Circles & Crystals - Peter Gallon (Telescope House) saw this 22° halo and sundog on the afternoon of 24th December in Ashdown Forest, East Sussex, England.   ©Peter Gallon, shown with permission.


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The 22° circular halo is said to be the most often seen of the ice halos.   That can depend on where you are. Sundogs are much more common at my location in England .   

The halo is formed by sun rays refracted between two side faces of hexagonal ice prisms inclined at 60° to each other.   To form the circle the prisms must somehow be randomly (or nearly so) tilted.

There lies a mystery because aerodynamic alignment is the norm for ice crystals large enough to form halos .   Short prisms (plates) drift downwards in cloud air currents with their large faces nearly horizontal.   Long column shaped crystals drift with their long axes horizontal.

Intermediate equi-dimensional crystals are sometimes invoked to explain the 22 degree halo but crystals samples do not have nearly enough of them. The crystals in the background image were sampled at the South Pole by Walter Tape during a bright high level 22° halo display with no other halos conspicuous. Equi-dimensional crystals are sparse.

How then is this common halo formed?   Hexagonal prisms in some guise are responsible – that we know.   It could be that tumbling clusters of prisms are responsible but we are not sure. Some potential halo forming clusters are at right in the South Pole sample.