NCERT Textbook Chapter 9 - Solar Radiation, Heat Balance and Temperature, Class 11, Geography UPSC Notes | EduRev

 Page 1


SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you
know that we live at the bottom of a
huge pile of air? We inhale and exhale
but we feel the air when it is  in motion. It means
air in motion is wind. You have already learnt
about the fact that earth is surrounded by air
all around. This envelop of air is atmosphere
which is composed of numerous gases. These
gases support life over the earth’s surface.
The earth receives almost all of its energy
from the sun. The earth in turn radiates back
to space the energy received from the sun. As
a result, the earth neither warms up nor does
it get cooled over a period of time. Thus, the
amount of heat received by different parts of
the earth is not the same. This variation causes
pressure differences in the atmosphere. This
leads to transfer of heat from one region to the
other by winds. This chapter explains the
process of heating and cooling of the
atmosphere and the resultant temperature
distribution over the earth’s surface.
SOLAR RADIATION
The earth’s surface receives most of its energy
in short wavelengths. The energy received by
the earth is known as incoming solar radiation
which in short is termed as insolation.
As the earth is a geoid resembling a sphere,
the sun’s rays fall obliquely at the top of the
atmosphere and the earth intercepts a very
small portion of the sun’s energy. On an
average the earth receives 1.94 calories per sq.
cm per minute at the top of its atmosphere.
The solar output received at the top of the
atmosphere varies slightly in a year due to the
variations in the distance between the earth and
the sun. During its revolution around the sun,
the earth is farthest from the sun (152 million
km) on 4th July. This position of the earth is
called aphelion. On 3rd January, the earth is
the nearest to the sun (147 million km). This
position is called perihelion. Therefore, the
annual insolation received by the earth on 3rd
January is slightly more than the amount
received on 4th July. However, the effect of this
variation in the solar output is masked by
other factors like the distribution of land and
sea and the atmospheric circulation. Hence, this
variation in the solar output does not have
great effect on daily weather changes on the
surface of the earth.
Variability of Insolation at
the Surface of the Earth
The amount and the intensity of insolation vary
during a day, in a season and in a year.  The factors
that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii)  the angle of
inclination of the sun’s rays; (iii) the length of the
day; (iv) the transparency of the atmosphere; (v)
the configuration of land in terms of its aspect.
The last two however, have less influence.
The fact that the earth’s axis makes an angle
of 66 with the plane of its orbit round the sun
has a greater influence on the amount of
insolation received at different latitudes. Note the
variations in the duration of the day at different
latitudes on solstices given in Table 9.1.
CHAPTER
© NCERT
not to be republished
Page 2


SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you
know that we live at the bottom of a
huge pile of air? We inhale and exhale
but we feel the air when it is  in motion. It means
air in motion is wind. You have already learnt
about the fact that earth is surrounded by air
all around. This envelop of air is atmosphere
which is composed of numerous gases. These
gases support life over the earth’s surface.
The earth receives almost all of its energy
from the sun. The earth in turn radiates back
to space the energy received from the sun. As
a result, the earth neither warms up nor does
it get cooled over a period of time. Thus, the
amount of heat received by different parts of
the earth is not the same. This variation causes
pressure differences in the atmosphere. This
leads to transfer of heat from one region to the
other by winds. This chapter explains the
process of heating and cooling of the
atmosphere and the resultant temperature
distribution over the earth’s surface.
SOLAR RADIATION
The earth’s surface receives most of its energy
in short wavelengths. The energy received by
the earth is known as incoming solar radiation
which in short is termed as insolation.
As the earth is a geoid resembling a sphere,
the sun’s rays fall obliquely at the top of the
atmosphere and the earth intercepts a very
small portion of the sun’s energy. On an
average the earth receives 1.94 calories per sq.
cm per minute at the top of its atmosphere.
The solar output received at the top of the
atmosphere varies slightly in a year due to the
variations in the distance between the earth and
the sun. During its revolution around the sun,
the earth is farthest from the sun (152 million
km) on 4th July. This position of the earth is
called aphelion. On 3rd January, the earth is
the nearest to the sun (147 million km). This
position is called perihelion. Therefore, the
annual insolation received by the earth on 3rd
January is slightly more than the amount
received on 4th July. However, the effect of this
variation in the solar output is masked by
other factors like the distribution of land and
sea and the atmospheric circulation. Hence, this
variation in the solar output does not have
great effect on daily weather changes on the
surface of the earth.
