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Just looking through photos we took last year, I came across this one which seems to be of a reverse rainbow, something I don't ever recall having seen before. Are they a rare occurrence? What conditions causes them?

Roger

It's a halo. These are caused by light refracting through ice crystals in cirriform clouds relatively high up. Ice crystals have a hexagonal form and refract light at approximately 22 degrees. As this figure differs slightly for different wavelengths, the light is split in the same way that it is in a rainbow. For blue light the angle is larger, for red light it is smaller - this causes the blue to be on the outside of the arc and the red on the inside.

In case you are wondering, this is a précis of a chunk from a Collins publication simply called "Weather". Very interesting and lots of good colour plates.

Dave

There's a good website about these phenomena at http://www.atoptics.co.uk/halosim.htm I once saw a particularly impressive display, including some of the rarer forms, over Albuquerque but, unfortunately, did not have a camera at the time

Mike

As stated previously, a halo has 22 degrees radius. A rainbow is 42 degrees with the centre directly opposite the sun. It is often not realised that as a consequence, a rainbow cannot be seen if the sun's azimuth (angle above horizon) is greater than 42 degrees. I have even seen TV weather presenters make this mistake and refer to rainbows throughout a showery summer's day - morning and evening, yes, but not throughout the day.

Exceptions to this are of course when seen from a high vantage point, eg a hill and from an aircraft when a full circle can often be seen (don't confuse with The Glory - that's another phenomenon).

Here is a below-horizon rainbow taken from a road on Mull about 100 metres above sea level. Jack