When can you see a rainbow upside down?

There is a rare optic event in nature that causes the rainbow to appear upside down. In order for this to occur there has to be very thin clouds on the height of 7-8 kilometers that consist of ice crystals. The sun light has to fall under a certain angle, so the spectrum of reflection goes up to the atmosphere. The colors of of the "upside down" rainbow also appear to be upside down with violet on top and red at the bottom.

SOURCE: http://www.dailymail.co.uk/news/article-1056500/Pictured-Rare-upside-rainbow-spotted-UK.html
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