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   "Odd Radius" Halos

Circular halos from poorly oriented pyramidal crystals swamp the everyday 22� halo in this 90 million ray simulation. Outwards from the sun are 9,18,20,23,24 and 35� radius halos. Sometimes called "Odd radius halos".

Sometimes circular halos appear that cannot be explained by invoking 'ordinary' hexagonal prism crystals.

These are the 'Odd radius halos' produced by pyramidal ice crystals. Pyramidal crystals have one or both end faces of ordinary ice crystals replaced by six sided pyramids. The pyramids are sometimes truncated and the central prism section is sometimes missing, creating 18 different variations with up to 20 sides. Very likely, all hexagonal ice crystals are actually pyramidal but with insignificantly small inclined facets.

In contrast to ordinary prism crystals where the angles between faces are either 60 or 90�, the angles between pyramidal crystal faces can also be 28, 52.4, 56, 62, 63.8 and 80.2�. Each inclination or dihedral angle forms a separate circular halo.

The odd radius halos are all minimum deviation phenomena like the more familiar 22 and 46� halos. Their relative intensities - or whether some appear at all - depend on the presence and relative sizes of particular sets of faces.

Odd radius halos are perhaps not so 'odd' or rare as usually thought. Make a point of routinely searching for them.