ABSTRACT
Since the earth is surrounded by an atmosphere which contains various gaseous constituents, suspended dust and other minute solid and liquid particulate matter and clouds of various types, marked depletion of solar energy takes place during its passage through the atmosphere to the surface of the earth. In a cloud-free atmosphere, the depletion occurs by three distinct physical processes operating simultaneously: (i) selective absorption by molecular oxygen, ozone, carbon dioxide, and water vapour in certain specific wavelengths, (ii) Rayleigh scattering by molecules of the different gases that constitute the atmosphere, and (iii) Mie scattering. In Rayleigh scattering, where the size of the scattering particles is small compared with the wavelength of light, the intensity of scattering follows the well-known λ-4 law, as a consequence of which a spatial redistribution of the energy of the incoming radiation takes place in the scattered light, the shorter wavelengths predominating and giving rise to the blue of the sky. Roughly one half of the scattered radiation is lost to space and the remaining half is directed downwards to the earth’s surface from different directions as diffuse radiation. In Mie scattering where the sizes of the scattering particles, mostly dust, are comparable with the wavelength of visible and infrared radiation, a depletion in the solar radiation occurs both by true scattering (involving a redistribution of incident energy) as well as by absorption by the particles, wherein a part of the radiant energy is transformed into heat. This type of scattering also leads to a fraction of the solar radiation being lost to space and another fraction being directed downwards as diffuse radiation. Unlike Rayleigh scattering, however, the dust-scattering process is more complicated and the redistribution of energy is much more asymmetrical
with respect to the different directions when compared to Rayleigh scattering. Because of absorption by oxygen and ozone at high levels of the atmosphere, the short-wavelength limit of solar radiation received at the earth’s surface is approximately 0.29 µ. In a cloudy atmosphere, considerable depletion of the direct solar radiation takes place. A large fraction is reflected back to space from the tops of clouds, another part transmitted downwards to the earth as diffuse radiation and a small fraction absorbed by the clouds. Dense dark clouds of appreciable vertical depth can cut off as much as 80 per cent of the incident energy, but thinner clouds deplete only 20-50 per cent of the radiant energy, depending on their depth and liquid water content. The effect of an increase in dust in the atmosphere is to decrease the direct solar radiation and increase the diffuse radiation. When the cloud amount increases from 0 to about 4 or 5 octas of the sky, there is a corresponding increase in diffuse radiation. However, when the cloud amount increases still further to overcast conditions, a decrease in diffuse radiation takes place, largely because of increased absorption and reflection from cloud tops. The fraction of the total solar radiant energy reflected back to space through reflection from the tops of clouds, scattering by atmospheric gases and dust particles and by reflection at the earth’s surface is called the albedo of the earth-atmosphere system and has a value of about 0.30 for the earth as a whole. The mean monthly value of the intensity of direct solar radiation normal to the solar beam actually received at the earth’s surface at noon time in India varies from 0.51 to 1.05 kW/m2, depending on latitude, altitude of the station, and season. The solar radiant energy in the shortwave spectrum falling on the earth’s surface as well as that absorbed by the atmosphere is re-emitted back into space as longwave radiation. This longwave radiation has the characteristics of blackbody radiation in the temperature range of about –60° to +30°C. Since, in the long run, the earth’s mean surface temperature as well as the mean temperature of the atmosphere remains unchanged, it follows that the net heat absorbed by the earth-atmosphere system from the shortwave solar radiation is equal to the net heat emitted as longwave radiation by the planet earth and its atmosphere.