Variability of Insolation at
the Surface of the Earth
The amount and the intensity of insolation vary
during a day, in a season and in a year.  The factors
that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii)  the angle of
inclination of the sun’s rays; (iii) the length of the
day; (iv) the transparency of the atmosphere; (v)
the configuration of land in terms of its aspect.
The last two however, have less influence.
The fact that the earth’s axis makes an angle
of 66 with the plane of its orbit round the sun
has a greater influence on the amount of
insolation received at different latitudes. Note the
variations in the duration of the day at different
latitudes on solstices given in Table 9.1.
CHAPTER
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 80
The second factor that determines the
amount of insolation received is the angle of
inclination of the rays. This depends on the
latitude of a place. The higher the latitude the
less is the angle they make with the surface of
the earth resulting in slant sun rays. The area
covered by vertical rays is always less than the
slant rays. If more area is covered, the energy
gets distributed and the net energy received
per unit area decreases. Moreover, the slant rays
are required to pass through greater depth of
the atmosphere resulting in more absorption,
scattering and diffusion.
Figure 9.1 : Summer Solstice
Latitude 0° 20° 40° 60° 90°
December 22 12h 00m 10h 48m 9h 8m 5h 33m 0
June 21 12 h 13h 12m 14h 52m 18h 27m 6 months
Table 9.1 : Length of the Day in Hours and Minutes on Winter and Summer Solstices in the Northern Hemisphere
colour of the sky are the result of scattering of
light within the atmosphere.
The Passage of Solar Radiation
through the Atmosphere
The atmosphere is largely transparent to short
wave solar radiation. The incoming solar
radiation passes through the atmosphere
before striking the earth’s surface. Within the
troposphere water vapour, ozone and other
gases absorb much of the near infrared
radiation.
Very small-suspended particles in the
troposphere scatter visible spectrum both to
the space and towards the earth surface. This
process adds colour to the sky. The red colour
of the rising and the setting sun and the blue
Spatial Distribution of Insolation
at the Earth’s Surface
The insolation received at the surface varies
from about 320 Watt/m
2
 in the tropics to about
70 Watt/m
2
 in the poles. Maximum insolation
is received over the subtropical deserts, where
the cloudiness is the least. Equator receives
comparatively less insolation than the tropics.
Generally, at the same latitude the insolation
is more over the continent than over the oceans.
In winter, the middle and higher latitudes
receive less radiation than in summer.
HEATING AND COOLING OF ATMOSPHERE
There are different ways of heating and cooling
of the atmosphere.
The earth after being heated by insolation
transmits the heat to the atmospheric layers near
to the earth in long wave form. The air in contact
with the land gets heated slowly and the upper
layers in contact with the lower layers also get
heated. This process is called conduction.
Conduction takes place when two bodies of
unequal temperature are in contact with one
another, there is a flow of energy from the warmer
to cooler body. The transfer of heat continues until
both the bodies attain the same temperature or
the contact is broken. Conduction is important
in heating the lower layers of the atmosphere.
The air in contact with the earth rises
vertically on heating in the form of currents
and further transmits the heat of the
atmsphere. This process of vertical heating of
the atmosphere is known as convection. The
convective transfer of energy is confined only
to the troposphere.
The transfer of heat through horizontal
movement of air is called advection. Horizontal
movement of the air is relatively more important
© NCERT
not to be republished
Page 3


SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you
know that we live at the bottom of a
huge pile of air? We inhale and exhale
but we feel the air when it is  in motion. It means
air in motion is wind. You have already learnt
about the fact that earth is surrounded by air
all around. This envelop of air is atmosphere
which is composed of numerous gases. These
gases support life over the earth’s surface.
The earth receives almost all of its energy
from the sun. The earth in turn radiates back
to space the energy received from the sun. As
a result, the earth neither warms up nor does
it get cooled over a period of time. Thus, the
amount of heat received by different parts of
the earth is not the same. This variation causes
pressure differences in the atmosphere. This
leads to transfer of heat from one region to the
other by winds. This chapter explains the
process of heating and cooling of the
atmosphere and the resultant temperature
distribution over the earth’s surface.
SOLAR RADIATION
The earth’s surface receives most of its energy
in short wavelengths. The energy received by
the earth is known as incoming solar radiation
which in short is termed as insolation.
As the earth is a geoid resembling a sphere,
the sun’s rays fall obliquely at the top of the
atmosphere and the earth intercepts a very
small portion of the sun’s energy. On an
average the earth receives 1.94 calories per sq.
cm per minute at the top of its atmosphere.
