Circumhorizontal Arc

A circumhorizontal arc is an optical phenomenon in the ice-halo family, produced when sunlight or moonlight refracts through flat, horizontally oriented hexagonal ice crystals suspended high in the atmosphere. It appears as a large, brightly spectrum-colored band, with red uppermost, running parallel to the horizon far below the sun or moon, created when light enters the vertical side face of a crystal and exits through its near-horizontal bottom face at a 90-degree angle that separates the colors. It can only be seen when the sun or moon sits very high in the sky, at an elevation of 58 degrees or more, which makes it impossible to see north of about 55 degrees north or south of 55 degrees south; it can appear several times each summer in parts of the United States but stays rare in northern Europe.