Study Notes on Heat Budget

Heat Budget

The Earth's climate is a solar powered system. The earth intercepts only ½ of the billionth fraction of the energy radiated from the Sun. The Earth in turn radiates back the energy received from the sun in the form of long wave terrestrial radiations. As a result, the earth neither warms up nor does it get cooled over a period of time. It maintains its temperature. This can happen only because the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation. This is known as heat budget of the earth. Let the solar energy radiated be taken as 100 units. Out of total incoming radiation entering the earth's atmosphere 35 per cent is sent back to space through scattering by dust particles (6%), reflection from clouds (27%) and from the ground surface (2%), 51 percent is received by the earth's surface and 14 per cent is absorbed by the atmospheric gases and water vapour.

Incoming shortwave solar radiation: equals to 100 units

a) Amount lost to space through scattering and reflection equals to 35% comprises of

• Clouds = 27%

• Reflected by ground = 2%

• Scattered by dust particles = 6%

b) Heat received by earth equals to 51% comprises of

• Through direct radiation = 34%

• Received as diffuse day light = 17%

c) Absorption by the atmospheric gases and water vapour equals to 14%

After receiving energy from the Sun the Earth also radiates energy out of its surface into the atmosphere through long-wave radiation. As we have seen above 51 per cent of heat is absorbed by the earth. Thus at the time of outgoing radiations 23 per cent of the energy is lost through direct long-wave terrestrial radiation (in which 6% absorbed by atmosphere and 17% goes directly to space). About 9 per cent out of 51 per cent is spent in convection and 19 per cent is spent through evaporation. Thus the total energy received by atmosphere from sun and the earth becomes 48 per cent. 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.

Outgoing long-wave terrestrial radiation

a) Reflected by Earth which was equal to 51 per cent as shown above

• 23% from radiation

• 9% through convection

• 19% through evaporation

b) 48% absorbed in atmosphere moved through radiation back into space.