Crepuscular rays
From Wikipedia, the free encyclopedia
Crepuscular rays, in atmospheric optics, also known as sun rays, God's rays or the Fingers of God, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds or between other objects, are diverging columns of sunlit air separated by darker cloud-shadowed regions. The name comes from their frequent occurrences during crepuscular hours (those being dawn and dusk), when the contrasts between light and dark are the most obvious. Various airborne compounds scatter the sunlight and make these rays visible, due to diffraction, reflection, and scattering.
Crepuscular rays are near-parallel, but appear to diverge because of linear perspective. They often occur when objects such as mountain peaks or clouds partially shadow the sun's rays like a cloud cover. There are three primary forms of crepuscular rays:
- Rays of light penetrating holes in low clouds (also called "Jacob's Ladder").
- 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.
The rays of the second and third types, 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 apparently odd directions are a perspective effect.
Crepuscular and anticrepuscular rays behave in the same manner. Crepuscular rays are usually red or yellow in appearance because the atmosphere acts as a giant lens, which refracts low sunset rays into long curved paths that pass through up to 40 times as much air as rays from a high midday sun. Particles in the air scatter short wavelength blue and green rays much more strongly than longer wavelength yellow and red.
Crepuscular rays can also occasionally be viewed underwater, particularly in arctic areas appearing from ice shelfs or cracks in the ice.
[edit] Alternative names
- Jacob's Ladder
- 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
- Sunbeams
- Light Rays/Volume light (Mostly used by the Computer Graphics industry)[citation needed]
- Gateways to Heaven
[edit] See also
[edit] External links
