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Crepuscular rays

 

Crepuscular rays

Crepuscular rays, taken near Waterberg Plateau, Namibia

Crepuscular rays (also known as sunbeams, Sun rays or God rays), in atmospheric optics, are 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 (particularly stratocumulus) or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions. Despite seeming to converge at a point, the rays are in fact near-parallel shafts of sunlight, and their apparent convergence is a perspective effect (similar, for example, 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 between light and dark are the most obvious. Crepuscular comes from the Latin word "crepusculum", meaning twilight.[1]

Contents

  • Anticrepuscular rays 1
  • Colour 2
  • Alternative names 3
  • Gallery 4
  • See also 5
  • References 6
  • External links 7

Anticrepuscular rays

The rays in some cases may extend 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 are called anticrepuscular or antisolar rays.[2] 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.[3]

Colour

Crepuscular rays are usually orange in appearance because 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. Particles in the air scatter short wavelength light (blue and green) through Rayleigh scattering much more strongly than longer wavelength yellow and red light.

Alternative names

  • Backstays of the sun—a nautical term, from the fact that backstays that brace the mast of a sailing ship converge in a similar way
  • Buddha rays
  • Cloud breaks
  • Jacob's Ladder
  • Ropes of Maui—originally. taura a Maui—from the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer
  • Shafts of light
  • Sun drawing water—from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)
  • Sunbeams
  • Sunburst
  • Volumetric lighting (used by the computer graphics industry)
  • God rays (used by the computer graphics industry[4])
  • Fingers of God
  • Jesus Rays
  • Devil's Rays (refers to anticrepuscular rays only)
  • Tyndall rays

Gallery

See also

References

  1. ^
  2. ^
  3. ^
  4. ^ E.g. this term is mentioned in:

External links

  • Sunrays - Crepuscular rays, Explanation & Images
  • detailed description of how Crepuscular rays occur
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