Chapter-9- SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE
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.
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.
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.(important )
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 .
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.
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 colour of the sky are the result of
scattering of light within the atmosphere.
Spatial Distribution of Insolation at the Earth’s Surface
The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about
70 Watt/m2 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 atmosphere. 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 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
The earth as a whole does not 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
why the earth neither warms up nor cools down despite the huge transfer of heat that takes
place??
Consider that the insolation received at the top of the atmosphere is 100 per cent.
Roughly 35 units are reflected back to space even before reaching the earth’s
surface. (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
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
Variation in the Net Heat Budget at the Earth’s Surface
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.
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.
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 (important )
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.
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 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.
3. 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.
4. 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, 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 Isotherms are lines joining places having equal temperature.
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.
The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more
or less parallel to the latitudes and the variation in temperature is more gradual than in the
northern hemisphere. The isotherm of 20° C, 10° C, and 0° C runs parallel to 35° S, 45° S and 60°
S latitudes respectively.
In July the isotherms generally run parallel to the latitude. The equatorial oceans record
warmer temperature, more than 27°C. Over the land more than 30°C is noticed in the
subtropical continental region of Asia, along the 30° N latitude. Along the 40° N runs the
isotherm of 10° C and along the 40° S the temperature is 10° C.
INVERSION OF TEMPERATURE
Normally, temperature decreases with increase in elevation. It is called normal lapse rate.
the situations 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 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.
Over polar areas, temperature inversion is normal throughout the year.
Surface inversion promotes stability in the lower layers of the atmosphere. Smoke
and dust particles get collected beneath the inversion layer and spread horizontally
to fill the lower strata of the atmosphere.
Dense fogs in mornings are common occurrences especially during winter season.
This inversion commonly lasts for few hours until the sun comes up and beings to
warm the earth.
The inversion takes place in hills and mountains due to air drainage. Cold air at the
hills and mountains, produced during night, flows under the influence of gravity.
Being heavy and dense, the cold air acts almost like water and moves down the slope to
pile up deeply in pockets and valley bottoms with warm air above. This is called air
drainage. It protects plants from frost damages.
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