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What is a Circumzenithal Arc?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 24 November 2014
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A circumzenithal arc is a fascinating atmospheric phenomenon, sometimes called a reverse rainbow, because at first glance it does indeed resemble a backward or upside-down rainbow. Many observers miss out on circumzenithal arcs, because they are located directly overhead; astronomers cite the circumzenithal arc as another reason for people to look up more, as if the stars weren't enough. Unlike a rainbow, which appears opposite the sun, a circumzenithal arc is centered around the zenith of the sky, and can only appear if the solar angle is less than 32 degrees.

In order for conditions to be right for a circumzenithal arc to form, small, flat, six sided ice crystals must be suspended high in the sky to create a field of tiny prisms. The sun's rays enter the ice crystals and reflect through them, projecting an arc in the sky which, if complete, would circle the zenith. Completely circular circumzenithal arcs are rare, however; most of them only take up a section of the sky, looking like a smile looking down from the heavens. The circumzenithal arc will remain until the solar angle changes, unless weather conditions change dramatically.

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The colors of a circumzenithal arc are also reversed from those of a rainbow; the violet end of the spectrum is closer to the zenith of the sky, while the red range is closest to Earth. Technically, the red range is still at the top of the arc, but because the arc is reversed, the colors seem upside down as well. When looking at a circumzenithal arc, observers may notice sun dogs, another atmospheric phenomenon, as well, as the conditions that prevail to permit the formation of circumzenithal arcs are also optimal for sun dogs.

In most cases, a circumzenithal arc will last at least half an hour, and sometimes more, plenty of time to admire and photograph the beautiful arc. They are most common in colder climates, where ice crystals tend to collect in the sky with abundance, although they can be seen in temperate zones as well, especially during cooler weather. Just like with a rainbow, it is also possible to see a circumzenithal arc from the inside of an airplane, although it may be difficult to distinguish from a rainbow due to the unique angle the observer is seeing it from.

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