The solar output received at the top of the
atmosphere varies slightly in a year due to the
variations in the distance between the earth and
the sun. During its revolution around the sun,
the earth is farthest from the sun (152 million
km) on 4th July. This position of the earth is
called aphelion. On 3rd January, the earth is
the nearest to the sun (147 million km). This
position is called perihelion. Therefore, the
annual insolation received by the earth on 3rd
January is slightly more than the amount
received on 4th July. However, the effect of this
variation in the solar output is masked by
other factors like the distribution of land and
sea and the atmospheric circulation. Hence, this
variation in the solar output does not have
great effect on daily weather changes on the
surface of the earth.
Variability of Insolation at
the Surface of the Earth
The amount and the intensity of insolation vary
during a day, in a season and in a year.  The factors
that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii)  the angle of
inclination of the sun’s rays; (iii) the length of the
day; (iv) the transparency of the atmosphere; (v)
the configuration of land in terms of its aspect.
The last two however, have less influence.
The fact that the earth’s axis makes an angle
of 66 with the plane of its orbit round the sun
has a greater influence on the amount of
insolation received at different latitudes. Note the
variations in the duration of the day at different
latitudes on solstices given in Table 9.1.
CHAPTER
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 80
The second factor that determines the
amount of insolation received is the angle of
inclination of the rays. This depends on the
latitude of a place. The higher the latitude the
less is the angle they make with the surface of
the earth resulting in slant sun rays. The area
covered by vertical rays is always less than the
slant rays. If more area is covered, the energy
gets distributed and the net energy received
per unit area decreases. Moreover, the slant rays
are required to pass through greater depth of
the atmosphere resulting in more absorption,
scattering and diffusion.
Figure 9.1 : Summer Solstice
Latitude 0° 20° 40° 60° 90°
December 22 12h 00m 10h 48m 9h 8m 5h 33m 0
June 21 12 h 13h 12m 14h 52m 18h 27m 6 months
Table 9.1 : Length of the Day in Hours and Minutes on Winter and Summer Solstices in the Northern Hemisphere
colour of the sky are the result of scattering of
light within the atmosphere.
The Passage of Solar Radiation
through the Atmosphere
The atmosphere is largely transparent to short
wave solar radiation. The incoming solar
radiation passes through the atmosphere
before striking the earth’s surface. Within the
troposphere water vapour, ozone and other
gases absorb much of the near infrared
radiation.
Very small-suspended particles in the
troposphere scatter visible spectrum both to
the space and towards the earth surface. This
process adds colour to the sky. The red colour
of the rising and the setting sun and the blue
Spatial Distribution of Insolation
at the Earth’s Surface
The insolation received at the surface varies
from about 320 Watt/m
2
 in the tropics to about
70 Watt/m
2
 in the poles. Maximum insolation
is received over the subtropical deserts, where
the cloudiness is the least. Equator receives
comparatively less insolation than the tropics.
Generally, at the same latitude the insolation
is more over the continent than over the oceans.
In winter, the middle and higher latitudes
receive less radiation than in summer.
HEATING AND COOLING OF ATMOSPHERE
There are different ways of heating and cooling
of the atmosphere.
The earth after being heated by insolation
transmits the heat to the atmospheric layers near
to the earth in long wave form. The air in contact
with the land gets heated slowly and the upper
layers in contact with the lower layers also get
heated. This process is called conduction.
Conduction takes place when two bodies of
unequal temperature are in contact with one
another, there is a flow of energy from the warmer
to cooler body. The transfer of heat continues until
both the bodies attain the same temperature or
the contact is broken. Conduction is important
in heating the lower layers of the atmosphere.
The air in contact with the earth rises
vertically on heating in the form of currents
and further transmits the heat of the
atmsphere. This process of vertical heating of
the atmosphere is known as convection. The
convective transfer of energy is confined only
to the troposphere.
The transfer of heat through horizontal
movement of air is called advection. Horizontal
movement of the air is relatively more important
© NCERT
not to be republished
SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 81
accumulate or loose heat. It maintains its
temperature. This can happen only if the
amount of heat received in the form of insolation
equals the amount lost by the earth through
terrestrial radiation.
Consider that the insolation received at the
top of the atmosphere is 100 per cent. While
passing through the atmosphere some amount
of energy is reflected, scattered and absorbed.
Only the remaining part reaches the earth
surface. Roughly 35 units are reflected back
to space even before reaching the earth’s
surface. Of these, 27 units are reflected back
from the top of the clouds and 2 units from the
snow and ice-covered areas of the earth. The
reflected amount of radiation is called the
albedo of the earth.
The remaining 65 units are absorbed, 14
units within the atmosphere and 51 units by
the earth’s surface. The earth radiates back
51 units in the form of terrestrial radiation.
