A circumhorizontal arc is an optical phenomenon - an ice-halo formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-colored band running parallel to the horizon, located below the Sun or Moon. The distance below the Sun or Moon is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.

FORMATION

A circumhorizontal arc is an optical phenomenon - an ice-halo formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-colored band running parallel to the horizon, located below the Sun or Moon. The distance below the Sun or Moon is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.

FREQUENCY

How often a circumhorizontal arc is seen, depends on the location and the latitude of the observer. In the United States it is a relatively common halo, seen several times each summer in any one place. In contrast, it is a rare phenomenon in northern Europe for several reasons. Apart from the presence of ice-containing clouds in the right position in the sky, the halo requires that the light source (Sun or Moon) be very high in the sky, at an elevation of 58° or greater. This means that the solar variety of the halo is impossible to see at locations north of 55°N or south of 55°S. 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. For example, in London, England the sun is only high enough for 140 hours between mid-May and late July, whereas Los Angeles has the sun higher than 58 degrees for 670 hours between late March and late September.

                                              Similar optical phenomenon

Circumhorizontal arcs, especially when only fragments can be seen, are sometimes confused with cloud iridescence. This phenomenon also causes clouds to appear multi-colored, but it originates from diffraction (typically by liquid water droplets or ice crystals) rather than refraction. The two phenomena can be distinguished by several features. Firstly, a circumhorizon arc always has a fixed location in the sky in relation to the Sun or Moon (namely below it at an angle of 46°), while iridescence can occur in different positions (often directly around the Sun or Moon). 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. Finally, the colors of a circumhorizon arc are pure and spectral (more so than in a rainbow), while the colors in cloud iridescence have a more washed-out, "mother of pearl" appearance.

Confusion with other members of the halo family, such as sun dogs or the circumzenithal arc, may also arise, but these are easily dismissed by their entirely different positions in relation to the Sun or Moon. More difficult is the distinction between the circumhorizontal arc and the infralateral arc, both of which almost entirely overlap when the Sun or Moon is at a high elevation. The difference is that the circumhorizontal arc always runs parallel to the horizon (although pictures typically show it as a curved line due to perspective distortion), whereas the infralateral arc 

curves upward at its ends.