Circumzenithal arc

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The circumzenithal arc, also called the Bravais' arc and abbreviated CZA, 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 it is present). However, the CZA can have any radius between 32.2° and 0°, but at either of these points it is vanishingly faint.

[edit] Sources

• David K. Lynch and William Livingston. Color and Light in Nature. 2^nd ed, 2004 printing.

[edit] See also

• Circumhorizontal arc