martes, 18 de marzo de 2008

Crepuscular Rays

You probably all saw something similar. It makes amazing images like this one below from Stow Lake in Golden Gate Park, San Francisco.

These are Crepuscular Rays.

"Crepuscular rays, in atmospheric optics, also known as sun rays or God's rays, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds, are diverging columns of sunlit air separated by darker cloud-shadowed regions. The name comes from their frequent occurrences during twilight, when the contrasts between light and dark are the most obvious. Various airborne compounds scatter the sunlight and make these rays visible. We see the light so defined because of 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. Three main forms of crepuscular rays are:

* 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 way. Crepuscular rays are usually red or yellow in appearance because the atmosphere acts as a giant lens, refracting low sunset rays into long curved paths passing through up to 40 times as much air than the 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.

Alternative names

* Jacob's Ladder

* Buddha's fingers
* hands of God (also. fingers of God, arms of God, touch of God)
* Jesus rays (also Jesus beams)

* 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

* god beams, godbeams
* Light Rays/Volume light (Mostly used by the Computer Graphics industry)