Heat Budget of The Earth
• The balance between the amount of solar radiation received by the earth’s surface and its atmosphere and the amount of heat lost by the outgoing terrestrial longwave radiation from the earth’s surface and loss of heat from the atmosphere is referred to as heat budget.
•The solar energy received at the earth’s surface is
converted into heat energy which heats the outer
the surface of the earth. The radiation from the sun
towards the earth is called incoming solar radiation
and radiation from the Earth towards the atmosphere and space
is called outgoing longwave terrestrial radiation.
• There is the energy balance of the earth and the atmosphere or global radiation balance
which is referred to the heat budget of the earth and the atmosphere that displays a
statement of receipt of solar radiation by the atmosphere and the earth’s surface and the
loss of energy by the earth and the atmosphere also known as terrestrial radiation.
• The total incident of solar radiation (energy) reaching the outer margin of Earth’s
atmosphere is taken to be 100 units or 100 percent for the explanation of heat/energy
budget of the earth and the atmosphere.
Incoming Shortwave Solar Radiation and the Heat Budget of the Earth and the Atmosphere:
- Most of the radiation energy of the earth is received from the sun through shortwave solar radiation. The solar energy radiated towards the earth’s surface (1/2 billionth part of the total energy radiated from the outer surface- photosphere-of the sun which is equivalent to 23 trillion horsepower) is taken as 100 percent or 100 units.
- Out of the total incoming solar radiation entering the earth’s atmosphere, 35 percent is reflected back to space through scattering by dust particles (6%), reflection from the clouds (27%) and from the ground surface (2%), 51 percent is received by the earth’s surface (received as direct radiation), and 14 percent is absorbed by the atmospheric gases (ozone, oxygen, etc.) and water vapour in different vertical zones of the atmosphere.
- Out of the 51 percent solar energy received by the earth, 34 percent is received as direct solar radiation and 17 percent as diffuse daylight. The heat budget of the atmosphere comprises 48 percent of solar radiation wherein 14 percent is received through absorption of the short-wave incoming solar radiation and 34 percent is received from the outgoing long-wave terrestrial radiation.
Outgoing Long-wave Terrestrial Radiation and Heat Balance:
- The energy received from the sun from the earth also radiates energy out of its surface into the atmosphere through long waves. Terrestrial radiation is also called ‘effective radiation’ as it helps in heating the bottom section of the atmosphere. 23% of the percent of energy (out of 51% of the energy that the earth has gained from the sun) is lost through direct longwave outgoing terrestrial radiation out of which 6 percent is absorbed by the atmosphere and 17 percent goes directly to space.
- About 9 percent of the terrestrial energy is exhausted in convection and turbulence and 19 percent is through evaporation which is further added to the atmosphere as a latent heat of condensation. Thus, the total energy received by the atmosphere from the sun (14%) and the earth (34%) becomes 48 percent which is reradiated to space in one way or the other.
Energy that is remitted back to the space = 35% + 17% (through radiation from the earth) +
48% (through radiation from the atmosphere) = 100%
- It may be pointed out that the mechanism of solar and terrestrial radiation is not as simple as mentioned above, rather it is highly complex. For example, not all the energy received by the atmosphere from the sun and the earth is re-radiated directly to space rather a sizeable amount of energy received by the atmosphere is counter-radiated to the earth’s surface which is again radiated to space and the atmosphere.
- Our planet maintains an ambient temperature because there is a balance between the influx & outflux of incoming solar radiation and outgoing terrestrial radiation.
- Heat budget is a mathematical statement, an expression to help us understand and appreciate how this balance is maintained.
I. Heat Budget: (based on G.T. Trewartha)
- (i) Incoming shortwave solar radiation & the heat budget of the earth & the atmosphere:
- The earth receives most of its energy from the sun through shortwave solar radiation.
- The solar energy radiated towards the earth’s surface is taken as 100% or 100 units. While passing through the atmosphere some amount of energy is reflected, scattered, and absorbed only the remaining part reaches the earth’s surface.
- Roughly 36 units are reflected in space even before reaching the earth’s surface.
- Reflected from the top of clouds 27 units
- Reflected by the atmosphere 06 units
- Reflected by ice field of earth 02 units
- Reflected from the top of clouds 27 units
- The remaining 65 units are absorbed as:
- Absorbed by the atmosphere 14 units
- Absorbed by the earth (scattered + direct radiation) 51 units
- Scattering takes place by gas molecules & dust particles. This takes place in all directions, overall, Earth receives 51 units of radiation which in turn radiates back in the form of terrestrial radiation.
- Radiated to space directly 17 units
- Radiated to atmosphere 34 units
- Where 34 units of radiation absorbed by the atmosphere from terrestrial radiation-
- Absorbed directly 06 units
- Absorbed through convection & turbulence 09 units
- Absorbed through latent heat of condensation 19 units
(ii) Outgoing Longwave terrestrial radiation & heat balance20
- The total units absorbed by the atmosphere are 48 (14 units insolation + 34 units terrestrial radiation). These are radiated back into space. Thus, the total radiation returning from the earth & the atmosphere respectively are:
- Radiation back by Earth 17 units
- Radiated back by atmosphere 48 units
- These returning 65 units balance the total of 65 units received from the sun. This account of incoming & outgoing radiation always maintains the balance of heat on the surface of the earth. This is termed the heat budget a heat balance of the earth.
(iii) Latitudinal Heat Budget
- Although the earth as a whole maintains a balance between incoming solar radiation and outgoing terrestrial radiation. But this is not true when we observe at different latitudes.
- There are some places where the receipt of solar energy is more than the energy lost because the solar energy comes at a faster rate than the terrestrial energy goes out & vis-à-vis.
- This mechanism results in the development of areas of energy surplus and energy deficit.
- There is a SURPLUS of net radiation balance between 400 N & S degrees of the regions near the poles that have a DEFICIT.
- The surplus heat energy from the tropics is redistributed pole wards and as a result, the tropics do not get progressively heated up due to accumulation of excess heat, or the high latitudes get permanently frozen due to excess deficit.
- This transfer of surplus heat from the tropics to the polar region is being performed by atmospheric and oceanic circulations such as winds & ocean currents.
Analysis of Heat Budget
- Balance of earth happens but it’s not instantaneous.
- Redistribution of heat is due to factors like Wind Movement, Ocean Water circulation, and Biological processes.
- Earth’s atmosphere system has a net gain/loss of zero. But balance is not for every latitude. Low latitudes are energy surplus & high latitudes are energy deficit.
- The heat budget mechanism is a delicate process distribution in the mechanism & earth’s component atmosphere system can alter the heat budget. The manifestation of this is climate change and global warming (global warming is a consequence of the Greenhouse effect.
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