Circumhorizontal arc
A circumhorizontal arc is an optical phenomenon, an ice-halo formed by plate shaped ice crystalsin high level cirrus clouds.
The current accepted technical names are circumhorizon arc or Lower symmetric 46° plate arc. The term 'fire rainbow' was coined recently by a journalist is not recognised. It is misleading as the arc is not a rainbow and is not related to fire.
The complete halo is a huge and beautiful 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.
There is a myth that the halo is rare. How often it is seen depends on location and in particular 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 in mid-latitude and northern Europe.
For the halo to form the sun must be very high in the sky, at an elevation of 58° or more. Cirrus cloud or haze containing relatively large plate-shaped ice crystals must also be present. The sun altitude requirement has the consequence that the halo is impossible to see at locations north of 55°N or south of 55°S (although a lunar circumhorizon arc might be visible). In 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 on graph.[1]. 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. When the cloudiness of Europe is also taken into account, the halo becomes more than 10-20X more likely to be seen in the United States.
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. There is no requirement that the plates be thick. 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 produces the well-separated spectral colours and, if the crystal alignment is just right, makes the entire cirrus cloud appear to shine.[2]
A circumhorizontal arc can be difficult to distiguish 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.[2]
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Photographed in Ravenna, Michigan -
Photographed in Hocking Hills -
See also
References
- ^ Les Cowley - Is the circumhorizon arc common or rare?, [1]
- ^ a b Les Cowley. "Circumhorizon arc". Atmospheric Optics. Retrieved 2007-04-22.
External links
