Insolation and Heat Budget

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Insolation and Heat Budget

1. Insolation is the energy received on the earth’s surface from the sun. It is the most important single source of atmospheric heat.

2. The earth’s surface does not absorb all the energy that it receives. The proportion of the solar radiation reflected from the surface is called Albedo.

3. On an average, insolation is highest near the tropics, marginally lower at the equator and lowest at the poles.

4. Although the earth receives energy continuously from the sun, its temperature remains fairly constant, the only variations being the long-term climatic changes.

This is so because the atmosphere loses an amount of heat equal to the gain through insolation. This mechanism of maintaining the same temperature by the atmosphere is called the Heat Budget or Heat Balance.

5. Assuming that 100 units of energy reach the top of the atmosphere of the earth, 14 units are absorbed directly by the atmosphere and 35 units are lost to space through reflection.

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The remaining 51 units reach the earth’s surface and are absorbed by the earth due to which the surface gets heated. The heated surface of the earth starts radiating energy in the form of long waves and this process is called Terrestrial Radiation.

Out of the total 51 units given up by the surface in the form of terrestrial radiation, the atmosphere (mainly carbon dioxide and water vapour) absorbs about 34 units and the remaining 17 units escape to space.

In this manner, the atmosphere receives a total of 14 + 34 = 48 units and this amount is radiated back to space by the atmosphere. The total loss of energy to space thus amounts to 100 units: 35 units reflected by the atmosphere, 17 units lost as terrestrial radiation and 48 units from the atmosphere. In this manner, no net gain or loss of energy occurs in the earth’s surface.

6. Although the earth and its atmosphere as a whole have a radiation balance, there are latitudinal variations. The heat/energy is transferred from the lower latitudes to the higher latitudes through winds and ocean currents.
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