In Finnish Halo Observers Network, FHON (homepage) has counted some halos as common (or frequently observed) halos. In the end of 2004 we have discussion about are rare halos really rare. After this depate I work out a rules which define common halo.

To get a list of common halos we can apply three (3) simple rules.

Rule 1 - Ice crystal rule

It is hexagonal ice, which produce practically all observed halos. The most simplies and the most common form of hexagonal ice crystal is hexagonal prism. So the common halo must arise from simple hexagonal prism with 2 basal faces and 6 side faces.

Rule 2 - Orientation rule

We know from statistics that most observed halos arise from three main orientations: plate orientation, column orientation and random orientation. So a common halo must arise from one of these three orientations. However, random orientation is in matter of fact a special case and we do not know excatly how common it is, but we can ignore this argument since it will not change the list.

Rule 3 - Ray path rule

Simpliest ray paths are single reflection on the surface of ice crystal and simple refraction through ice crystal. So, if ray path of halo has internal reflections, we can rule it out from our list of common halos.

These rules seems to work well, since rules did not change much FHON's old list of common halos based on statistics.

List of common halos based on rules above

• 22° halo
• sun pillar *
• (22°) parhelia
• (22°) tangent arcs 
  • upper tangent arc
  • lower tangent arc
  • circumscribed halo
• 46° plate arcs 
  • circumzenith arc
  • circumhorizon arc
• 46° halo
• 46° column arcs
  • (46°) infralateral arcs
  • (46°) supralateral arc
• parhelic circle
• subsun

*) Note that sun pillar from plate orientation does not fully fit into these rules, since sun pillar from plate orientation has ray path 3-2-1-6 (internal reflections are agains rule 3) if sun is not at horizon. Ray path 1 or 2 from nearly horizontal plate crystals produce elongated subsun but not pillar near the sun except when sun is close to horizon. In theoretical point of view we could easily ignore internal reflections, but because idea of these rules, we can not ignore them.

However, sun pillar can be produced by oriented columns and then it fits these rules and that is why sun pillar stays in the list of common halos. 

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© Jarmo Moilanen 2005