IAS/ HPAS / Geography Notes / IAS Preparation/ Civil services Preparation / Climate / Solar Radiation and Heat Budget of Planet Earth
Incoming solar radiation received by earth in the form of short wavelengths is called insolation. Earth receives 1.94 calorie per square cm per minute at the top its atmosphere but it varies slightly in a year due to variation in distance between earth and sun.When the earth is farthest from the sun(152 million Kilometre) on 4th July,this position is called aphelion and when the earth is nearer to the sun (147 million Kilometre) on 3rd January then this position is called perihelion.Therefore heat received by the earth is slightly more on 3rd January than the amount received on 4th July.
Variability of Insolation at the surface of the Earth
There are following factors causing variations in insolation are:-
1)the rotation of earth on its axis as earth axis makes an angle of 66 and half degree with the plane of its orbit round the sun and influence the amount of insolation at different latitude.
2)the angle of inclination of the sun’s rays which also depend upon latitude of a place. Higher is the latitude ,lesser will be the angle they will make with surface of earth resulting into slant rays.
3)the length of the day.
4)the transparency of the atmosphere.
Heating and Cooling of Atmosphere
The earth after being heated by insolation released heat in the form of long wave to the atmosphere and these long waves is absorbed by atmospheric gases particularly carbon dioxide and other green gases.This energy heats up the atmosphere from below.This process is known as terrestrial radiation.
It takes place when two bodies of unequal temperature are in contact one another.There is flow of energy from warmer to cooler body.
The vertical movement of heat/energy in the form of currents from surface to the atmosphere is called convection.But this phenomenon is confined to the troposphere.
Horizontal movement of energy in the form of air is called advection.
Heat Budget of the Planet Earth
Consider that insolation received at the top of the atmosphere is 100%.while passing through atmosphere some amount of energy is reflected,scattered and absorbed.35 units are reflected back to the space without reaching surface of earth out of which 27 units are reflected back from the top of the clouds,2 units from the snow and ice covered areas of the earth.The reflected amount of radiation is called the albedo.
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 remaining 34 units absorbed by atmosphere (6 units absorbed directly by the atmosphere,9 units through convection and turbulence, 19 units through latent heat of condensation.48 units absorbed by the atmosphere(14 units from insolation+34 units from terrestrial radiation) are also radiate back to the surface.
35%=reflected back without reaching surface of the earth=27%(clouds)+2%(shining surface of the earth)+6%(Scattering)=Albedo
65%=absorbed by the atmosphere(14%)+absorbed by the earth’s surface(51%)
51%=radiated by surface of earth in the form of terrestrial radiation=17%(radiated directly to the space)+34%(absorbed by the atmosphere=6% absorbed by the atmosphere+9% through convection and turbulence+19% through latent heat of condensation)
48% =absorbed by the atmosphere(14% from insolation+34% from terrestrial radiation)
Hence,total radiation returning from the earth and the atmosphere is 17+48=65% which balance the total of 65% received from the sun.This termed as heat budget of the earth.
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