Upper tangent arc

An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo centred around the sun.

The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low (less than 29–32°) it appears as an arc over the sun forming a sharp angle. As the sun rises, the curved wings of the arc lower towards the 22° halo while gradually becoming longer. As the sun rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.cite web
url = http://www.meteoros.de/arten/ee05e.htm | language = English
title = Upper Tangent Arc | publisher = Arbeitskreis Meteore e.V.
accessdate = 2007-04-15 | author =
]

Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally, while otherwise rotated in any direction. This orientation of the crystals also produces other halos, including 22° halos and sun dogs, but a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like many other halos, upper tangent arcs grade from a red inner edge to a blue outer edge because red light is refracted more strongly than blue light.cite web
url = http://www.atoptics.co.uk/halo/column.htm
title = Tangent Arcs | publisher = Atmospheric Optics
accessdate = 2007-04-15 | author = Les Cowley (?)
]

See also

* Circumzenithal arc
* Circumhorizontal arc

References

External links

* [http://www.engl.paraselene.de/html/tangent_arcs.html www.paraselene.de - Tangent Arcs] (including several HaloSim simulations.)


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