Wednesday, 24 December 2014

Crepuscular Rays

Crepuscular rays are also called as “Sun rays” in atmospheric optics, are actually rays of sunlight that appear to radiate from the point in the sky where the sun is located. These rays, which stream through gaps in clouds mainly stratocumulus or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions. In spite of seeming to converge at a point, the rays are actually fact near-parallel shafts of sunlight, and their apparent convergence is a perspective effect alike, i.e., to the way that parallel railway lines seem to converge at a point in the distance.

The name comes from their frequent occurrences during twilight hours those around dawn and dusk, when the contrasts in the middle of light and dark are the most obvious. The name “Crepuscular” comes from the Latin word "Crepusculum", which means twilight. The rays in some cases may spread across the sky and appear to converge at the antisolar point, the point on the sky sphere directly opposite the sun. In this case they’re called anticrepuscular rays. These are not as easily spotted as crepuscular rays. This apparent dual convergence to both the solar and antisolar points is a perspective effect analogous to railway tracks appearing to converge to opposite points in opposite directions.

Crepuscular rays are frequently red or yellow in appearance since the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high midday sun. However; particles in the air scatter short wavelength light blue and green through Rayleigh scattering much more strongly than longer wavelength yellow and red light.

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