Of these, 17 units are radiated to space
directly and the remaining 34 units are
absorbed by the atmosphere (6 units
absorbed directly by the atmosphere, 9 units
through convection and turbulence and 19
units through latent heat of condensation).
48 units absorbed by the atmosphere
(14 units from insolation +34 units from
than the vertical movement. In middle latitudes,
most of dirunal (day and night) variation in
daily weather are caused by advection alone.
In tropical regions particularly in northern
India during summer season local winds called
‘loo’ is the outcome of advection process.
Terrestrial Radiation
The insolation received by the earth is in short
waves forms and heats up its surface. The earth
after being heated itself becomes a radiating
body and it radiates energy to the atmosphere
in long wave form. This energy heats up the
atmosphere from below. This process is known
as terrestrial radiation.
The long wave radiation is absorbed by the
atmospheric gases particularly by carbon
dioxide and the other green house gases. Thus,
the atmosphere is indirectly heated by the
earth’s radiation.
The atmosphere in turn radiates and
transmits heat to the space. Finally the amount
of heat received from the sun is returned to
space, thereby maintaining constant temperature
at the earth’s surface and in the atmosphere.
Heat Budget of the Planet Earth
Figure 9.2 depicts the heat budget of the planet
earth. The earth as a whole does not
Figure 9.2 : Heat budget of the earth
© NCERT
not to be republished
Page 4


SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you
know that we live at the bottom of a
huge pile of air? We inhale and exhale
but we feel the air when it is  in motion. It means
air in motion is wind. You have already learnt
about the fact that earth is surrounded by air
all around. This envelop of air is atmosphere
which is composed of numerous gases. These
gases support life over the earth’s surface.
The earth receives almost all of its energy
from the sun. The earth in turn radiates back
to space the energy received from the sun. As
a result, the earth neither warms up nor does
it get cooled over a period of time. Thus, the
amount of heat received by different parts of
the earth is not the same. This variation causes
pressure differences in the atmosphere. This
leads to transfer of heat from one region to the
other by winds. This chapter explains the
process of heating and cooling of the
atmosphere and the resultant temperature
distribution over the earth’s surface.
SOLAR RADIATION
The earth’s surface receives most of its energy
in short wavelengths. The energy received by
the earth is known as incoming solar radiation
which in short is termed as insolation.
As the earth is a geoid resembling a sphere,
the sun’s rays fall obliquely at the top of the
atmosphere and the earth intercepts a very
small portion of the sun’s energy. On an
average the earth receives 1.94 calories per sq.
cm per minute at the top of its atmosphere.
The solar output received at the top of the
atmosphere varies slightly in a year due to the
variations in the distance between the earth and
the sun. During its revolution around the sun,
the earth is farthest from the sun (152 million
km) on 4th July. This position of the earth is
called aphelion. On 3rd January, the earth is
the nearest to the sun (147 million km). This
position is called perihelion. Therefore, the
annual insolation received by the earth on 3rd
January is slightly more than the amount
received on 4th July. However, the effect of this
variation in the solar output is masked by
other factors like the distribution of land and
sea and the atmospheric circulation. Hence, this
variation in the solar output does not have
great effect on daily weather changes on the
surface of the earth.
Variability of Insolation at
the Surface of the Earth
The amount and the intensity of insolation vary
during a day, in a season and in a year.  The factors
that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii)  the angle of
inclination of the sun’s rays; (iii) the length of the
day; (iv) the transparency of the atmosphere; (v)
the configuration of land in terms of its aspect.
The last two however, have less influence.
The fact that the earth’s axis makes an angle
of 66 with the plane of its orbit round the sun
has a greater influence on the amount of
insolation received at different latitudes. Note the
variations in the duration of the day at different
latitudes on solstices given in Table 9.1.
CHAPTER
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 80
The second factor that determines the
amount of insolation received is the angle of
inclination of the rays. This depends on the
latitude of a place. The higher the latitude the
less is the angle they make with the surface of
the earth resulting in slant sun rays. The area
covered by vertical rays is always less than the
slant rays. If more area is covered, the energy
gets distributed and the net energy received
per unit area decreases. Moreover, the slant rays
are required to pass through greater depth of
the atmosphere resulting in more absorption,
scattering and diffusion.
Figure 9.1 : Summer Solstice
Latitude 0° 20° 40° 60° 90°
December 22 12h 00m 10h 48m 9h 8m 5h 33m 0
June 21 12 h 13h 12m 14h 52m 18h 27m 6 months
Table 9.1 : Length of the Day in Hours and Minutes on Winter and Summer Solstices in the Northern Hemisphere
colour of the sky are the result of scattering of
light within the atmosphere.
