Circumhorizon Arc

Ice crystal glints of this huge halo fill wispy cirrus fragments. Image by Del Zane (photofolio, photogallery) at Anacortes, WA.

Image ©Del Zane

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The circumhorizon arc is huge and very colourful. When seen as a fragment it is oft mistaken for iridescent cloud. To distinguish it, look for structured colours. The CHA is beneath the sun and always has red at top.

It is a late Spring/Summer arc because the sun needs to be higher than 58°. That does not necessarily make it rare. Its extreme rarity is an Internet myth. In any one location in the USA it will be seen several times a year.

However, Europe is much closer t the pole. It is never seen north of Copenhagen and rarely in UK or northern Germany.

Visibility charts here.

CHAs form by sun rays entering the side faces of plate oriented hexagonal ice crystals and leaving through their lowest face. A crystal is shown here as a side cross-section.

In 'A' a colourful CHA has formed. In 'B' the sun is lower and the CHA ray only just manages to escape from the lower face'.  In 'C' the sun is lower still and the ray is totally internally reflected rather than leaving to form a CHA.

What of the ray externally reflected going downwards to the right?  That contributes to the parhelic circle.

Made using iPad/iPhone App "Prism HD"