high above the horizon over Grand Forks, ND, Jan 2007
Often described as a '' smile in the sky '' , a circumzenithal arc occurs when the sun shines at a right angle through high clouds ( approximately 6100-7600 metres high ) made of ice crystals about the size of salt grains, which refract the light into a brilliant spectrum of colour. The arc is only visible when the sky is free of low-level clouds and when the sun sits less than about 33degrees above the horizon.
[Taken from the Discovery Channel Magazine, issue 7, March 09]
Here's a news paper article I found on the subject Dated 17th September 2008
An astronomer caught this unusual upside-down rainbow on camera near her home in Cambridge
'' 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.
Renowned astronomer and writer Dr Jacqueline Mitton was lucky enough to capture the optical phenomenon on camera near her home in Cambridge.
The 60-year-old who has a doctorate in astrophysics from Cambridge University said: 'I've never seen anything like it before - and I'm 60. ''
read more here
WOW!!! i saw this type of a rainbow for the 1st time... 
