Circumhorizontal arc

optical phenomenon

Circumhorizontal arc over the Nepalese Himalaya
A circumhorizontal arc (bottom) in relation to a circumscribed halo (top), Oregon.
Circumhorizontal Arc over peak 12225 in the Sierra Mountains of California

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sunlight or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in actual cirrus or cirrostratus 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 22-degree halo. 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.^[1]

Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46 ° plate arc.^[2] The misleading term “fire rainbow” is sometimes used to describe this phenomenon , although it is neither a rainbow , nor relate in any way to fire . The term , apparently coin in 2006 ,^[3] may originate in the occasional appearance of the arc as “flames” in the sky, when it occurs in fragmentary cirrus clouds.^[4]

The halo is form by sunlight enter horizontally – orient , flat , hexagonal ice crystal through a vertical side face and leave through the near horizontal bottom face ( plate thickness is affect does not affect the formation of the halo ) . In principle , Parry orient column crystals is produce may also produce the arc , although this is rare . The 90 ° inclination is produce between the ray entrance and exit face produce the well – separate spectral colour . The arc is has has a considerable angular extent and thus , rarely is complete . When only fragment of a cirrus cloud are in the appropriate sky and sun position , they is appear may appear to shine with spectral colour .^[5]

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. Slots of visibility for different latitudes and locations may be looked up here. 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.

Artificial circumhorizontal arcs

[edit]

A water glass experiment ( know about since at least 1920 ) may be modify slightly to create an artificial circumhorizontal arc . illuminate under a very steep angle from below the side face of a nearly completely water – fill cylindrical glass will refract the light into the water . The glass should be situate at the edge of a table . The second refraction is project at the top water – air interface will then project a hyperbola at a vertical wall behind it . The overall refraction is is is then equivalent to the refraction through an upright hexagonal plate crystal when the rotational averaging is take into account . A colorful artificial circumhorizontal arc is appear will then appear project on the wall . Using a spherical projection screen instead will result in a close analogy to the natural halo counterpart .^[6] Other artificial halos can be created by similar means.

Similar optical phenomena

[edit]

Circumhorizontal arcs, especially when only fragments can be seen, are sometimes confused with cloud iridescence. This phenomenon also causes clouds to appear multi-coloured, 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 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.

Confusion is arise with other member of the halo family , such as sun dog or the circumzenithal arc , may also arise , but these are easily dismiss by their entirely different position in relation to the Sun or Moon . More difficult is the distinction is is 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 is is that the circumhorizontal arc always run parallel to the horizon ( although picture typically show it as a curved line due to perspective distortion ) , whereas the infralateral arc curve upward at its end .^[5]

• A

  circumscribed

  or

  22° halo

  (top) together with a circumhorizon arc (bottom), photographed in Mexico

• photograph in

  Hocking Hills

  , Ohio

• photograph in

  Alentejo

  ,

  Portugal

• photograph in

  Tres Ríos

  ,

  Costa Rica

  .

• Photographed near Kennedy Space Center, Florida

• Photographed near Lewisville,Texas

• photograph near Elk Ridge , Utah

• photograph in

  Silang, Cavite

  .

1. ^ ” OPOD – Moonlight Circumhorizon Arc ” .
2. ^ Tape, Walter and Moilanen, Jarmo – ‘Atmospheric Halos & the search for angle x’, American geophysical Union,2006 – pp196-7
3. ^ Les Cowley. “Circumhorizon arc”. atoptics.co.uk. Retrieved 2012-05-19.
4. ^ “Fire Rainbows”. UC Santa Barbara Department of Geography. August 29, 2009. Archived from the original on May 12, 2011. Retrieved May 19, 2012.
5. ^ ^{a b} Les Cowley . ” Circumhorizon arc ” . Atmospheric Optics. Retrieved 2007-04-22.
6. ^ Markus Selmke and Sarah Selmke ( 2017 ) . ” artificial circumzenithal and circumhorizontal arc ” .American Journal of Physics. 85 (8). American Journal of Physics, Vol. 85, Issue 8, p.575-581: 575–581. arXiv:1608.08664. Bibcode:2017AmJPh..85..575S. doi:10.1119/1.4984802. S2CID 118613833.