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Formation of the halo requires that the sun be very high in the sky, at an elevation of 58° or greater, and that a cirrus cloud or haze be present and contain plate-shaped ice crystals. The sun's altitude determines the visibility of the halo; it is impossible to see at locations north of 55°N or south of 55°S (although a lunar circumhorizon arc might be visible at other latitudes). At other latitudes the phenomenon is visible, for a greater or lesser time, around the summer solstice. Slots of visibility for different latitudes and locations may be looked up here. For example, in London, England the sun is only high enough for 140 hours between mid-May and late July. Contrast that with Los Angeles, with the sun higher than 58 degrees for 670 hours between late March and late September.
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.