The Passage of Solar Radiation
through the Atmosphere
The atmosphere is largely transparent to short
wave solar radiation. The incoming solar
radiation passes through the atmosphere
before striking the earth’s surface. Within the
troposphere water vapour, ozone and other
gases absorb much of the near infrared
radiation.
Very small-suspended particles in the
troposphere scatter visible spectrum both to
the space and towards the earth surface. This
process adds colour to the sky. The red colour
of the rising and the setting sun and the blue
Spatial Distribution of Insolation
at the Earth’s Surface
The insolation received at the surface varies
from about 320 Watt/m
2
 in the tropics to about
70 Watt/m
2
 in the poles. Maximum insolation
is received over the subtropical deserts, where
the cloudiness is the least. Equator receives
comparatively less insolation than the tropics.
Generally, at the same latitude the insolation
is more over the continent than over the oceans.
In winter, the middle and higher latitudes
receive less radiation than in summer.
HEATING AND COOLING OF ATMOSPHERE
There are different ways of heating and cooling
of the atmosphere.
The earth after being heated by insolation
transmits the heat to the atmospheric layers near
to the earth in long wave form. The air in contact
with the land gets heated slowly and the upper
layers in contact with the lower layers also get
heated. This process is called conduction.
Conduction takes place when two bodies of
unequal temperature are in contact with one
another, there is a flow of energy from the warmer
to cooler body. The transfer of heat continues until
both the bodies attain the same temperature or
the contact is broken. Conduction is important
in heating the lower layers of the atmosphere.
The air in contact with the earth rises
vertically on heating in the form of currents
and further transmits the heat of the
atmsphere. This process of vertical heating of
the atmosphere is known as convection. The
convective transfer of energy is confined only
to the troposphere.
The transfer of heat through horizontal
movement of air is called advection. Horizontal
movement of the air is relatively more important
© NCERT
not to be republished
SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 81
accumulate or loose heat. It maintains its
temperature. This can happen only if the
amount of heat received in the form of insolation
equals the amount lost by the earth through
terrestrial radiation.
Consider that the insolation received at the
top of the atmosphere is 100 per cent. While
passing through the atmosphere some amount
of energy is reflected, scattered and absorbed.
Only the remaining part reaches the earth
surface. Roughly 35 units are reflected back
to space even before reaching the earth’s
surface. Of these, 27 units are reflected back
from the top of the clouds and 2 units from the
snow and ice-covered areas of the earth. The
reflected amount of radiation is called the
albedo of the earth.
The remaining 65 units are absorbed, 14
units within the atmosphere and 51 units by
the earth’s surface. The earth radiates back
51 units in the form of terrestrial radiation.
Of these, 17 units are radiated to space
directly and the remaining 34 units are
absorbed by the atmosphere (6 units
absorbed directly by the atmosphere, 9 units
through convection and turbulence and 19
units through latent heat of condensation).
48 units absorbed by the atmosphere
(14 units from insolation +34 units from
than the vertical movement. In middle latitudes,
most of dirunal (day and night) variation in
daily weather are caused by advection alone.
In tropical regions particularly in northern
India during summer season local winds called
‘loo’ is the outcome of advection process.
Terrestrial Radiation
The insolation received by the earth is in short
waves forms and heats up its surface. The earth
after being heated itself becomes a radiating
body and it radiates energy to the atmosphere
in long wave form. This energy heats up the
atmosphere from below. This process is known
as terrestrial radiation.
The long wave radiation is absorbed by the
atmospheric gases particularly by carbon
dioxide and the other green house gases. Thus,
the atmosphere is indirectly heated by the
earth’s radiation.
The atmosphere in turn radiates and
transmits heat to the space. Finally the amount
of heat received from the sun is returned to
space, thereby maintaining constant temperature
at the earth’s surface and in the atmosphere.
Heat Budget of the Planet Earth
Figure 9.2 depicts the heat budget of the planet
earth. The earth as a whole does not
Figure 9.2 : Heat budget of the earth
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 82
terrestrial radiation) are also radiated back
into space. Thus, the total radiation
returning from the earth and the atmosphere
respectively is 17+48=65 units which
balance the total of 65 units received from
the sun. This is termed the heat budget or
heat balance of the earth.
This explains, why the earth neither warms
up nor cools down despite the huge transfer of
heat that takes place.
Variation in the Net Heat Budget at the
Earth’s Surface
As explained earlier, there are variations in the
amount of radiation received at the earth’s
surface. Some part of the earth has surplus
radiation balance while the other part has
deficit.
