Moments in the life of a circumzenithal arc by Olivier Vandeginste (site, cza).  ŠOlivier Vandeginste, shown with permission.
More vividly than a thousand stills the video captures the ephemeral "once here, next gone, then back again" character of circumzenithal arcs. Multi-layered cirrus crosses the sweet spots in the sky where plate crystals can, just for a moment, glint their CZA colours towards the eye. The arc hints its presence, blazes with full brilliance only to fade and then shine forth once more.

CZAs seem to alter, shine and fade more quickly than other halos. Their position near the zenith is responsible. There, clouds and crystals sweep more quickly across the sky than elsewhere.
Atmospheric
Optics

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