About heat budget:
Earth's heat budget is also called Earth's heat balance. The heat budget refers to a detailed mathematical description of insolation (short-wave solar radiation) and terrestrial radiation (long-wave radiation).
Earth neither stores nor loses heat over time, it maintains its temperature. This can only happen when the amount of insolation (short-wave solar radiation) is equal to the long-wave Earth radiation (terrestrial radiation).
Over a long period of time, the difference between total mean insolation and terrestrial irradiance is zero.
The following are the details of the heat budget of the earth:
We assume that the top of the Earth's atmosphere (about 1000 km from the Earth's surface) receives 100 units of insolation.
Some units of insolation are reflected, scattered, and absorbed while passing through the atmosphere, the rest of the insolation reaches the Earth's surface.
About 35 units of insolation are reflected back into space before reaching the Earth's surface.
The remaining 65 units of insolation are absorbed either by the atmosphere or by the Earth's surface. About 14 units of insolation are absorbed by the atmosphere and 51 units of insolation are absorbed by the Earth's surface.
The Earth radiates back 51 units as long-wave terrestrial radiation. Of the 51 units of long-wave terrestrial radiation, 17 units are radiated directly into space and the remaining 34 units are absorbed by the atmosphere. Of the 34 units, 6 units are absorbed directly by the atmosphere, 9 units through convection and turbulence, and 19 units through latent heat of condensation.
A total of 48 units (14 units from insolation and 34 units from terrestrial radiation) are absorbed by the atmosphere.
Thus the total radiation returning from the Earth is about 65 units that were received on the Earth, of which 17 units are radiated directly from the Earth into space and 48 units are radiated from the atmosphere.
This is called the heat balance or heat budget of the earth. This is the reason why the earth neither gets hotter nor colder despite the transfer of excessive heat.
The following diagram explains the heat budget on the earth:
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