Figure 9.3 depicts the latitudinal variation
in the net radiation balance of the earth — the
atmosphere system. The figure shows that
there is a surplus of net radiation balance
between 40 degrees north and south and the
regions near the poles have a deficit. The
surplus heat energy from the tropics is
redistributed pole wards and as a result the
tropics do not get progressively heated up due
to the accumulation of excess heat or the high
latitudes get permanently frozen due to excess
deficit.
Figure 9.3 : Latitudinal variation in net
    radiation balance
heat which is measured in terms of
temperature. While heat represents the
molecular movement of particles comprising a
substance, the temperature is the measurement
in degrees of how hot (or cold) a thing (or a
place) is.
Factors Controlling Temperature Distribution
The temperature of air at any place is influenced
by  (i) the latitude of the place;  (ii) the altitude
of the place; (iii) distance from the sea, the air-
mass circulation; (iv) the presence of warm and
cold ocean currents; (v) local aspects.
The latitude : The temperature of a place
depends on the insolation received. It has been
explained earlier that the insolation varies
according to the latitude hence the
temperature also varies accordingly.
The altitude : The atmosphere is indirectly
heated by terrestrial radiation from below.
Therefore, the places near the sea-level record
higher temperature than the places situated
at higher elevations. In other words, the
temperature generally decreases with
increasing height. The rate of decrease of
temperature with height is termed as the
normal lapse rate. It is 6.5°C per 1,000 m.
Distance from the sea : Another factor that
influences the temperature is the location of a
place with respect to the sea. Compared to land,
the sea gets heated slowly and loses heat
slowly. Land heats up and cools down quickly.
Therefore, the variation in temperature over the
sea is less compared to land. The places
situated near the sea come under the
moderating influence of the sea and land
breezes which moderate the temperature.
Air-mass and Ocean currents : Like the land
and sea breezes, the passage of air masses also
affects the temperature. The places, which
come under the influence of warm air-masses
experience higher temperature and the places
that come under the influence of cold air-
masses experience low temperature. Similarly,
Temperature
The interaction of insolation with the
atmosphere and the earth’s surface creates
© NCERT
not to be republished
Page 5


SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you
know that we live at the bottom of a
huge pile of air? We inhale and exhale
but we feel the air when it is  in motion. It means
air in motion is wind. You have already learnt
about the fact that earth is surrounded by air
all around. This envelop of air is atmosphere
which is composed of numerous gases. These
gases support life over the earth’s surface.
The earth receives almost all of its energy
from the sun. The earth in turn radiates back
to space the energy received from the sun. As
a result, the earth neither warms up nor does
it get cooled over a period of time. Thus, the
amount of heat received by different parts of
the earth is not the same. This variation causes
pressure differences in the atmosphere. This
leads to transfer of heat from one region to the
other by winds. This chapter explains the
process of heating and cooling of the
atmosphere and the resultant temperature
distribution over the earth’s surface.
SOLAR RADIATION
The earth’s surface receives most of its energy
in short wavelengths. The energy received by
the earth is known as incoming solar radiation
which in short is termed as insolation.
As the earth is a geoid resembling a sphere,
the sun’s rays fall obliquely at the top of the
atmosphere and the earth intercepts a very
small portion of the sun’s energy. On an
average the earth receives 1.94 calories per sq.
cm per minute at the top of its atmosphere.
The solar output received at the top of the
atmosphere varies slightly in a year due to the
variations in the distance between the earth and
the sun. During its revolution around the sun,
the earth is farthest from the sun (152 million
km) on 4th July. This position of the earth is
called aphelion. On 3rd January, the earth is
the nearest to the sun (147 million km). This
position is called perihelion. Therefore, the
annual insolation received by the earth on 3rd
January is slightly more than the amount
received on 4th July. However, the effect of this
variation in the solar output is masked by
other factors like the distribution of land and
sea and the atmospheric circulation. Hence, this
variation in the solar output does not have
great effect on daily weather changes on the
surface of the earth.
Variability of Insolation at
the Surface of the Earth
The amount and the intensity of insolation vary
during a day, in a season and in a year.  The factors
that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii)  the angle of
inclination of the sun’s rays; (iii) the length of the
day; (iv) the transparency of the atmosphere; (v)
the configuration of land in terms of its aspect.
The last two however, have less influence.
The fact that the earth’s axis makes an angle
of 66 with the plane of its orbit round the sun
has a greater influence on the amount of
insolation received at different latitudes. Note the
variations in the duration of the day at different
latitudes on solstices given in Table 9.1.
