Halo Arcs ~ A 22 degree halo surrounded by a circumscribed halo.  Lower picture also shows a pyramidal crystal 9° halo.  Images by Ian Jacobs at Pathum Thani, Bangkok, Thailand.  ŠIan Jacobs, shown with permission.

When the sun is high and an apparently circular halo is seen in the sky it is often mistaken for the common 22 degree halo.

Here, a 22 degree halo is also present. The outer halo, touching the 22 degree at top and bottom, is a 'circumscribed halo'. As the sun climbs even higher the circumscribed halo becomes more and more circular and eventually the two halos overlap.

Both halos are formed by the same ray path passing between the side faces of hexagonal prism ice crystals. The critical difference comes from crystal alignment.

The 22 degree halo is generated by crystals that take different alignments about three axes. In other words they are essentially randomly tilted in the sky.

The circumscribed halo is from 'singly oriented columns'.   These crystals are aligned by aerodynamic drag as they drift slowly downwards in cloud air currents and the long axis of the crystal stays nearly horizontal.

Circumscribed halos tend to be sharper and have brighter colours than 22 degree halos. But that is not an infallible guide.

The image at left shows a bonus - A 9 degree 'odd-radius halo' formed by pyramidal ice crystals.


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