation — dew, frost, fog,
mist and cloud; rainfall — types and world distributon
• World climates — classification (Koeppen), greenhouse effect,
global warming and climatic changes
UNIT
IV
© NCERT
not to be republished
COMPOSITION AND STRUCTURE
OF ATMOSPHERE
C
an a person live without air? We eat
food two - three times a day and drink
water more frequently but breathe
every few seconds. Air is essential to the
survival of all  organisms. Some organisms like
humans may survive for some time without
food and water but can’t survive even a few
minutes without breathing air. That shows the
reason why we should understand the
atmosphere in greater detail. Atmosphere is a
mixture of different gases and it envelopes the
earth all round. It contains life-giving gases like
oxygen for humans and animals and carbon
dioxide for plants. The air is an integral part of
the earth’s mass and 99 per cent of the total
mass of the atmosphere is confined to the
height of 32 km from the earth’s surface. The
air is colourless and odourless and can be felt
only when it blows as wind.
Can you imagine what will happen
to us in the absence of ozone in the
atmosphere?
COMPOSITION OF THE ATMOSPHERE
The atmosphere is composed of gases, water
vapour and dust particles. Table 8.1 shows
details of various gases in the air, particularly
in the lower atmosphere.  The proportion of
gases changes in the higher layers of the
atmosphere in such a way that oxygen will be
almost in negligible quantity at the height of
120 km.  Similarly, carbon dioxide and water
vapour are found only up to 90 km from the
surface of the earth.
Table 8.1 : Permanent Gases of the Atmosphere
Constituent Formula Percentage by Volume
Nitrogen N
2
78.08
Oxygen O
2
20.95
Argon Ar 0.93
Carbon dioxide CO
2
0.036
Neon Ne 0.002
Helium He 0.0005
Krypto Kr 0.001
Xenon Xe 0.00009
Hydrogen H
2
0.00005
Gases
Carbon dioxide is meteorologically a very
important gas as it is transparent to the
incoming solar radiation but opaque to the
outgoing terrestrial radiation. It absorbs a part
of terrestrial radiation and reflects back some
part of it towards the earth’s surface. It is
largely responsible for the green house effect.
The volume of other gases is constant but the
volume of carbon dioxide has been rising in
the past few decades mainly because of the
burning of fossil fuels. This has also increased
the temperature of the air. Ozone is another
important component of the atmosphere found
between 10 and 50 km above the earth’s
surface and acts as a filter and absorbs the
ultra-violet rays radiating from the sun and
prevents them from reaching the surface of the
earth.
Water Vapour
Water vapour is also a variable gas in the
atmosphere, which decreases with altitude. In
the warm and wet tropics, it may account for
CHAPTER
© NCERT
not to be republished
COMPOSITION AND STRUCTURE OF ATMOSPHERE 77
four per cent of the air by volume, while in the
dry and cold areas of desert and polar regions,
it may be less than one per cent of the air. Water
vapour also decreases from the equator
towards the poles. It also absorbs parts of the
insolation from the sun and preserves the
earth’s radiated heat. It thus, acts like a blanket
allowing the earth neither to become too cold
nor too hot. Water vapour also contributes to
the stability and instability in the air.
Dust Particles
Atmosphere has a sufficient capacity to keep
small solid particles, which may originate from
different sources and include sea salts, fine soil,
smoke-soot, ash, pollen, dust and disintegrated
particles of meteors. Dust particles are
generally concentrated in the lower layers of
the atmosphere; yet, convectional air currents
may transport them to great heights. The
higher concentration of dust particles is found
in subtropical and temperate regions due to
dry winds in comparison to equatorial and
polar regions. Dust and salt particles act as
hygroscopic nuclei around which water vapour
condenses to produce clouds.
STRUCTURE OF THE ATMOSPHERE
The atmosphere consists of different layers with
varying density and temperature. Density is
highest near the surface of the earth and
decreases with increasing altitude.  The column
of atmosphere is divided into five different
layers depending upon the temperature
condition. They are: troposphere, stratosphere,
mesosphere, thermosphere and exosphere.
The troposphere is the lowermost layer of
the atmosphere. Its average height is 13 km
and extends roughly to a height of 8 km near
the poles and about 18 km at the equator.
Thickness of the troposphere is greatest at the
equator because heat is transported to great
heights by strong convectional currents. This
layer contains dust particles and water vapour.
All changes in climate and weather take place
in this layer. The temperature in this layer
decreases at the rate of 1
°
C for every 165m of
height. This is the most important layer for all
biological activity.
