Heat Budget of Earth

 

Solar Radiation & Heat Budget of Earth

Sun is the supreme source of heat and light on the surface of the Earth. Most of the energy received by earth’s surface known as insolation (incoming solar radiation) is in the form of short wavelengths. The atmosphere is largely transparent to short wave solar radiation. The insolation received is not same at all the places over the globe.

Factors Causing Variability of Insolation:-

ü Rotation of earth about its own axis.

ü Angle of inclination of sun’s rays associated with latitude of a place.

ü The length of the day.

ü The transparency of the atmosphere.

ü The configuration of land in terms of its aspect.

ü Tilt of the Earth

Spatial Distribution of Insolation:-

The insolation received at the tropics is about 320 Watt/m² and 70 Watt/m² at the poles. Maximum insolation is received at the subtropical deserts due to less cloudiness (clouds absorb the radiations). Equator receives less insolation as compared to the tropics due to presence of clouds. Further, at the same latitude, the insolation is more over the continents and less over the oceans.

Heating and Cooling of Atmosphere:-

There are four ways in which the atmosphere heats and cools namely-

ü Conduction: - Conduction is the process of flow of heat, provided the two bodies are in contact and these bodies have unequal temperature. The direction of flow of heat is from hotter to colder body and the process continues until contact is broken or both the bodies attain same temperature. The lower layers of the atmosphere are heated by this process. The layer of air just above the surface of earth gets heated through the process of conduction and so does the layer of air just above this layer.

ü Convection: - The process of vertical transfer of heat occurs through convection. The air that rises after the process of conduction in the form of air currents further transmits heat to the atmosphere. The process of convection is limited to troposphere (first layer of atmosphere).

ü Advection: - The horizontal transfer of heat occurs through the process of advection. Horizontal transfer of heat is more significant than vertical transfer. The weather conditions in middle latitude and ‘loo’ in northern India is a result of advection.

ü Terrestrial radiation: - The earth after receiving heat from the sun in short wave form gets heated up and itself becomes a radiating body. It radiates back the heat in the form of long wavelengths to the atmosphere and outer space. Thus the atmosphere is indirectly heated by the earth’s radiation known terrestrial radiation. It is also called effective radiation because the lower part of atmosphere gets heat through it.  

Heat Budget of Earth:-

Most of the terrestrial radiation is absorbed by carbon dioxide and other gases in the atmosphere and the remaining is radiated back to the space thus maintaining a natural balance of heat. This is known as heat budget of the earth. It is worth mentioning that balance doesn’t  mean that each unit of heat is radiated back. Let’s see how the heat budget is exactly maintained.

(a)             Incoming short wave solar radiation

The solar energy radiated towards the earth is taken as 100 percent or 100 units. Out of the total insolation 35% is radiated back to space through scattering- dust particles (6%), reflection from clouds (27%) and from ground surface (2%), 51% is received by the earth’s surface as direct radiation, 14% is absorbed by the atmospheric gases and water vapors in different vertical zones. The 51% solar energy received by the earth comprises 34% as direct solar radiation and 17% as diffused day light.

(b)            Outgoing long wave radiation

23% heat energy out of 51% that the Earth has gained from sun is lost through direct long wave outgoing terrestrial radiation. Out of this 23%, 6% is absorbed by the atmosphere and 17% goes directly to the space. About 9% of terrestrial energy is spent in convection and turbulence and 19% is spent through evaporation which is added to the atmosphere as latent heat of condensation. Thus the total energy received by the atmosphere becomes 48% which is re radiated to space in one way or the other.

Incoming solar radiation                                   percentage

Total amount of energy at top of atmosphere            100

Diffused to space by scattering                                  5

Reflection from clouds to space                                21

Direct reflection from earth’s atmosphere                 6

Absorbed by molecules, dust, water vapor, CO₂,

clouds etc                                                                    18

Absorbed by earth’s surface                                      50

Sum of absorption and reflection                              100

Outgoing long wave terrestrial radiation        percentage

Loss of energy directly to the space                             8

Loss of energy to atmosphere                                      90

Net outgoing radiation from earth’s surface                 21

Gain of energy from the radiation from surface           90

Loss of energy through counter radiation from surface 77

Net gain of energy from earth’s surface                        13

Gain of energy from direct shortwave absorption         18

Gain of energy from latent heat transfer                       20

Gain of energy from mechanical heat transfer             9

Total net gain by the atmosphere                                 60

Radiated to space from atmosphere                            68

Radiated directly from earth’s surface                       8

Total radiation from planet to space                         68

Source of data- W.D. Sellers