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.