How a Circumhorizontal Arc Forms

The sky puts on an amazing light show everyday. But sometimes, it produces something special like a circumhorizontal arc.

Often mistaken for a rainbow, a circumhorizontal arc is an entirely different optical phenomenon. It is formed by the refraction, or bending, of sunlight through plate-like hexagonal ice crystals that are situated horizontally in cirrus or cirrostratus clouds. More specifically, light enters through the vertical side of the crystals and exits through their horizontal bottoms.This angled pathway produces the well-separated colors of the spectrum that we see in the sky.  They are brightest where the cirrus clouds are thickest. Oriented parallel to the horizon, a circumhorizontal arc always sits below the sun.

Rainbows, by contrast, are produced by the combination of refraction and reflection of sunlight in liquid water droplets. These arching bands of color always appear in the part of the sky that is opposite the sun.

Circumhorizontal arcs are somewhat rare. In addition to the appropriate cloud conditions, they require the sun to be very high in the sky – at least 58° above the horizon.  They are usually only seen during the summer in the mid-latitudes.

A circumhorzontal arc seen by the author last summer in Colorado. Image Credit: Melissa Fleming

A circumhorizontal arc seen by the author last summer in Colorado.  Credit: Melissa Fleming