Circumhorizontal arc

In its full form, the arc has the appearance of a large, brightly spectrum-coloured band (red being the topmost colour) running parallel to the horizon, located far below the Sun or Moon.

The term, apparently coined in 2006,[3] may originate in the occasional appearance of the arc as "flames" in the sky, when it occurs in fragmentary cirrus clouds.

When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colours.

A lunar circumhorizon arc might be visible at other latitudes, but is much rarer since it requires a nearly full Moon to produce enough light.

At other latitudes the solar circumhorizontal arc is visible, for a greater or lesser time, around the summer solstice.

Illuminating under a very steep angle from below the side face of a nearly completely water-filled cylindrical glass will refract the light into the water.

This phenomenon also causes clouds to appear multi-coloured, but it originates from diffraction (typically by liquid water droplets or ice crystals) rather than refraction.

Secondly, the colour bands in a circumhorizon arc always run horizontally with the red on top, while in iridescence they are much more random in sequence and shape, which roughly follows the contours of the cloud that causes it.