Tuesday, November 3, 2009

Crepuscular rays

Crepuscular 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 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 is a term used to describe some animals that are primarily active during twilight, that is at dawn and at dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is thus in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be active on a bright moonlit night. Many animals that are casually described as nocturnal are in fact crepuscular. Within the definition of crepuscular are the terms matutinal (or "matinal") and vespertine, denoting species active in the dawn and dusk respectively.
The patterns of activity are thought to be an antipredator adaptation. Many predators forage most intensely at night, while others are active at mid-day and see best in full sun. Thus the crepuscular habit may reduce predation. Additionally, in hot areas, it may be a way of avoiding thermal stress while capitalizing on available light. Some crepuscular mammals include the Red Panda, cats, deer, moose, rabbits, chinchillas, ferrets, guinea pigs, hamsters, the common mouse, skunks, rats, wombats, quolls and capybaras. Crepuscular birds include the Common Nighthawk, Chimney Swift, American Woodcock, and Spotted Crake.
Some animals that are extinct were also crepuscular such as the Tasmanian Tiger.
Some species have different habits in the absence of predators. For example, the Short-eared Owl is crepuscular on those of the Galápagos Islands that have buzzard species, but diurnal on those without.
Many moths, beetles, flies, and other insects are crepuscular as well.

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. 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:
i. Rays of light penetrating holes in low clouds (also called "Jacob's Ladder").
ii. Beams of light diverging from behind a cloud.
iii. Pale, pinkish or reddish rays that radiate from below the horizon. These are often mistaken for light pillars.
They are most common in Antarctica.

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.
Anticrepuscular rays are similar to crepuscular rays, but seen opposite the sun in the sky. Sunlight travels in straight lines, but the projections of these lines on Earth's spherical atmosphere are great circles. Hence, straight-line crepuscular rays from a setting (or rising) sun can appear to re-converge at the antisolar point. Anticrepuscular rays are most frequently visible near sunrise or sunset. Crepuscular rays are usually much brighter than anticrepuscular rays. This is because for crepuscular rays, seen on the same side of the sky as the sun, the atmospheric light scattering and making them visible is taking place at small angles (see Mie theory).
Although anticrepuscular rays appear to converge onto a point opposite the sun, the convergence is actually an illusion. The rays are in fact parallel, and the apparent convergence is to the vanishing point at infinity.

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
a) Sunbeams
b) Sunburst
c) Sun rays
d) Sun drawing water - from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)
e) 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
f) Ropes of Maui - (originally. taura a Maui) from the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer
g) Jacob's Ladder
h) Gateways to Heaven and Stairways to Heaven
i) God Rays
j) Cloud breaks
k) Fingers of God (or God's Fingers)
l) Volumetric lighting (used by the computer graphics industry
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