Heat Budget | Geography for UPSC CSE PDF Download

Heat budget of the Earth

Heat Budget of the Earth balances incoming solar insolation and outgoing terrestrial radiation which maintains the average annual temperature of the earth at 14-degree celsius.

Solar energy received by earth’s surface is called solar insolation.

Heat Budget of the Earth and Atmosphere

• The total solar radiation reaching a horizontal surface on the ground is called global radiation. 
• It comprises the direct shortwave radiation from the Sun + the diffuse radiation scattered by the atmosphere.
• It may be pointed out that the solar energy received at the earth’s surface is converted into heat energy, which heats the earth's outer surface.
• Thus, the earth, after being heated, also radiates energy in the form of long-wave radiation.
• The Radiation from the sun towards the earth is called incoming shortwave solar radiation and from the earth towards the atmosphere is called
  outgoing longwave terrestrial radiation.

  Question for Heat Budget

  Try yourself:What is the average annual temperature of the earth?

  • A.

    14 degrees celsius
  • B.

    15 degrees celsius
  • C.

    13 degrees celsius
  • D.

    16 degree celsius

  Explanation

  Mean annual temperature of the earth is 14 degrees celsius. However, due to global warming and climate change, it is expected to reach 15.6 degrees celsius in the near future if global emissions are not controlled.

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➤ How is it calculated?

Suppose incoming solar insolation is = 100 units Amount lost through scattering and reflection.

a) Through Clouds- 27units

b) By dust particles – 6units

c) By Ice Caps and Glaciers- 2units

Total of 35 units are reflected into space. (known as albedo of the earth) Now, the units received by earth and its atmosphere = 100 – 35 = 65units.

➤ Heat budget of the Earth

The earth receives 51 units of solar insolation as direct radiation which can be segregated as follows:

• Received through direct Radiation = 34units
• Received as diffused daylight =. 17units

Which comes out to be 51 units.

➤ Heat Budget of Atmosphere

• Absorption by atmospheric gases in different vertical zones of atmosphere-14 units
  Now 51 units + 14 units= 65 units (total solar insolationreceived by earth andatmosphere)
• Out of the solar radiation received directly by earth i.e. 51 units, 17 units are re-radiated back into outer space, and rest 34 units (51-17units) is absorbed by the atmosphere in the form of outgoing terrestrial radiation.
  Which comes out to be 48 units (14+ 34 = 48)

➤ Albedo

• Albedo can be defined as measuring how much light that hits a surface is reflected without being absorbed.
• It is a reflection coefficient and has a value of less than one.
• When the solar radiation passes through the atmosphere, some amount is reflected, scattered, and absorbed.
• The reflected amount of radiation is called the albedo of the earth.
• Because of albedo's effect, highly developed areas such as urban cities can experience higher average temperatures than the surrounding suburban or rural areas, a phenomenon known as the Urban Heat Island Effect.
• The higher average temperature can be attributed to less vegetation, higher population densities, and more infrastructures with dark surfaces (asphalt roads, brick buildings, etc.).