- The global heat budget is the balance between the heat receivedby solar radiations and outgoing heat.
- The net difference between the incoming solar radiations and outgoing heat is zero.
- The average temperature of earth is around 15oC which is conducive for the growth of life on earth. Had there been a surplus of heat i.e. the heat trapped by the earth is more than the heat released, than the temperature would have been ever increasing making earth inhabitable.
- Also if the heat released was more than the heat received, the temperature would have gradually reduced again making the earth lifeless.
The global radiation budget has three major components:
- 1) Solar radiation in coming at the outer limits of the atmosphere(Q)
- 2) The planetary albedo(a)
- 3) Outgoing long wave radiation from the earth to space.(L)
Thus the basic form of equation for the earth and its atmosphere is
R = Q (1-a) – L
- Here, R is radiation balance (surplus or deficit) and (1-a) is the percentage of total insolation which is absorbed by the earth and atmosphere.
- Of the total incoming radiation about 26% is reflected by the clouds and gas molecules without heating the air, 4% is reflected to space from the earth’s surface.
- Hence the mean albedo of earth is taken as 30%.
- Almost 20% of insolation is absorbed in the atmosphere by gases, clouds and suspended particles.
- The earth’s surface absorbs 50% of the insolation either directly or after scattering by the clouds and atmosphere.
- Thus, approximately 70% of total insolation is effective in heating the earth and its atmosphere.
- Everybody in the universe radiates at certain wavelength. Higher the temperature of the body, shorter is the wavelength hence higher energy.
- The earth is also a radiating body.
- In contrast to sun which radiates at 6000oC, earth has an average temperature of 15oC.
- It emits terrestrial radiation at much larger wavelength.
- The atmosphere has the ability to absorb a large part of this long wave radiation.
- This ability of atmosphere to allow the short infrared radiations coming from sun to penetrate while absorbing the long wave radiations and not allowing them to escape easily is called Greenhouse effect or in more appropriate terms Atmospheric effect.
- Besides the long wave radiations from the earth’s surface, there is also radiation from cloud layers, gases (especially water vapour and carbon dioxide) and dust to space.
- The total amount of energy reaching the earth over a considerable period of time is equaled to total outward losses.
- However, at a particular place the radiation balance is rarely balanced.
- At lower latitudes there is an annual surplus of energy receive while at higher latitudes there is an annual deficit.
- At higher latitudes the energy received is much lower than the tropics as the radiations received are slant.
- Energy received is lower than the energy radiated.
- While at tropics the case is opposite.
- The complete weather phenomenon takes place in order to distribute this heat on earth.
Note- read about Albedo and insolation as other topic.