The circumhorizon arc (CHA) is a huge and colourful ice halo produced by sunlight refracted through horizontal plate crystals high in cirrus cloud.

Look for it when the sun is high (above 58°). Peg's picture at left is looking sunward and the sun is off the top of the frame. The upper halo is the familiar 22° halo (or maybe a circumscribed halo - it is hard to tell sometimes). The CHA is roughly twice as far below the sun.

The CHA is not uncommon at latitudes below 45° which have many hours per year with the sun shining and above the required altitude. Northern Europe is far less favoured and there the halo is hardly ever seen. Perhaps it is this that gives the halo its misleading reputation for being a rarity.

Why is it so colourful? The light rays forming it are refracted through ice facets inclined at 90° which widely separates the colours. Furthermore the incoming sun rays are parallel leading to very little colour overlap. An equally colourful halo is its close relation the circumzenithal arc.