Tuesday, October 27, 2009

GLOBAL RADIATION AND HEAT BALANCES

GLOBAL RADIATION AND HEAT BALANCES
The flow of energy from sun to earth and then into space is a complex system but of vital importance to us.

INSOLATION
Insolation is the radiant energy that reaches the surface of the earth from the sun. The sun emits a wide variety of energy waves from very short X-rays to longer infrared rays. Only about 1 part in 2,bOO million of the total of sun's radiation reaches the earth; it is essential for the sustenance of life here. Insolation is the most important single source of atmospheric heat.

At any par­ticular place on earth, the amount of insolation received each day depends upon (i) the angle at which the sun's rays strike the earth, and (il) the length of time of exposure to the rays. Thus, the greatest amount is received at places where the skies are generally clear,.the angle of sun is high and the days relatively long, e.g., around the tropics. On an average, throughout the year, the amount of insolation received by the surface decreases from the equator towards the poles and while the equatorial areas have little variation throughout the year, there is great variation near the poles. The unit of measurement of solar insolation is langely, one langely being equal to one gram-calorie per square centimetre. Measurement by satellites indicates that the
radiation rate from the sun is two langleys per minute.

HEAT BALANCE The moving solar radiation is bal­anced by some sort of equal energy losses in the atmo­sphere which prevent the earth from becoming intolerably hot. The balance is achieved by a complex series of energy transfers involving three common types. (a) Radiation is the transference of heat by electromagnetic waves including X­rays, heat rays and radiowaves. The sun, having a very hot surface, radiates short wavelengths while earth, having a cool surface; re-radiates heat at much larger wavelengths. (b) Convection involves the mass movement of gases or liquids, the heat acquired by the liquid or gas being transported with the medium. (c) Conduction is the trans­ference of heat by actual contact.

As insolation arrives in different wavelengths, different diversions by the atmosphere result. The most significant changes are as follows: (a) Absorption of insolation takes place in the lower layers of atmosphere by carbon dioxide and water vapour, and in the upper layers by oxygen and ozone. Absorption leads to a rise in the air temperature. (b) Scattering takes place by gas molecules and dust particles in all directions. (c) Radiation takes place by clouds and water droplets. Presence or absence of clouds is an important determinant in the amount of radiation reaching the earth; thick clouds are capable of reflecting (back into space) up to 60 per cent of insolation. (d) Reflection of radiation takes place from earth's surface also, varying in amount according to the nature of the ground.

The percentage of radiant energy reflected back by a surface is called the albedo. While water has a low albedo, land surfaces have a much higher albedo. The total amount of energy lost by scattering and reflection of various kinds and returned to space is called earth's albedo, which amounts to about 36 per cent of insolation while the absorption that takes place directly by the atmosphere is about 17 per cent of insolation. Together, they constitute about 53 per cent of solar radiation which gets reflected back into space. Thus, only about 47 per cent of the original insolation received at the top of the atmosphere actually reaches the ground.

Terrestrial Radiation
is the energy which is re-radiated by the earth into atmosphere at long wavelengths. Some of it is directly lost into space and a great deal is absorbed by the atmosphere, especially the clouds. The atmosphere, in turn, radiates or reflects much of this heat back again to the earth and thus a continuous interchange of energy exists with the ground. Further heat is lost from earth through evaporation. There is also a small amount of conduction of heat between the ground and the atmo­sphere. The terrestrial radiation equals and balances with the incoming radiation. The most significant fact in the energy budget is that atmosphere is largely heated from below: while it reflects or lets through the short-wave incoming solar radiation, it absorbs a great amount of the outgoing terrestrial energy. This results in warming up of the atmosphere.

A similar principle is applied in the greenhouse effect where the glass lets in insolation but does not allow the warm air inside to escape readily. While equatorial areas have a positive heat budget (surplus heat), the poles have a negative budget. But the mean tempera­ture of both areas remains fairly constant owing to the presence of horizontal circulation systems. The excess heat received at low latitudes is transferred to poles through various media, including wind systems and ocean currents.

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