Circumzenithal Arc

A capture by Jane Moore-Taylor from the Chilterns, England on 25th June ’14.

Over the past few days a northern air stream has rendered English skies the clearest blue with little extinction near to the horizon, a blue deepened by contrast with low cumulus fractus   Humidity has been just right to spread contrails into wide long-lasting cirrus ribbons that as the day progressed thickened and spread more.    This cirrus has so far failed to show a much sought circumhorizon arc in the narrow time slot available around noon near the solstice.   However, an equally pure coloured circumzenithal arc is here an ample compensation.
All images ©Jane Moore-Taylor, shown with permission
Inside view:
Rays enter the roof and leave through the right wall. The wall reflects some rays and these leave through the floor to help light the parhelic circle.

When the sun climbs higher than 32° the rays from the roof strike the wall at too shallow an angle to be transmitted. To them, the wall closes and becomes a perfect mirror (total internal reflection). There is no longer any CZA.

Contrails to dense cirrus ribbons. Image by Les Cowley

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Almost parallel sun rays enter the top face of a hexagonal plate crystal. They are refracted and pass through the ice to leave through a side face. The path is equivalent to passage through a 90° prism in a spectrometer and gives nearly pure spectral colours