CHAPTER
SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you The solar output received at the top of the
know that we live at the bottom of a atmosphere varies slightly in a year due to the
huge pile of air? We inhale and exhale variations in the distance between the earth and
but we feel the air when it is in motion. It means the sun. During its revolution around the sun,
air in motion is wind. You have already learnt the earth is farthest from the sun (152 million
about the fact that earth is surrounded by air km) on 4th July. This position of the earth is
all around. This envelop of air is atmosphere called aphelion. On 3rd January, the earth is
which is composed of numerous gases. These the nearest to the sun (147 million km). This
gases support life over the earth’s surface. position is called perihelion. Therefore, the
The earth receives almost all of its energy annual insolation received by the earth on 3rd
from the sun. The earth in turn radiates back January is slightly more than the amount
to space the energy received from the sun. As received on 4th July. However, the effect of this
a result, the earth neither warms up nor does variation in the solar output is masked by
it get cooled over a period of time. Thus, the other factors like the distribution of land and
amount of heat received by different parts of sea and the atmospheric circulation. Hence, this
the earth is not the same. This variation causes variation in the solar output does not have
pressure differences in the atmosphere. This great effect on daily weather changes on the
leads to transfer of heat from one region to the surface of the earth.
other by winds. This chapter explains the
Variability of Insolation at
process of heating and cooling of the
the Surface of the Earth
atmosphere and the resultant temperature
distribution over the earth’s surface. The amount and the intensity of insolation vary
during a day, in a season and in a year. The factors
SOLAR RADIATION that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii) the angle of
The earth’s surface receives most of its energy inclination of the sun’s rays; (iii) the length of the
in short wavelengths. The energy received by day; (iv) the transparency of the atmosphere; (v)
the earth is known as incoming solar radiation the configuration of land in terms of its aspect.
which in short is termed as insolation. The last two however, have less influence.
As the earth is a geoid resembling a sphere, The fact that the earth’s axis makes an angle
the sun’s rays fall obliquely at the top of the of 66½ with the plane of its orbit round the
atmosphere and the earth intercepts a very sun has a greater influence on the amount of
small portion of the sun’s energy. On an insolation received at different latitudes.
average the earth receives 1.94 calories per sq. The second factor that determines the
cm per minute at the top of its atmosphere. amount of insolation received is the angle of
2022-23
, 76 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
inclination of the rays. This depends on the insolation is received over the subtropical
latitude of a place. The higher the latitude deserts, where the cloudiness is the least.
the less is the angle they make with the surface Equator receives comparatively less insolation
of the earth resulting in slant sun rays. The than the tropics. Generally, at the same
area covered by vertical rays is always less latitude the insolation is more over the
than the slant rays. If more area is covered, continent than over the oceans. In winter, the
the energy gets distributed and the net energy middle and higher latitudes receive less
received per unit area decreases. Moreover, radiation than in summer.
the slant rays are required to pass through
greater depth of the atmosphere resulting in HEATING AND COOLING OF ATMOSPHERE
more absorption, scattering and diffusion.
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
Figure 9.1 : Summer Solstice temperature or the contact is broken. Conduction
is important in heating the lower layers of the
The Passage of Solar Radiation atmosphere.
through the Atmosphere The air in contact with the earth rises
The atmosphere is largely transparent to short vertically on heating in the form of currents
wave solar radiation. The incoming solar and further transmits the heat of the
radiation passes through the atmosphere atmsphere. This process of vertical heating
before striking the earth’s surface. Within the of the atmosphere is known as convection.
troposphere water vapour, ozone and other The convective transfer of energy is confined
gases absorb much of the near infrared only to the troposphere.
radiation. The transfer of heat through horizontal
Very small-suspended particles in the movement of air is called advection. Horizontal
troposphere scatter visible spectrum both to movement of the air is relatively more
the space and towards the earth surface. This important than the vertical movement. In
process adds colour to the sky. The red colour middle latitudes, most of dirunal (day and
of the rising and the setting sun and the blue night) variation in daily weather are caused
colour of the sky are the result of scattering by advection alone. In tropical regions
of light within the atmosphere. particularly in northern India during summer
season local winds called ‘loo’ is the outcome
Spatial Distribution of Insolation of advection process.
