Heat Budget of the Earth

What is heat budget of the earth ?

Heat Budget of the Earth is the balance between incoming solar  insolation and outgoing terrestrial radiation which maintains the average annual temperature of earth at 15 degree celcius.

What do you mean by solar insolation ?

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

What are the factors that affect solar insolation ?

The amount of solar insolation decreases as we move towards poles from equator . Few factors responsible are mentioned below :

a. Angle of the Sun’s Rays : The sun’s rays are verticle (90 Degree ) at the equator which becomes slanting as we move pole wards. This is due to decrease in angle of sun’s rays reaching earth.As a result of which equator receives more solar insolation as compared to poles. Moreover, vertical rays spread over minimum surface area( in equator) thereby increasing the energy received per unit area. Oblique rays spread over larger area(in poles) of earth’s surface, thereby decreasing energy received per unit area.

b. Sunspots : The solar energy radiated from sun is maximum when solar spots number increases. Sunspots increase and decrease in a cycle of 11 years. In other words, the sun spots are maximum every 11th year.

What is terrestrial radiation ?

The energy which is received by earth in the form of short waves are radiated back into the atmosphere in the form of long waves known as terrestrial radiation. It helps in heating lower portion of atmosphere.

How it is calculated ?

It can be shown by a simple illustration :

First let us understand the amount of solar insolation received by earth and atmosphere separately.

Suppose incoming solar insolation is = 100 units

Amount lost through scattering and reflection

a) Through Clouds- 27 units

b) By dust particles – 6 units

c) By Ice Caps and Glaciers- 2 units

Total 35 units are reflected back into space.(known as albedo of earth)

Now, the units received by earth and its atmosphere = 100- 35 = 65 units

Heat Budget of the Earth

51 units of solar insolation is received by earth as direct radiation which can be segregated as follows:

(i) Received through direct Radiation= 34 units

(ii) Received as diffused day light= 17 units

Which comes out to be 51 units.

Heat Budget of Atmosphere

(i) Absorption by atmospheric gases in different vertical zones of atmosphere- 14 units

Now 51 units + 14 units= 65 units (total solar insolation received by earth and atmosphere)

(ii) 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 (51-17 units) is absorbed by atmosphere in the form of outgoing terrestrial radiation.

Which comes out to be 48 units.(14+ 34 = 48)

Now lets have a look at the outgoing terrestrial radiation and heat balance.

1) Terrestrial Heat Balance

Energy Received Energy Lost
a)      34 units Through direct radiationb)      17 units through diffuse day light

Total= 51 units

a)      23 units by radiation (6 units absorbed by atmosphere in next step, only 17 units escape into space)b)      9 units through convection and turbulence (absorbed by atmosphere in next step)c)       19 units through evaporation (absorbed by atmosphere in next step)

 

Total= 51 units

 

2) Atmospheric Heat Balance

Energy Received Energy Lost
a)      14 units through absorbtion of incoming solar insolationb)      6 units through terrestrial radiation ( explained above in terrestrial heat balance step)c)       9 units through convection and turbulence  ( explained above in terrestrial heat balance step)

d)      19 units through evaporation (explained above in terrestrial heat balance step)

Total= 48 units

48 units radiated into the space

Energy sent back to space= 35 units + 17 units (through radiation from earth) + 48 units (through radiation from atmosphere)

Since atmosphere is a fluidic medium, the flow of energy through it maintains the average annual temperature of earth at 15 degree celcius.

However the above illustration is to make the readers understand the heat budget concept clearly. In actual, mechanism of solar and terrestrial radiation is quite complex.

What do you mean by Albedo ?

Albedo may be defined as the reflectivity of earth.

It can be calculated by a simple formula :

Albedo = ( Reflected solar insolation /  Total solar insolation) * 100

from the above illustration,

Albedo= (35/100)*100 = 35 unit

We can arrange the following in decreasing order of albedo.

Ice caps/ snow > Barren Land > Crop > Forest > Water

Hence, Antarctica (due to predominance of ice caps) has maximum albedo on earth, and equatorial region has lowest albedo on earth.

In upcoming articles we shall discuss about latitudinal Heat Budget and Oceanic Heat Budget.

 

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  1. By bj

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