Circumhorizontal arc
Encyclopedia
A circumhorizontal arc is an optical phenomenon
- an ice-halo
formed by plate-shaped ice crystals in high level cirrus clouds.
The current accepted names are circumhorizon arc or lower symmetric 46° plate arc The complete halo is a huge, multi-coloured band running parallel to the horizon with its center beneath the sun. The distance below the sun 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.
How often a circumhorizontal arc is seen depends on the location and the latitude. In the United States it is a relatively common halo seen several times each summer in any one place. In contrast, it is rare to non-observable in northern Europe.
Formation of the halo requires that the sun be very high in the sky, at an elevation of 58° or more, and that the cirrus cloud or haze contain plate-shaped ice crystals. The sun's altitude determines the visibility of the halo; it is impossible to see at locations north of 55°N or south of 55°S (although a lunar circumhorizon arc might be visible). At other latitudes it is visible for a greater or lesser time around the summer solstice. Slots of visibility for different latitudes and locations can be looked up here. For example, in London, England the sun is only high enough for 140 hours between mid May and late July. Contrast that with Los Angeles with the sun higher than 58 degrees for 670 hours between late March and late September.
The halo is formed by sunlight entering horizontally-oriented flat hexagon ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo). In principle, Parry oriented
column crystals can also produce the arc, although this is rare.
The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colours.
The arc has a considerable angular extent and is thus rarely complete. When only fragments of cirrus cloud are in the appropriate sky/sun position they can appear to shine with spectral colors.
A circumhorizontal arc can be difficult to distinguish from an infralateral arc
when the sun is high in the sky. The former is always parallel to the horizon whereas the latter curves upwards at its ends.
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
- an ice-halo
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
formed by plate-shaped ice crystals in high level cirrus clouds.
The current accepted names are circumhorizon arc or lower symmetric 46° plate arc The complete halo is a huge, multi-coloured band running parallel to the horizon with its center beneath the sun. The distance below the sun is twice as far as the common 22-degree halo
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen.
How often a circumhorizontal arc is seen depends on the location and the latitude. In the United States it is a relatively common halo seen several times each summer in any one place. In contrast, it is rare to non-observable in northern Europe.
Formation of the halo requires that the sun be very high in the sky, at an elevation of 58° or more, and that the cirrus cloud or haze contain plate-shaped ice crystals. The sun's altitude determines the visibility of the halo; it is impossible to see at locations north of 55°N or south of 55°S (although a lunar circumhorizon arc might be visible). At other latitudes it is visible for a greater or lesser time around the summer solstice. Slots of visibility for different latitudes and locations can be looked up here. For example, in London, England the sun is only high enough for 140 hours between mid May and late July. Contrast that with Los Angeles with the sun higher than 58 degrees for 670 hours between late March and late September.
The halo is formed by sunlight entering horizontally-oriented flat hexagon ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo). In principle, Parry oriented
Parry arc
A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc.- Discovery :...
column crystals can also produce the arc, although this is rare.
The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colours.
The arc has a considerable angular extent and is thus rarely complete. When only fragments of cirrus cloud are in the appropriate sky/sun position they can appear to shine with spectral colors.
A circumhorizontal arc can be difficult to distinguish from an infralateral arc
Infralateral arc
An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...
when the sun is high in the sky. The former is always parallel to the horizon whereas the latter curves upwards at its ends.