Upsidedown Rainbow

I was sitting outside having tea on a lovely evening and looking up did a double take as I noticed a rainbow which on closer inspection was actually upsidedown.
Looking on the net it seems that this is what is known as a circumzenithal arc and is caused by ice crystals in high level clouds instead of raindrops for an ordinary rainbow.
Ive never seen one before and didn't even know there was such a thing until tonight.