CHA's need a high sun.
Circumhorizon arcs form by sun rays entering the side faces of plate oriented hexagonal ice crystals and leaving through their lowest face. A crystal is shown at right as a side cross-section.
In 'A' the sun is higher than 58° and a colourful CHA has formed.
In 'B' the sun is lower and the CHA ray only just manages to escape from the lower face'.
In 'C' the sun is lower still and the ray is totally internally reflected rather than leaving to form a CHA.
Made using iPad/iPhone App "Prism HD"