Crepuscular rays

Crepuscular rays ( /krɨˈpʌskjələr/; also known as God Rays), in atmospheric optics, are rays of sunlight that appear to radiate from a single point in the sky. 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. The name comes from their frequent occurrences during crepuscular 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]

Appearance

Crepuscular rays are near-parallel, but appear to diverge because of linear perspective.^[2] They often occur when objects such as mountain peaks or clouds partially shadow the sun's rays like a cloud cover. Various airborne compounds scatter the sunlight and make these rays visible, due to diffraction, reflection, and scattering.

Crepuscular rays can also occasionally be viewed underwater, particularly in arctic areas, appearing from ice shelves or cracks in the ice.

There are three primary forms of crepuscular rays^{[citation needed]}:

• Rays of light penetrating holes in low clouds (also called "Jacob's Ladder"^{[citation needed]}).
• Beams of light diverging from behind a cloud.
• Pale, pinkish or reddish rays that radiate from below the horizon. These are often mistaken for light pillars.

They are commonly seen near sunrise and sunset, when tall clouds such as cumulonimbus and mountains can be most effective at creating these rays.^[3]

Anticrepuscular rays

Crepuscular and anticrepuscular rays are generated in the same way. The rays in some cases may extend across the sky and appear to converge at the antisolar point, which is the point on the sky sphere directly opposite the sun, and they are called anticrepuscular rays. Like crepuscular rays, they are parallel shafts of sunlight from holes in the clouds, and their apparent convergence is a perspective effect. They are not as easily spotted as crepuscular rays.^[2]

Color

Crepuscular rays are usually red or yellow in appearance because the path through the atmosphere at sunrise and sunset pass 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

• Sunbeams
• Sunburst
• Sun rays
• Sun drawing water - from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)
• 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
• Ropes of Maui - (originally. taura a Maui) from the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer
• Jacob's Ladder
• Gateways to Heaven, Stairways to Heaven and Paths to Heaven^{[citation needed]}
• God Rays, Fingers of God (or God's Fingers), so called because they appear often in the background on religious pictures.
• Jesus Beams, Beams of Jesus, or Jesus Rays
• Cloud breaks
• Volumetric lighting (used by the computer graphics industry)
• Buddha's Rays
• Divine Light

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  Crepuscular rays at sunset.

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  Crepuscular rays at Black Mountain Tower, Canberra.

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  Crepuscular rays in Golden Gate Park.

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  Anticrepuscular rays opposite the setting sun off the Florida Gulf Coast of the United States.

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  Artificial reflection-induced crepuscular rays in San Francisco.

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  Crepuscular rays at noon in Saint Peter's Basilica, Vatican City.

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  Crepuscular rays in steam from hot springs at Yellowstone National Park.

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  Wide crepuscular rays over Santa Clarita, CA.

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  Lunar crepuscular rays.

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  Crepuscular rays at the Dago Waterfall near Bandung, date 1920-1932.

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  This astronaut photograph from the ISS provides an unusual viewing perspective from above some crepuscular rays and a clear illustration of their parallel nature.

See also

• Airglow
• Anticrepuscular rays
• Light beam
• Rayleigh scattering
• Tyndall effect

References

1. ^ Edens, Harald, "Crepuscular rays", Weather Photography lightning, clouds, atmospheric optics & astronomy, http://www.weatherscapes.com/album.php?cat=optics&subcat=crepuscular_rays, retrieved November 1, 2011. 
2. ^ ^{a b} John A. Day (2005), The Book of Clouds, Sterling Publishing Company, Inc., pp. 124–127, ISBN 9781402728136, http://books.google.com/books?id=0cpQGHqxQBUC&pg=PA124&dq=crepuscular+rays+book#v=onepage&q=crepuscular%20rays%20book&f=false, retrieved 2010-10-09. 
3. ^ Cowley, Les, "Formation & Perspective", Atmospheric Optics, http://www.atoptics.co.uk/atoptics/rayform.htm, retrieved November 1, 2011. 

External links

• Sunrays - Crepuscular rays, Explanation & Images
• detailed description of how Crepuscular rays occur
• Photographic illustration of Crespuscular Rays morning time in Goa
• "Sun drawing water" as a music piece