Tangent arc: Difference between revisions
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[[Image:Upper tangent1.jpg|thumb|An upper tangent arc with the sun at a low altitude.]] |
[[Image:Upper tangent1.jpg|thumb|An upper tangent arc with the sun at a low altitude.]] |
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[[Image:Sun halo optical phenomenon edit.jpg|thumb|A halo phenomenon observed over the South Pole. Featured in the photo are several distinct phenomena: A [[parhelic circle]] (horizontal line), a [[22° halo]] (circle) with a [[sundog]] (bright spot), and an '''upper tangent arc'''.<br /><small>Photo: Cindy McFee, [[NOAA]], December 1980.</small><ref name="NOAA-McFee">{{cite web | url = http://www.photolib.noaa.gov/historic/nws/wea00195.htm | title = A magnificent halo | publisher = [[NOAA]] | date = 1980-12-21 | accessdate = 2007-04-14 |archiveurl = http://web.archive.org/web/20061213123117/http://www.photolib.noaa.gov/historic/nws/wea00195.htm <!-- Bot retrieved archive --> |archivedate = 2006-12-13}}</ref>]] |
[[Image:Sun halo optical phenomenon edit.jpg|thumb|A [[halo (optical phenomenon)|halo]] display observed over the South Pole. Featured in the photo are several distinct phenomena: A [[parhelic circle]] (horizontal line), a [[22° halo]] (circle) with a [[sundog]] (bright spot), and an '''upper tangent arc'''.<br /><small>Photo: Cindy McFee, [[NOAA]], December 1980.</small><ref name="NOAA-McFee">{{cite web | url = http://www.photolib.noaa.gov/historic/nws/wea00195.htm | title = A magnificent halo | publisher = [[NOAA]] | date = 1980-12-21 | accessdate = 2007-04-14 |archiveurl = http://web.archive.org/web/20061213123117/http://www.photolib.noaa.gov/historic/nws/wea00195.htm <!-- Bot retrieved archive --> |archivedate = 2006-12-13}}</ref>]] |
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An ''' upper tangent arc''' is a [[Halo (optical phenomenon)|halo]], an atmospheric [[optical phenomenon]] which appears over and [[tangent]] to the [[22° halo]] around the [[ |
An ''' upper tangent arc''' is a [[Halo (optical phenomenon)|halo]], an atmospheric [[optical phenomenon]] which appears over and [[tangent]] to the [[22° halo]] around the [[Sun]] or [[Moon]]. |
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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]].<ref name="meteoros">{{cite web |
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]].<ref name="meteoros">{{cite web |
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Revision as of 09:57, 15 February 2015
Photo: Cindy McFee, NOAA, December 1980.[1]
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the Sun or Moon.
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.[2]
Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally. Each crystal can have its long axis oriented in a different horizontal direction, and can rotate around the long axis. Such a crystal configuration also produces other halos, including 22° halos and sun dogs; 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.[3]
See also
References
- ^ "A magnificent halo". NOAA. 1980-12-21. Archived from the original on 2006-12-13. Retrieved 2007-04-14.
- ^ "Upper Tangent Arc". Arbeitskreis Meteore e.V. Retrieved 2007-04-15.
- ^ Les Cowley (?). "Tangent Arcs". Atmospheric Optics. Retrieved 2007-04-15.
External links
- www.paraselene.de - Tangent Arcs (including several HaloSim simulations.)
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