4 Dec 2006 Jarmo Moilanen

A short introduction into halos

Celestial Pattern of Arcs and Spots

Halo phenomena are white or coloured arcs and spots on day or night sky. They are caused by tiny ice crystals that usually hower in thin cirrus clouds. Halos can appear even 200 days a year - still, few people are aware of them as usually they are faint and located near the glare of the sun. A few times a year halos can occupy the whole of sky creating a bright, symmetrical pattern.


A major halo display in Oulunsalo Northern Finland on 27.4.-96 observed and drawn by Jarmo Moilanen. This display is one of the best ever reported St. Andrew arcs displays outside Antarctica.

Frequently Observed halos

The most common halo is the 22° halo that is caused by randomly oriented hexagonal ice crystals. The dimension 22° refers to the constant radius of the ring - its angular distance from the sun or moon is always the same, 22°. At least a piece of this ring should be able to be seen a couple of times a week, in spring more often.

A list of the most common halos. The parhelic circle should be the rarest one of these. It can however be seen about dozen times a year.

• 22° halo

• 22° tangent arcs / circumscribed halo

• sun pillar

• (22°) parhelia

• 46° halo

• circumzenith arc & circumhorizon arc

• (46°) supralateral arc & (46°) infralateral arc

• subsun

• parhelic circle

Rare Halos

There are dozens of separate halos documented in nature. The large majority of these are very rare. They might appear only once or twice during a century at any observing location. The displays that include these halos, St. Andrew arcs for instance, are those that create an all-sky dazzling web pattern and are seen also by laymen.

A list of documented rare halos caused by normal hexagonal ice crystals (the rarest ones come in the latter part of the list):

• Parry arcs (upper sunvex and suncave, lower sunvex and suncave)

• 120° parhelia

• Lowitz arcs (upper, lower and circular)

• subparhelia

• sub-parhelic circle

• Wegener anthelic arcs

• Greenler anthelic arcs (diffuse anthelic arcs or anthelion)

• Tricker anthelic arcs

• "reflected Lowitz arcs"

• heliac arc

• subhelic arc

• Tape arcs (= 46° Parry supralateral and 46° Parry infralateral arcs)

• subanthelic arc

• "sub-Lowitz arcs"

• "120° parhelic Lowitz arcs"

• "120° parhelic arcs" (not accepted as a separate halo form by some researhers)

• Liljequist parhelia

• Hastings anthelic arcs

• Liljequist subparhelia

• 46° contact arcs