at the Earth’s Surface
Terrestrial Radiation
The insolation received at the surface varies
from about 320 Watt/m2 in the tropics to The insolation received by the earth is in short
about 70 Watt/m2 in the poles. Maximum waves forms and heats up its surface. The
2022-23
SOLAR RADIATION, HEAT BALANCE
AND TEMPERATURE
D
o you feel air around you? Do you The solar output received at the top of the
know that we live at the bottom of a atmosphere varies slightly in a year due to the
huge pile of air? We inhale and exhale variations in the distance between the earth and
but we feel the air when it is in motion. It means the sun. During its revolution around the sun,
air in motion is wind. You have already learnt the earth is farthest from the sun (152 million
about the fact that earth is surrounded by air km) on 4th July. This position of the earth is
all around. This envelop of air is atmosphere called aphelion. On 3rd January, the earth is
which is composed of numerous gases. These the nearest to the sun (147 million km). This
gases support life over the earth’s surface. position is called perihelion. Therefore, the
The earth receives almost all of its energy annual insolation received by the earth on 3rd
from the sun. The earth in turn radiates back January is slightly more than the amount
to space the energy received from the sun. As received on 4th July. However, the effect of this
a result, the earth neither warms up nor does variation in the solar output is masked by
it get cooled over a period of time. Thus, the other factors like the distribution of land and
amount of heat received by different parts of sea and the atmospheric circulation. Hence, this
the earth is not the same. This variation causes variation in the solar output does not have
pressure differences in the atmosphere. This great effect on daily weather changes on the
leads to transfer of heat from one region to the surface of the earth.
other by winds. This chapter explains the
Variability of Insolation at
process of heating and cooling of the
the Surface of the Earth
atmosphere and the resultant temperature
distribution over the earth’s surface. The amount and the intensity of insolation vary
during a day, in a season and in a year. The factors
SOLAR RADIATION that cause these variations in insolation are : (i)
the rotation of earth on its axis; (ii) the angle of
The earth’s surface receives most of its energy inclination of the sun’s rays; (iii) the length of the
in short wavelengths. The energy received by day; (iv) the transparency of the atmosphere; (v)
the earth is known as incoming solar radiation the configuration of land in terms of its aspect.
which in short is termed as insolation. The last two however, have less influence.
As the earth is a geoid resembling a sphere, The fact that the earth’s axis makes an angle
the sun’s rays fall obliquely at the top of the of 66½ with the plane of its orbit round the
atmosphere and the earth intercepts a very sun has a greater influence on the amount of
small portion of the sun’s energy. On an insolation received at different latitudes.
average the earth receives 1.94 calories per sq. The second factor that determines the
cm per minute at the top of its atmosphere. amount of insolation received is the angle of
2022-23
, 76 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
inclination of the rays. This depends on the insolation is received over the subtropical
latitude of a place. The higher the latitude deserts, where the cloudiness is the least.
the less is the angle they make with the surface Equator receives comparatively less insolation
of the earth resulting in slant sun rays. The than the tropics. Generally, at the same
area covered by vertical rays is always less latitude the insolation is more over the
than the slant rays. If more area is covered, continent than over the oceans. In winter, the
the energy gets distributed and the net energy middle and higher latitudes receive less
received per unit area decreases. Moreover, radiation than in summer.
the slant rays are required to pass through
greater depth of the atmosphere resulting in HEATING AND COOLING OF ATMOSPHERE
more absorption, scattering and diffusion.
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
Figure 9.1 : Summer Solstice temperature or the contact is broken. Conduction
is important in heating the lower layers of the
The Passage of Solar Radiation atmosphere.
through the Atmosphere The air in contact with the earth rises
The atmosphere is largely transparent to short vertically on heating in the form of currents
wave solar radiation. The incoming solar and further transmits the heat of the
radiation passes through the atmosphere atmsphere. This process of vertical heating
before striking the earth’s surface. Within the of the atmosphere is known as convection.
troposphere water vapour, ozone and other The convective transfer of energy is confined
gases absorb much of the near infrared only to the troposphere.
radiation. The transfer of heat through horizontal
Very small-suspended particles in the movement of air is called advection. Horizontal
troposphere scatter visible spectrum both to movement of the air is relatively more
the space and towards the earth surface. This important than the vertical movement. In
process adds colour to the sky. The red colour middle latitudes, most of dirunal (day and
of the rising and the setting sun and the blue night) variation in daily weather are caused
colour of the sky are the result of scattering by advection alone. In tropical regions
of light within the atmosphere. particularly in northern India during summer
season local winds called ‘loo’ is the outcome
Spatial Distribution of Insolation of advection process.
at the Earth’s Surface
Terrestrial Radiation
The insolation received at the surface varies
from about 320 Watt/m2 in the tropics to The insolation received by the earth is in short
about 70 Watt/m2 in the poles. Maximum waves forms and heats up its surface. The
2022-23
