The sun was 65 high and the width of this portion of his image is about 40 . The circumhorizontal arc is a very large halo, always near to the horizon and only visible when the sun is more than 58 high. Look for it around noon in mid summer when the sun is at its highest. Usually only fragments are visible where there happen to be cirrus clouds. It is a close relation of the circumzenithal arc and is produced by plate and Parry oriented crystals.
Light rays enter the almost vertical side faces and leave via the lower horizontal face.
The refraction of the almost parallel sun's rays through faces inclined at 90 produces pure, bright and well separated prismatic colours. The colours are at their most pure when the crystal tilts are smallest. Large crystal tilts produce less intense pastel hues. An infralateral arc can have a similar shape and appearance when the sun is high. The CHA is everywhere the same distance above the horizon - the infralateral arc is lowest beneath the sun then curves upwards.