• Question: How are circumzenithal arcs formed (they look like upside-down rainbows)? I actually saw one once, and I know it is to do with refraction by ice crystals, but I'm not what exactly is going on...

    Asked by doppler to Adam, Sheila on 24 Mar 2011 in Categories: .
    • Photo: Adam Tuff

      Adam Tuff answered on 24 Mar 2011:

      Ah yeah, I’ve seen a few of these – they’re caused by the really strand like clouds called Cirrus clouds. They also cause “sun dogs” as well which are quite frequent too. The angle that they refract at has more to do with the prism-shape of the ice crystals which are orientated to the incoming light.

    • Photo: Sheila Kanani

      Sheila Kanani answered on 24 Mar 2011:

      Ah jealous that you’ve seen one! Where did you see it?

      The light that forms the CZA enters an ice crystal rather than a raindrop, and the light enters through the crystals flat top face, and exits through a side prism face. Unlike a rainbow, which appears opposite the sun, a cza 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 cza to form, small, flat, six sided ice crystals must be suspended high in the sky to create a field of tiny prisms. Because the crystals are flat and hexagonal, they invert the light and create an upside-down curve. 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. However; most of them only take up a section of the sky, looking like a smile looking down from the heavens.

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