Sky radiation

This is otherwise known as ‘diffused’ radiation or ‘scattered’ radiation. Its presence constitutes a heat gain to the earth, in addition to that from direct radiation, and it arises from the translucent nature of the atmosphere.

The atmospheric losses in direct radiation which produce the sky radiation stem from four principal phenomena:

(i) A scattering of the direct radiation in all directions which occurs when the radiant energy encounters the actual molecules of the ideal gases (nitrogen and oxygen) in

, the atmosphere. This effect is more pronounced for the short wavelengths, and accounts for the blue appearance of the sky.

(ii) Scattering resulting from the presence of molecules of water vapour.

(iii) Selective absorption by the ideal gases and by water vapour. Asymmetrical gaseous molecules such as ozone, water vapour, carbon dioxide etc. have a greater ability to

Absorb (and hence to emit) radiation than do gaseous molecules of a symmetrical structure, such as nitrogen and oxygen.

(iv) Scattering caused by dust particles.

The losses incurred are largely due to the permanent gases and to water vapour. There is also some diffusion of the direct sunlight caused when direct (and indirect) radiation encounters cloud. Not all the solar energy removed from direct radiation by the processes of scattering and absorption reaches the surface of the earth. Some is scattered back to space, and the absorbed energy which is re-radiated at a longer wavelength (mostly from water vapour) is also partly lost to outer space. Some direct energy is additionally lost by reflection from the upper surfaces of clouds. However, a good deal of the energy which direct radiation loses does eventually reach the surface of the earth in a scattered form.

The amount of sky radiation varies with the time of day, the weather, the cloud cover and the portion of the sky from which it is received, the amount from the sky in the vicinity of the sun exceeding that from elsewhere, for example. Sky radiation cannot, however, be assigned a specific direction and, hence, it casts no directional shadow outdoors.

Equations are available (see section 7.15) for calculating the total scattered solar radiation received from the sky and the ground. The intensity of such radiation depends on seasonal variations of moisture content and the earth-sun distance (given by Allen (1973) as 149.5 x 106 km), the angular relation of the receiving surface with respect to the surroundings, and the reflectances of the relevant surfaces.

Values of monthly mean daily irradiance on inclined, plane surfaces, at various angles to the horizontal, for both direct and diffuse solar radiation are provided by the CIBSE Guide A2 (1999). Several locations are given, including Bracknell, over various orientations. The intensity of diffuse radiation depends mostly on the solar altitude but also, to a lesser extent, on the orientation when the receiving surface is vertical.

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