circumzenithal arc


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circumzenithal arc

[¦sər·kəm¦zē·nə·thəl ′ärk]
(optics)
A brilliant rainbow-colored arc of about a quarter of a circle with its center at the zenith and about 46° above the sun, produced by refraction and dispersion of the sun's light striking the top of prismatic ice crystals in the atmosphere, and usually lasting only a few minutes.
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References in periodicals archive ?
This leads me to believe that it is indeed a circumzenithal arc captured in the image, an optical effect formed as light refracts off ice particles in thin cloud at high levels.
We call them pseudo-circumzenithal arcs, or PCZAs, because the ray paths are similar to those that form the beautiful, rainbowlike circumzenithal arcs on Earth.
Today, it's true, we seldom hear the circumzenithal arc described as being "of Bravais," or a well-developed fogbow referred to as "Ulloa's ring." But some of these sky effects are too complex or rare for the names of their classic observers to be deleted.
The spectrally colored arc with its back to this may have been the faint "upper tangential arc" of the "circumscribed halo." If it was not nearly in contact with the first arc, it could have been the more colorful "circumzenithal arc," which really does seem like an out-of-place rainbow in a rainless sky.
Using only a point-and-shoot camera he was able to capture a richly colored segment of the circumzenithal arc that appeared about 65 [degrees] above the Sun (see page 103).
Like its more common cousin the circumzenithal arc, this display is formed by refraction of sunlight in ice crystals of cirrus and high haze, which can be present year around.
Apart from circumhorizontal arcs, the right combination of the sun and the cirrus clouds can also create other kinds of optical illusions that resemble rainbows, such as cloud iridescence (a diffraction phenomenon caused by small ice crystals individually scattering sunlight), infralateral arcs (formed when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides), and circumzenithal arcs (located at a considerable distance above the sun, they usually form a quarter of a circle centered on the zenith).
Common rainbows are caused by light shining through raindrops but circumzenithal arcs appear when light is refracted through ice crystals higher up in the atmosphere.
Unlike rainbows, where sunlight refracts through raindrops, circumzenithal arcs occur when sunlight refracts through ice crystals in cirrus clouds.
When was the last time you glanced up on a sunny day to check for rings around the Sun, or any colorful circumzenithal arcs or iridescent clouds?