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From the foregoing discussion it becomes clear that the earth has been continuously receiving solar energy. Most of it is received at the earth’s surface in the form of short-wave solar radiation where it is converted into heat by the process of absorption.

It is also true that through long-wave terrestrial radiation the earth sends the same amount of energy back into space, except for a small amount of energy stored fossil fuels.

The earth-atmosphere system receives on an average 0.30 calorie of heat per square centimeter per minute. But the most interesting thing is that there exists a balance between the amount of incoming solar radiation and the amount of terrestrial radiation returned to space.

In the absence of such a balance the earth would be getting progressively colder or progressively warmer. This balance between the amount of insolation received from the sun and the outgoing terrestrial radiation is known as the earth’s heat budget.

Diagrammatically illustrates this balance by presuming 100 units to represent the total amount of incoming solar radiation received at the outer boundary of our atmosphere.

This exchange of energy between the earth and the sun is made possible by the complex processes of radiation. That is why the mean temperature of the earth remains constant without any substantial variation whatsoever.

On the right side in the figure the details of the incoming solar radiation, and on the left side, the details of the outgoing terrestrial radiation have been represented.

What is to be kept in mind is that the energy statistics used in the earth’s heat budget are all based on estimates, and secondly, the calculations made in this regard by different scientists may also differ slightly from one another.

Keeping in view the complexity of the processes of exchange of heat between the earth and the various layers of the atmosphere, such a difference in calculations seems to be quite natural.

Of the total of 100 units of solar radiation reaching the outer atmosphere, about 34 units are returned to space without heating the atmosphere. Of the total 34 units returned to space, 25 units are reflected to space by clouds, 2 units are directly reflected to space by the earth’s land-sea surface and 7 units are scattered by molecules of air and fine dust.

Of the remaining 66 units, 19 units are absorbed within the atmosphere (17 units by gases in the atmosphere and 2 units by clouds).

The remaining 47 units are transmitted through the atmosphere and absorbed at the land- sea surface, out of which 19 units are received at the surface as direct sunlight, 23 units as diffused radiation through clouds, and 5 units as scattered radiation.

The incoming solar radiation absorbed directly by the earth’s land and sea surface forms the largest percentage of the total solar energy intercepted by the entire earth-atmosphere System.

In all, 66 units absorbed by the atmosphere and the earth’s surface become effective in heating the atmosphere. Obviously, the atmosphere gets a larger part of its heat only indirectly.

The fact that the atmosphere is heated mainly from below is of great climatic significance. Since factors like the amount of cloudiness and contrasts in the reflective power of different land surfaces are characterized by temporal and regional variations, the amount of radiant energy absorbed at the earth’s surface shows great temporal and spatial variations.

Besides, the land-water surface serves as a great heat reservoir, because large amount of incident solar radiation is absorbed and stored there to be released consequently to the atmosphere.

If all the solar energy absorbed at the earth’s surface were re- radiated directly back to space, the terrestrial heat budget would be a simple affair. But the matter is complicated.

The earth’s surface radiates as a black body, since it absorbs and radiates with nearly 100 per cent efficiency for its temperatures. Gases, on the contrary, are selective absorbers and radiators.

Thus, the atmosphere which is nearly transparent to certain wave-lengths of radiation is nearly opaque to visible light. However, in long-wave terrestrial radiation, the radiant energy from the earth that is absorbed by the atmosphere is reradiated, mainly upward and downward.

If the earth is warmer than the overlying atmosphere, the atmosphere will receive more terrestrial radiation than it emits.

However, there are occasions when a large portion of the atmosphere is found to be warmer than the ground, and in such a case the net radiation is downward. This is in addition to and at a different wave-length from the radiation coming downward from the sun.

It is thus because of the greenhouse effect of the atmosphere as well as the interplay of radiation between the surface of the earth and the atmosphere that the earth’s land-sea surface radiates more than it receives from the sun. But actually this is but natural in view of the complex streams of up-and-down radiation involved in the atmosphere’s greenhouse effect.

It has already been noted that 66 units of the incoming solar radiation received at the outer margin of the atmosphere is absorbed either by the atmosphere or by the earth’s land-sea surface.

Actually these 66 units of solar radiation (called the effective solar radiation) are available for heating the atmosphere. In order to maintain the terrestrial heat balance, the 66 units of solar radiation gained must be balanced by the same amount of energy radiated back to space in the form of long-wave terrestrial radiation.

The left-hand side of illustrates the operation of the terrestrial heat balance. The earth radiates 120 units of energy upward out of which 6 units are radiated back to space direct. The atmosphere absorbs the remaining 114 units of long-wave earth radiation.

Out of these units, the atmosphere retains only 8 units and the remaining 106 units are re-radiated back to the earth’s surface. Further, 10 units of heat are transported upward by convection or turbulence. Thus, the atmosphere gains these 10 units of heat.

The atmosphere also gains 23 units as latent heat carried to it in the hydrologic cycle (evaporation, condensation, precipitation). 19 units of solar energy were already absorbed directly by the atmosphere.

Hence, the total units absorbed by the atmosphere are equal to 166 (19+114+10+23). Out of 166 units, 106 units are re-radiated back to the earth’s surface and 60 are radiated back to space.

Thus, including 6 units radiated directly to space by the earth’s surface, 66 units of energy are ultimately radiated back to space. In this way, the total incoming solar radiation is balanced by an equal amount of outgoing radiation.

So far the heat budget for the planet as a whole has been considered. But at any location, the heat budget undergoes variation throughout the year according to the seasons.

There is a tendency towards a surplus in the summer and a deficit in the low sun seasons. Seasonal variations may be negligible in the equatorial region, but they are more pronounced in the middle and high-latitude regions.