DIFFUSE SKY RADIATION

Diffuse sky radiation is solar radiation reaching the earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. Also called skylight, diffuse skylight, sky radiation.

Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25 percent of the incident radiation), about two-thirds ultimately reaches the earth as diffuse sky radiation.

In optics, scattering usually refers to the deflection of photons that occurs when they are absorbed and re-emitted by atoms or molecules. The sky is blue because molecules in the air preferentially scatter blue light.

Scattering is the process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation. Also called scatter.

Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.

The Babinet point is one of the three commonly detectable points of zero polarization of diffuse sky radiation, neutral points, lying along the vertical circle through the sun; the other two are the Arago point and the Brewster point.

The Babinet point typically lies only 15° to 20° above the sun, and hence is difficult to observe because of solar glare. The existence of this neutral point was discovered by Babinet in 1840.

The Arago point, so named for its discoverer, is customarily located at about 20° above the antisolar point; but it lies at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity.

The Brewster point, discovered by Brewster in 1840, is located about 15° to 20° directly below the sun; hence it is difficult to observe because of the glare of the sun.