The zone separating the tropsophere from
stratosphere is known as the tropopause. The
air temperature at the tropopause is about
minus 80
0
C over the equator and about minus
45
o
C over the poles.  The temperature here is
nearly constant, and hence, it is called the
tropopause. The stratosphere is found above
the tropopause and extends up to a height of
50 km. One important feature of the
stratosphere is that it contains the ozone layer.
This layer absorbs ultra-violet radiation and
shields life on the earth from intense, harmful
form of energy.
The mesosphere lies above the stratosphere,
which extends up to a height of 80 km. In this
layer, once again, temperature starts
decreasing with the increase in altitude and
reaches up to  minus 100
°
C at the height of 80
km. The upper limit of mesosphere is known
as the mesopause. The ionosphere is located
between 80 and 400 km above the mesopause.
It contains electrically charged particles known
as ions, and hence, it is known as ionosphere.
Radio waves transmitted from the earth are
reflected back to the earth by this layer.
Temperature here starts increasing with height.
The uppermost layer of the atmosphere above
Figure 8.1 : Structure of atmosphere
© NCERT
not to be republished
Page 4


CLIMATE
This unit deals with
• Atmosphere — compositions and structure; elements of weather
and climate
• Insolation — angle of incidence and distribution; heat budget
of the earth — heating and cooling of atmosphere (conduction,
convection, terrestrial radiation, advection); temperature — factors
controlling temperature; distribution of temperature — horizontal
and vertical; inversion of temperature
• Pressure — pressure belts; winds-planetary seasonal and local,
air masses and fronts; tropical and extra tropical cyclones
• Precipitation — evaporation; condensation — dew, frost, fog,
mist and cloud; rainfall — types and world distributon
• World climates — classification (Koeppen), greenhouse effect,
global warming and climatic changes
UNIT
IV
© NCERT
not to be republished
COMPOSITION AND STRUCTURE
OF ATMOSPHERE
C
an a person live without air? We eat
food two - three times a day and drink
water more frequently but breathe
every few seconds. Air is essential to the
survival of all  organisms. Some organisms like
humans may survive for some time without
food and water but can’t survive even a few
minutes without breathing air. That shows the
reason why we should understand the
atmosphere in greater detail. Atmosphere is a
mixture of different gases and it envelopes the
earth all round. It contains life-giving gases like
oxygen for humans and animals and carbon
dioxide for plants. The air is an integral part of
the earth’s mass and 99 per cent of the total
mass of the atmosphere is confined to the
height of 32 km from the earth’s surface. The
air is colourless and odourless and can be felt
only when it blows as wind.
Can you imagine what will happen
to us in the absence of ozone in the
atmosphere?
COMPOSITION OF THE ATMOSPHERE
The atmosphere is composed of gases, water
vapour and dust particles. Table 8.1 shows
details of various gases in the air, particularly
in the lower atmosphere.  The proportion of
gases changes in the higher layers of the
atmosphere in such a way that oxygen will be
almost in negligible quantity at the height of
120 km.  Similarly, carbon dioxide and water
vapour are found only up to 90 km from the
surface of the earth.
Table 8.1 : Permanent Gases of the Atmosphere
Constituent Formula Percentage by Volume
Nitrogen N
2
78.08
Oxygen O
2
20.95
Argon Ar 0.93
Carbon dioxide CO
2
0.036
Neon Ne 0.002
Helium He 0.0005
Krypto Kr 0.001
Xenon Xe 0.00009
Hydrogen H
2
0.00005
Gases
Carbon dioxide is meteorologically a very
important gas as it is transparent to the
incoming solar radiation but opaque to the
outgoing terrestrial radiation. It absorbs a part
of terrestrial radiation and reflects back some
part of it towards the earth’s surface. It is
largely responsible for the green house effect.
The volume of other gases is constant but the
volume of carbon dioxide has been rising in
the past few decades mainly because of the
burning of fossil fuels. This has also increased
the temperature of the air. Ozone is another
important component of the atmosphere found
between 10 and 50 km above the earth’s
surface and acts as a filter and absorbs the
ultra-violet rays radiating from the sun and
prevents them from reaching the surface of the
earth.
Water Vapour
Water vapour is also a variable gas in the
atmosphere, which decreases with altitude. In
the warm and wet tropics, it may account for
CHAPTER
© NCERT
not to be republished
COMPOSITION AND STRUCTURE OF ATMOSPHERE 77
four per cent of the air by volume, while in the
dry and cold areas of desert and polar regions,
it may be less than one per cent of the air. Water
vapour also decreases from the equator
towards the poles. It also absorbs parts of the
insolation from the sun and preserves the
earth’s radiated heat. It thus, acts like a blanket
allowing the earth neither to become too cold
nor too hot. Water vapour also contributes to
the stability and instability in the air.
