A circumzenithal arc is a fascinating atmospheric phenomenon, sometimes called a reverse rainbow, because at first glance it does indeed resemble a backwards 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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El arco iris invertido, el único fenómeno capaz de dibujar una sonrisa de colores en el cielo, requiere de unas circunstancias distintas al arco iris normal para formarse. La luz solar debe rebotar en los cristales de hielo de la alta atmósfera, la cual envía los rayos del sol hacia arriba.

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