158:. 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 colours of a circumhorizon arc are pure and spectral (more so than in a rainbow), while the colours in cloud iridescence have a more washed-out, "mother of pearl" appearance.
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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. Slots of visibility for different latitudes and locations may be looked up
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should be situated at the edge of a table. The second refraction at the top water-air interface will then project a hyperbola at a vertical wall behind it. The overall refraction is then equivalent to the refraction through an upright hexagonal plate crystal when the rotational averaging is taken into account. A colorful artificial circumhorizontal arc will then appear projected on the wall. Using a spherical projection screen instead will result in a closer analogy to the natural halo counterpart. Other
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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
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A water glass experiment (known about since at least 1920) may be modified slightly to create an artificial circumhorizontal arc. 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. The glass
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column crystals may 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 thus, rarely is complete. When only fragments of a cirrus cloud are in the
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clouds. 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 distance between the arc and the Sun or Moon is twice as far as the common
173:, 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
125:. For example, in London 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.
169:, 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
95:, nor related in any way to fire. The term, apparently coined in 2006, may originate in the occasional appearance of the arc as "flames" in the sky, when it occurs in fragmentary cirrus clouds.
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76:. 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) the Moon.
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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,
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Tape, Walter and
Moilanen, Jarmo - 'Atmospheric Halos & the search for angle x', American geophysical Union,2006 - pp196-7
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Circumhorizontal arcs, especially when only fragments can be seen, are sometimes confused with
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Markus Selmke and Sarah Selmke (2017). "Artificial circumzenithal and circumhorizontal arcs".
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appropriate sky and sun position, they may appear to shine with spectral colours.
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Images of artificial circumhorizontal, circumzenithal and suncave Parry arcs
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452:. UC Santa Barbara Department of Geography. August 29, 2009. Archived from
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91:" is sometimes used to describe this phenomenon, although it is neither a
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Circumhorizontal Arc over peak 12225 in the Sierra
Mountains of California
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508:(8). American Journal of Physics, Vol. 85, Issue 8, p.575-581: 575–581.
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Gilbert light experiments for boys - (1920), p. 98, Experiment No. 94
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The halo is formed by sunlight entering horizontally-oriented, flat,
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formed by the refraction of sunlight or moonlight in plate-shaped
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154:(typically by liquid water droplets or ice crystals) rather than
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Other currently accepted names for the circumhorizontal arc are
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Confusion with other members of the halo family, such as
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Circumhorizontal Arc - Harald Edens
Weather Photography
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63:suspended in the atmosphere, typically in actual
31:A circumhorizontal arc (bottom) in relation to a
571:Circumhorizontal Arc - Arbeitskreis Meteore e.V.
298:Photographed near Kennedy Space Center, Florida
23:Circumhorizontal arc over the Nepalese Himalaya
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561:How rare are they? When to see them.
556:Atmospheric Optics - Circumhorizon Arc
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405:"OPOD - Moonlight Circumhorizon Arc"
566:Atmospheric Optics - Image gallery
310:Photographed near Lewisville,Texas
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338:Photographed near Elk Ridge, Utah
138:can be created by similar means.
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129:Artificial circumhorizontal arcs
55:that belongs to the family of
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601:Atmospheric optical phenomena
85:lower symmetric 46° plate arc
285:Emerald Isle, North Carolina
502:American Journal of Physics
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324:Wrightsville, Pennsylvania
386:Polar stratospheric cloud
366:Halo (optical phenomenon)
142:Similar optical phenomena
87:. The misleading term "
175:perspective distortion
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482:. Atmospheric Optics
49:circumhorizontal arc
524:2017AmJPh..85..575S
480:"Circumhorizon arc"
429:"Circumhorizon arc"
381:Circumzenithal arc
167:circumzenithal arc
53:optical phenomenon
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33:circumscribed halo
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16:Optical phenomenon
532:10.1119/1.4984802
376:Cloud iridescence
216:Ravenna, Michigan
148:cloud iridescence
81:circumhorizon arc
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136:artificial halos
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89:fire rainbow
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69:cirrostratus
61:ice crystals
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266:Banjarmasin
152:diffraction
515:1608.08664
486:2007-04-22
435:2012-05-19
392:References
156:refraction
540:118613833
270:Indonesia
116:Frequency
105:hexagonal
99:Formation
57:ice halos
595:Category
360:See also
251:Portugal
247:Alentejo
200:22° halo
163:sun dogs
520:Bibcode
460:May 19,
371:Sundogs
181:Gallery
165:or the
93:rainbow
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233:, Ohio
65:cirrus
51:is an
536:S2CID
510:arXiv
462:2012
123:here
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
