It has been found that the yearly mean temperature of the earth remains constant. The incoming short- wave solar radiation is balanced by an equal amount of long-wave Earth radiation going back to space. This is termed heat balance or heat budget of the Earth.

The radiation from the sun towards the earth and from the earth towards the atmosphere and the space is called incoming short-wave solar radiation (radiation from the sun) and outgoing long-wave terrestrial radiation (from the earth) respectively.

Incoming Shortwave Solar Radiation and the Heat Budget:

If total incoming solar radiation is taken as 100 unit or 100% then out of his 100%, 35% is sent back to space through scattering by dust particles (6%) reflection from the cloud (27%) and from the surface (2%), 51% is received by the earth’s surface (received as direct radiation) and 14% is absorbed by the atmospheric gases and water vapour in different zones of the atmosphere.


The 51% solar energy received by the earth comprises 34% as direct solar radiation and 17% as diffuse day light. The heat budget of atmosphere comprises 48% of solar radiation where in 14% is received through absorption of the short-wave incoming solar radiation and 34% is received from the outgoing long-wave terrestrial radiation.

It can be summarised as:

(i) Incoming short-wave Solar Radiation = 100%

(ii) Amount lost to space through scattering and reflection.


(a) Reflected by the clouds = 27%

(b) Reflected by the ground = 2% 35%

(c) Scattered energy lost to space = 6%

Remaining Solar energy = 65%


[I] Heat Budget of the Earth

(i) Received through direct radiation =34%

(ii) Received as diffuse day light = 17%

Total =51%


[II] Heat Budget of the Atmosphere

(i) Absorption of incoming solar radiation =14%

(ii) Terrestrial radiation = 34%

Total = 48%