Dust Particles
Atmosphere has a sufficient capacity to keep
small solid particles, which may originate from
different sources and include sea salts, fine soil,
smoke-soot, ash, pollen, dust and disintegrated
particles of meteors. Dust particles are
generally concentrated in the lower layers of
the atmosphere; yet, convectional air currents
may transport them to great heights. The
higher concentration of dust particles is found
in subtropical and temperate regions due to
dry winds in comparison to equatorial and
polar regions. Dust and salt particles act as
hygroscopic nuclei around which water vapour
condenses to produce clouds.
STRUCTURE OF THE ATMOSPHERE
The atmosphere consists of different layers with
varying density and temperature. Density is
highest near the surface of the earth and
decreases with increasing altitude.  The column
of atmosphere is divided into five different
layers depending upon the temperature
condition. They are: troposphere, stratosphere,
mesosphere, thermosphere and exosphere.
The troposphere is the lowermost layer of
the atmosphere. Its average height is 13 km
and extends roughly to a height of 8 km near
the poles and about 18 km at the equator.
Thickness of the troposphere is greatest at the
equator because heat is transported to great
heights by strong convectional currents. This
layer contains dust particles and water vapour.
All changes in climate and weather take place
in this layer. The temperature in this layer
decreases at the rate of 1
°
C for every 165m of
height. This is the most important layer for all
biological activity.
The zone separating the tropsophere from
stratosphere is known as the tropopause. The
air temperature at the tropopause is about
minus 80
0
C over the equator and about minus
45
o
C over the poles.  The temperature here is
nearly constant, and hence, it is called the
tropopause. The stratosphere is found above
the tropopause and extends up to a height of
50 km. One important feature of the
stratosphere is that it contains the ozone layer.
This layer absorbs ultra-violet radiation and
shields life on the earth from intense, harmful
form of energy.
The mesosphere lies above the stratosphere,
which extends up to a height of 80 km. In this
layer, once again, temperature starts
decreasing with the increase in altitude and
reaches up to  minus 100
°
C at the height of 80
km. The upper limit of mesosphere is known
as the mesopause. The ionosphere is located
between 80 and 400 km above the mesopause.
It contains electrically charged particles known
as ions, and hence, it is known as ionosphere.
Radio waves transmitted from the earth are
reflected back to the earth by this layer.
Temperature here starts increasing with height.
The uppermost layer of the atmosphere above
Figure 8.1 : Structure of atmosphere
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 78
the thermosphere is known as the exosphere.
This is the highest layer but very little is known
about it. Whatever contents are there, these are
extremely rarefied in this layer, and it gradually
merges with the outer space. Although all
layers of the atmosphere must be exercising
influence on us, geographers are concerned
with the first two layers of the atmosphere.
Elements of Weather and Climate
The main elements of atmosphere which are
subject to change and which influence human
life on earth are temperature, pressure, winds,
humidity, clouds and precipitation. These
elements have been dealt in detail in Chapters
9, 10 and 11.
EXERCISES
1. Multiple choice questions.
  (i) Which one of the following gases constitutes the major portion of the
atmosphere?
(a) Oxygen (c) Argon
(b) Nitrogen (d) Carbon dioxide
 (ii) Atmospheric layer important for human beings is:
(a) Stratosphere (c) Troposphere
(b) Mesosphere (d) Ionosphere
(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:
(a) Gases (c) Water vapour
(b) Dust particles (d) Meteors
(iv) Oxygen gas is in negligible quantity at the height of atmosphere:
(a) 90 km (c) 100 km
(b) 120 km (d) 150 km
(v) Which one of the following gases is transparent to incoming solar radiation
and opaque to outgoing terrestrial radiation?
(a) Oxygen (c) Helium
(b) Nitrogen (d) Carbon dioxide
2. Answer the following questions in about 30 words.
(i) What do you understand by atmosphere?
(ii) What are the elements of weather and climate?
(iii) Describe the composition of atmosphere.
(iv) Why is troposphere the most important of all the layers of the atmosphere?
3. Answer the following questions in about 150 words.
(i) Describe the composition of the atmosphere.
(ii) Draw a suitable diagram for the structure of the atmosphere and label it
and describe it.
© NCERT
not to be republished