CHAPTER
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 80
The second factor that determines the
amount of insolation received is the angle of
inclination of the rays. This depends on the
latitude of a place. The higher the latitude the
less is the angle they make with the surface of
the earth resulting in slant sun rays. The area
covered by vertical rays is always less than the
slant rays. If more area is covered, the energy
gets distributed and the net energy received
per unit area decreases. Moreover, the slant rays
are required to pass through greater depth of
the atmosphere resulting in more absorption,
scattering and diffusion.
Figure 9.1 : Summer Solstice
Latitude 0° 20° 40° 60° 90°
December 22 12h 00m 10h 48m 9h 8m 5h 33m 0
June 21 12 h 13h 12m 14h 52m 18h 27m 6 months
Table 9.1 : Length of the Day in Hours and Minutes on Winter and Summer Solstices in the Northern Hemisphere
colour of the sky are the result of scattering of
light within the atmosphere.
The Passage of Solar Radiation
through the Atmosphere
The atmosphere is largely transparent to short
wave solar radiation. The incoming solar
radiation passes through the atmosphere
before striking the earth’s surface. Within the
troposphere water vapour, ozone and other
gases absorb much of the near infrared
radiation.
Very small-suspended particles in the
troposphere scatter visible spectrum both to
the space and towards the earth surface. This
process adds colour to the sky. The red colour
of the rising and the setting sun and the blue
Spatial Distribution of Insolation
at the Earth’s Surface
The insolation received at the surface varies
from about 320 Watt/m
2
 in the tropics to about
70 Watt/m
2
 in the poles. Maximum insolation
is received over the subtropical deserts, where
the cloudiness is the least. Equator receives
comparatively less insolation than the tropics.
Generally, at the same latitude the insolation
is more over the continent than over the oceans.
In winter, the middle and higher latitudes
receive less radiation than in summer.
HEATING AND COOLING OF ATMOSPHERE
There are different ways of heating and cooling
of the atmosphere.
The earth after being heated by insolation
transmits the heat to the atmospheric layers near
to the earth in long wave form. The air in contact
with the land gets heated slowly and the upper
layers in contact with the lower layers also get
heated. This process is called conduction.
Conduction takes place when two bodies of
unequal temperature are in contact with one
another, there is a flow of energy from the warmer
to cooler body. The transfer of heat continues until
both the bodies attain the same temperature or
the contact is broken. Conduction is important
in heating the lower layers of the atmosphere.
The air in contact with the earth rises
vertically on heating in the form of currents
and further transmits the heat of the
atmsphere. This process of vertical heating of
the atmosphere is known as convection. The
convective transfer of energy is confined only
to the troposphere.
The transfer of heat through horizontal
movement of air is called advection. Horizontal
movement of the air is relatively more important
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SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 81
accumulate or loose heat. It maintains its
temperature. This can happen only if the
amount of heat received in the form of insolation
equals the amount lost by the earth through
terrestrial radiation.
Consider that the insolation received at the
top of the atmosphere is 100 per cent. While
passing through the atmosphere some amount
of energy is reflected, scattered and absorbed.
Only the remaining part reaches the earth
surface. Roughly 35 units are reflected back
to space even before reaching the earth’s
surface. Of these, 27 units are reflected back
from the top of the clouds and 2 units from the
snow and ice-covered areas of the earth. The
reflected amount of radiation is called the
albedo of the earth.
The remaining 65 units are absorbed, 14
units within the atmosphere and 51 units by
the earth’s surface. The earth radiates back
51 units in the form of terrestrial radiation.
Of these, 17 units are radiated to space
directly and the remaining 34 units are
absorbed by the atmosphere (6 units
absorbed directly by the atmosphere, 9 units
through convection and turbulence and 19
units through latent heat of condensation).
48 units absorbed by the atmosphere
(14 units from insolation +34 units from
than the vertical movement. In middle latitudes,
most of dirunal (day and night) variation in
daily weather are caused by advection alone.
In tropical regions particularly in northern
India during summer season local winds called
‘loo’ is the outcome of advection process.
Terrestrial Radiation
The insolation received by the earth is in short
waves forms and heats up its surface. The earth
after being heated itself becomes a radiating
body and it radiates energy to the atmosphere
in long wave form. This energy heats up the
atmosphere from below. This process is known
as terrestrial radiation.
The long wave radiation is absorbed by the
atmospheric gases particularly by carbon
dioxide and the other green house gases. Thus,
the atmosphere is indirectly heated by the
earth’s radiation.
