Pictured: Rare upside-down rainbow spotted in the UK

By | September 17, 2008

It appeared in the sky for the briefest of moments.

A dazzling arc of psychedelic colour reminiscent of the Cheshire Cat’s grin in Lewis Carroll’s Alice in Wonderland.

But this is no fantasy or trick of the light, it is known as a circumzenithal arc. Seen here shimmering in the sky over Cambridge in the afternoon sunshine, it is often mistaken for a rainbow hanging upside down…. – dailymail

The circumzenithal arc or cicumzenith arc (CZA), also called the Bravais’ arc, is an optical phenomenon similar in appearance to a rainbow and arising from refraction of sunlight through non-terminated, horizontally-oriented ice crystals in certain clouds. It takes the shape of one-quarter of a circle centered at the zenith and parallel to the horizon, on the same side as the sun. Its colors run from blue near the zenith to red towards the horizon; it is one of the brightest and most colorful halos.

The light that forms the CZA enters an ice crystal through its horizontal top face and exits through a vertical side face (the angle of separation must be 90°). This means that it occurs only when the sun is at a lower altitude than 32.2° above the horizon. – wiki

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