The upper bright ice halo, very often mistaken for a rainbow, is a circumzenithal arc. Horizontal hexagonal plates form it. Sunlight enters the uppermost face and leaves via a side face. The combination of refraction between faces at right angles and near parallel sun rays gives pure, well separated, colours.

Whenever a CZA is seen, look for a fainter downward curving halo that just touches it. This is the rarer supralateral arc. Column crystals with their long axes horizontal form it. Its rarity stems in part from the ray path. Rays exit through a hexagonal column end face. these are rarely optically perfect.