The atmosphere in turn radiates and
transmits heat to the space. Finally the amount
of heat received from the sun is returned to
space, thereby maintaining constant temperature
at the earth’s surface and in the atmosphere.
Heat Budget of the Planet Earth
Figure 9.2 depicts the heat budget of the planet
earth. The earth as a whole does not
Figure 9.2 : Heat budget of the earth
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 82
terrestrial radiation) are also radiated back
into space. Thus, the total radiation
returning from the earth and the atmosphere
respectively is 17+48=65 units which
balance the total of 65 units received from
the sun. This is termed the heat budget or
heat balance of the earth.
This explains, why the earth neither warms
up nor cools down despite the huge transfer of
heat that takes place.
Variation in the Net Heat Budget at the
Earth’s Surface
As explained earlier, there are variations in the
amount of radiation received at the earth’s
surface. Some part of the earth has surplus
radiation balance while the other part has
deficit.
Figure 9.3 depicts the latitudinal variation
in the net radiation balance of the earth — the
atmosphere system. The figure shows that
there is a surplus of net radiation balance
between 40 degrees north and south and the
regions near the poles have a deficit. The
surplus heat energy from the tropics is
redistributed pole wards and as a result the
tropics do not get progressively heated up due
to the accumulation of excess heat or the high
latitudes get permanently frozen due to excess
deficit.
Figure 9.3 : Latitudinal variation in net
    radiation balance
heat which is measured in terms of
temperature. While heat represents the
molecular movement of particles comprising a
substance, the temperature is the measurement
in degrees of how hot (or cold) a thing (or a
place) is.
Factors Controlling Temperature Distribution
The temperature of air at any place is influenced
by  (i) the latitude of the place;  (ii) the altitude
of the place; (iii) distance from the sea, the air-
mass circulation; (iv) the presence of warm and
cold ocean currents; (v) local aspects.
The latitude : The temperature of a place
depends on the insolation received. It has been
explained earlier that the insolation varies
according to the latitude hence the
temperature also varies accordingly.
The altitude : The atmosphere is indirectly
heated by terrestrial radiation from below.
Therefore, the places near the sea-level record
higher temperature than the places situated
at higher elevations. In other words, the
temperature generally decreases with
increasing height. The rate of decrease of
temperature with height is termed as the
normal lapse rate. It is 6.5°C per 1,000 m.
Distance from the sea : Another factor that
influences the temperature is the location of a
place with respect to the sea. Compared to land,
the sea gets heated slowly and loses heat
slowly. Land heats up and cools down quickly.
Therefore, the variation in temperature over the
sea is less compared to land. The places
situated near the sea come under the
moderating influence of the sea and land
breezes which moderate the temperature.
Air-mass and Ocean currents : Like the land
and sea breezes, the passage of air masses also
affects the temperature. The places, which
come under the influence of warm air-masses
experience higher temperature and the places
that come under the influence of cold air-
masses experience low temperature. Similarly,
Temperature
The interaction of insolation with the
atmosphere and the earth’s surface creates
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SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 83
the places located on the coast where the warm
ocean currents flow record higher temperature
than the places located on the coast where the
cold currents flow.
Distribution of Temperature
The global distribution of temperature can well
be understood by studying the temperature
distribution in January and July. The
temperature distribution is generally shown
on the map with the help of isotherms. The
Isotherms are lines joining places having equal
temperature. Figure 9.4 (a) and (b) show the
distribution of surface air temperature in the
month of January and July.
In general the effect of the latitude on
temperature is well pronounced on the map,
as the isotherms are generally parallel to the
latitude.  The deviation from this general trend
is more pronounced in January than in July,
especially in the northern hemisphere. In the
northern hemisphere the land surface area is
much larger than in the southern hemisphere.
Hence, the effects of land mass and the ocean
currents are well pronounced. In January the
isotherms deviate to the north over the ocean
and to the south over the continent. This can
be seen on the North Atlantic Ocean. The
presence of warm ocean currents, Gulf Stream
and North Atlantic drift, make the Northern
Atlantic Ocean warmer and the isotherms bend
towards the north. Over the land the
temperature decreases sharply and the
isotherms bend towards south in Europe.
It is much pronounced in the Siberian
plain. The mean January temperature along
60° E longitude is minus 20° C both at 80° N
and 50° N latitudes. The mean monthly
temperature for January is over 27° C, in
equatorial oceans over 24° C in the tropics
and 2° C - 0° C in the middle latitudes
and –18° C to –48° C in the Eurasian
continental interior.
Figure 9.4 (a) : The distribution of surface air temperature in the month of January
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