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25/10 2008

Upside down rainbows

An original advertising? Photomontage? No, it a rare but possible phenomenon.

An upside down rainbow appeared last Sunday on September the 14th in the skies over Cambridge. It is a natural phenomenon typical of the polar zone. The astronomer that first saw it and photographed it, tells us its secrets (Alessandro Bolla, October 2008).

Some days ago an enormous multicolour smile appeared in the skies of Cambridge, astonishing thousands of people, they wonder if that great colourful smile was something natural or the new idea of a really creative advertising agent. The photo of this rainbow has been realized on September the 14th by Jaqueline Mitton, astronomer of the Royal Astronomical Society that has explained the causes of this strange phenomenon to the british daily paper Daily Telegraph. The upside down rainbows are really rare and their formation requires specific atmospheric conditions. Their convex form, with the cavity towards the sun, derives by the light refraction on the ice crystals set in anomalous way. Usually they have intense colours. Instead the traditional rainbows are formed from the solar beams that cross the small water drops suspended in the air after the rain. These particular phenomena are also called circumzenithal rainbows and are typical of the polar zones:
usually they are very high, near the sun and so it is difficult to locate them.
If you want to see other photos of these spectacular rainbows, see them on the pages of iFocus.



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