Apparently the sky has to be clear of rain and low level clouds for this phenomenon to be seen. They are created by sunlight bouncing off ice crystals in thin wispy cirrus clouds at high altitude. The ice crystals have to be at a specific angle to the sun and the observer also has to be in the right spot to see the effect. Thanks to Chris for identifying this rainbow. More examples and information can be found here, here and here.
Friday, 10 October 2014
I have been informed by a friend that the picture of an upside down rainbow I posted on 29th September and simply described as a rainbow of sorts is in fact known as a circumzenithal arc or Bravais' arc. A few searches on the internet also indicate it is quite rare to see one in the UK and the more defined they are the rarer they are.