Summer is coming ~ A huge circumhorizon arc, a summer only halo, imaged during the Southern Hemisphere summer by Ignacio Diaz Bobillo from the beach at Punta del Este, Uruguay, February '12. Images ©Ignacio Diaz Bobillo, shown with permission
The bright inner halos around the Uruguay sun. There is a 22° circular halo from randomly aligned crystals and an oval circumscribed halo from horizontal column crystals.

The latter also produce an outer infralateral arc which partially overlaps the circumhorizon arc and is often difficult to distinguish from it.

It is possible that the colourful arc seen here was a mixture of both. When you see one, look carefully to see whether the ends of the arc curve upwards from the horizon - if so it is an infralateral arc. Distortions by camera lenses make that judgment from photographs far less easy.

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This giant and colourful ice halo is becoming visible from the Northern Hemisphere as the sun climbs there towards summer.   In the USA it can already be seen from as far north as Richmond VA but from more northerly Europe a trip to southern Spain, Italy or Greece is currently needed.

The halo is produced by hexagonal plate crystals in cold high cloud. The plates drift with their large faces nearly horizontal. Sunlight enters a side face and leaves from the lower face. The refraction through the equivalent of a 90° prism widely disperses the colours.

Why only a summer halo?
When the sun is lower than ~58° its rays cannot escape from the lower crystal face. Instead, they are totally internally reflected and eventually leave through another near vertical side face.