Define insolation.
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.
Which factor is responsible for the varied distribution of energy?
As the earth is a Geoid resembling a sphere, the sun‘s rays fall obliquely at the top of the
the atmosphere and the earth intercepts a very small portion of the sun‘s energy.
What is the average amount of energy received by the earth?
On average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
Give the reasons why it is summer when the earth is far away from the sun and winter when it is nearest
to the Sun.
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 the 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
a 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 the 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 around 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 are given in the
Table below.
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 that covered by 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 a greater depth of the
atmosphere resulting in more absorption, scattering, and diffusion.
The incoming radiation is not fully reached the earth's surface. Why?
1. The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation
passes through the atmosphere before striking the earth‘s surface.
2. Within the troposphere water vapor, ozone, and other gases absorb much of the near-infrared
radiation.
3. Very small-suspended particles in the troposphere scatter visible spectrum both to space and
towards the earth's surface.
CHAPTER -9 SOLAR RADIATION, HEAT BALANCE, AND TEMPERATURE
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4. This process adds color to the sky.
5. The red color of the rising and the setting sun and the blue color of the sky are the results of
scattering of light within the atmosphere.
What is the average distribution of insolation on the surface? Give the reasons for such
variation.
Spatial Distribution of Insolation on the Earth’s Surface
The insolation received at the surface varies
from about 320 Watt/m in the tropics to about 70 Watt/min in the poles. Maximum insolation is
received over the subtropical deserts, where the cloudiness is the least. The Equator receives
comparatively less insolation than the tropics. Generally, at the same latitude, the insolation is moreover the continent than over the oceans. In winter, the middle and higher latitudes receive less
radiation than in summer.
HEATING AND COOLING OF ATMOSPHERE
Name the ways of heating the atmosphere.
1. Radiation 2. Conduction 3. Advection 4. convection
1. Horizontal movement of the air is relatively more important than vertical movement.
2. In middle latitudes, most diurnal (day and night) variation in daily weather are caused by
advection alone.
3. In tropical regions particularly in northern India during the summer season local winds called ‗loo‘ is
the outcome of the advection process.
Terrestrial Radiation
1. The insolation received by the earth is in shortwaves forms and heats up its surface.
2. The earth after being heated itself becomes a radiating body and it radiates energy to the
the atmosphere in the long waveform.
3. This energy heats up the atmosphere from below.
4. This process is known as terrestrial radiation.
5. The longwave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and
the other greenhouse gases. Thus, the atmosphere is indirectly heated by the earth‘s radiation.
The atmosphere in turn radiates and transmits heat to space. Finally the amount of heat
received from the sun is returned to space, thereby maintaining a constant temperature at the earth‘s
surface and in the atmosphere.
With the help of a diagram explain the Heat Budget of Planet Earth.
INCOMING SOLAR RADIATION
1. Figure 9.2 depicts the heat budget of the planet earth. The earth as a whole does
Not accumulate or lose heat. It maintains its temperature.
2. 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.
3. Consider that the insolation received at the top of the atmosphere is 100 percent.
4. While passing through the atmosphere some amount of energy is reflected,
scattered and absorbed.
5. Only the remaining part reaches the earth's surface.
6. Roughly 35 units are reflected back to space even before reaching the earth‘s surface.
7. Of these, 27 units are reflected back from the top of the clouds
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8. Only 2 units from the snow and ice-covered areas of the earth.
9. The remaining 65 units are absorbed,
10. 14 units within the atmosphere and 51 units by the earth‘s surface.
TERRESTRIAL RADIATION
1. The earth radiates back 51 units in the form of terrestrial radiation.
2. 17 units are radiated to space directly
3. the remaining 34 units are absorbed by the atmosphere
4.6 units absorbed directly by the atmosphere,
5.9 units through convection and turbulence
6. 19 units through latent heat of condensation
7.48 units absorbed by the atmosphere(14 units from insolation +34 units from
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 balances 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.
What do you mean by ‗Albedo‘?
The reflected amount of radiation is called the albedo of the earth.
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 a 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 polewards 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.
WHAT IS THE DIFFERENCE BETWEEN HEAT AND TEMPERATURE
Temperature
The interaction of insolation with the atmosphere and the earth‘s surface creates
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 the 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.
1.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.
2.The altitude: The atmosphere is indirectly
heated by terrestrial radiation from below.
Therefore, the places near the sea-level record
a 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
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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
concerning 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: 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.
Ocean currents
Similarly, the places located on the coast where the warm ocean currents flow record higher
temperatures than the places located on the coast where the cold currents flow.
Isotherms are lines joining places having an equal
temperature.
Fig no. 9.4 (a) distribution of surface
the temperature in January
Figure 9.4 (a) and (b) show the distribution of
surface air temperature in January
and July.
1. In general the effect of the latitude on
temperature is well pronounced on the map,
2. the isotherms are generally parallel to the
latitude.
3. The deviation from this general trend is more pronounced in January than in July, especially in
the northern hemisphere.
4. In the northern hemisphere the land surface area is much larger than in the southern hemisphere.
5 . Hence, the effects of landmass and 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.
INVERSION OF TEMPERATURE
Normally, temperature decreases with an increase in elevation. It is called the normal lapse rate. At times,
the situation is reversed and the normal lapse rate is inverted. It is called Inversion of temperature.
Inversion is usually of short duration but quite common nonetheless. A long winter night with clear
skies and still air is an ideal situation for inversion. The heat of the day is radiated off during the night,
and by early morning hours, the earth is cooler than the air above.