However, according to Wikipedia this term is misleading, because they are neither rainbows, nor related in any way to fire. The name comes from its appearance as a rainbow taking the shape of flames in the sky.
Other currently accepted names for the phenomenon are circumhorizon arc or lower symmetric 46° plate arc.
The complete halo is a huge, multi-coloured band running parallel to the horizon with its centre beneath the sun.
The distance below the sun is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen.
Formation of the halo requires that the sun be very high in the sky and that a cirrus cloud or haze be present and contain plate-shaped ice crystals.
The halo is formed by sunlight entering horizontally oriented, flat, hexagon ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo).
In principle, Parry oriented column crystals may also produce the arc, although this is rare. The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colours.
The arc has a considerable angular extent and thus, rarely is complete. When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colours.
A circumhorizontal arc may be difficult to distinguish from an infralateral arc when the sun is high in the sky. The former is always parallel to the horizon, whereas the latter curves upward at its ends.
A fire rainbow... or a phenomenon called circumhorizon arc? Photos: Nellie Venter, Suid-Kaap FORUM correspondent.
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