NCERT Book Class 11 Geography (Fundamental of Physical Geography) Chapter 8 Solar Radiation, Heat balance and Temperature

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