Circumzenithal arc

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July 10, 2005.

High above the horizon over Grand Forks, ND, January 2007.

The circumzenithal arc or cicumzenith arc (CZA), also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow and arising from refraction of sunlight through non-terminated, horizontally-oriented ice crystals in certain clouds. It takes the shape of one-quarter of a circle centered at the zenith and parallel to the horizon, on the same side as the sun. Its colors run from blue near the zenith to red towards the horizon; it is one of the brightest and most colorful halos.

The light that forms the CZA enters an ice crystal through its horizontal top face and exits through a vertical side face (the angle of separation must be 90°). This means that it occurs only when the sun is at a lower altitude than 32.2° above the horizon. The CZA is brightest when the sun is at 22° above the horizon (causing sunlight to enter and exit the crystals at the minimum deviation angle), and then it is about 22° in radius, 3° in width, and tangent to the supralateral arc (if present). However, the CZA can have any radius between 32.2° and 0°, but at either of these points it is vanishingly faint. As the sun rises above 32.2°, light exits the crystals through the horizontal bottom face instead to contribute to the colorless parhelic circle.

The circumzenithal arc can be confused with the 22° halo when the sun is located near zenith.^example

[edit] References

David K. Lynch and William Livingston. Color and Light in Nature. 2^nd ed, 2004 printing.
Les Cowley. Circumzenithal Arc. Atmospheric optics. Retrieved on 2007-04-23.
Les Cowley. CZA - Effect of solar altitude. Atmospheric Optics. Retrieved on 2007-04-23.