Jump to content

Tangent arc: Difference between revisions

Edit links
From Wikipedia, the free encyclopedia
Browse history interactively
← Previous edit
Content deleted Content added
VisualWikitext
GreenC bot (talk | contribs)
Bots
2,385,909 edits
Adding short description: "Atmospheric optical phemonenon"
 
(14 intermediate revisions by 11 users not shown)
Line 1: Line 1:
{{Short description|Atmospheric optical phemonenon}}
⚫
[[File:Upper tangent1.jpg|thumb|An upper tangent arc with the sun at a low altitude.]]
[[File:Tangent arc over San Francisco Bay.jpg|thumb|An upper tangent arc appearing above San Francisco Bay and Marin County in California, at sunset on April 9, 2023.]]


[[File:HaloSantaRosa.png|thumb|An upper tangent arc seen over the setting Sun, Santa Rosa Beach, Florida November 24th, 2015.]]
[[File:Upper tangent1.jpg|thumb|An upper tangent arc with the Sun at an observed low altitude.]]

⚫
[[File:Sun halo optical phenomenon edit.jpg|thumb|upright=0.65|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>]]
⚫
[[File:HaloSantaRosa.png|thumb|An upper tangent arc seen over the setting Sun at Santa Rosa Beach, Florida.]]
⚫
'''Tangent arcs''' are a type of [[Halo (optical phenomenon)|halos]], an atmospheric [[optical phenomenon]], which appear above and below the [[Sun]] or [[Moon]], [[tangent]] to the [[22° halo]]. To produce these arcs, rod-shaped [[hexagon]]al [[ice crystal]]s need to have their long axis aligned horizontally.<ref name="atoptics-tangent-arcs">{{cite web | url = http://www.atoptics.co.uk/halo/column.htm | title = Tangent Arcs | publisher = Atmospheric Optics
⚫
[[File:Sun halo optical phenomenon edit.jpg|thumb|upright=0.65|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 |access-date=2007-04-14 |archive-url=https://web.archive.org/web/20061213123117/http://www.photolib.noaa.gov/historic/nws/wea00195.htm |archive-date=2006-12-13 |url-status=dead }}</ref>]]
⚫
| accessdate = 2015-04-07 | author = Les Cowley (?)}}</ref>
⚫
'''Tangent arcs''' are a type of [[Halo (optical phenomenon)|halo]], an atmospheric [[optical phenomenon]], which appears above and below the observed [[Sun]] or [[Moon]], [[tangent]] to the [[22° halo]]. To produce these arcs, rod-shaped [[hexagonal]] [[ice crystals]] need to have their long axis aligned horizontally.<ref name="atoptics-tangent-arcs">{{cite web | url = http://www.atoptics.co.uk/halo/column.htm | title = Tangent Arcs | publisher = Atmospheric Optics
⚫
| access-date = 2015-04-07 | author = Les Cowley (?)}}</ref>


== Description ==
== Description ==
=== Upper arc ===
=== Upper arc ===
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 observed Sun forming a sharp angle. As the Sun is seen to rise above the Earth's horizon, 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
| url = http://www.meteoros.de/arten/ee05e.htm | title = Upper Tangent Arc | publisher = Arbeitskreis Meteore e.V.
| url = http://www.meteoros.de/arten/ee05e.htm | title = Upper Tangent Arc | publisher = Arbeitskreis Meteore e.V.
| accessdate = 2007-04-15 | author = Les Cowley}}</ref>
| access-date = 2007-04-15 | author = Les Cowley}}</ref>


=== Lower arc ===
=== Lower arc ===
The lower tangent arc is rarely observable, appearing under and [[tangent]] to a [[22° halo]] centred on the sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc forms a sharp, wing-shaped angle below the sun. As the sun rises above over the horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo.<ref name="meteoros2">{{cite web
The lower tangent arc is rarely observable, appearing under and [[tangent]] to a [[22° halo]] centred on the Sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the Sun. As the Sun is observed slipping over Earth's horizon the lower tangent arc forms a sharp, wing-shaped angle below the Sun. As the Sun rises over Earth's horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the Sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo.<ref name="meteoros2">{{cite web
| url = http://www.meteoros.de/arten/ee06e.htm | title = Lower Tangent Arc | publisher = Arbeitskreis Meteore e.V.
| url = http://www.meteoros.de/arten/ee06e.htm | title = Lower Tangent Arc | publisher = Arbeitskreis Meteore e.V.
| accessdate = 2015-04-07 | author = Les Cowley}}</ref>
| access-date = 2015-04-07 | author = Les Cowley}}</ref>


Since by definition, the sun elevation must exceeds 22° above the horizon, most observations are from elevated observation points such as mountains and planes.<ref name="meteoros2"/>
Since by definition, the Sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.<ref name="meteoros2"/>


== Origin ==
== Origin ==


Both the upper and lower tangent arc form when [[hexagon]]al rod-shaped [[ice crystal]]s in [[cirrus cloud]]s 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° halo]]s and [[sun dog]]s; a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is [[refraction|refracted]] less strongly than blue light.<ref name="atoptics-tangent-arcs"/>
Both the upper and lower tangent arc form when [[hexagon]]al rod-shaped [[ice crystal]]s in [[cirrus cloud]]s 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° halo]]s and [[sun dog]]s; a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is [[refraction|refracted]] less strongly than blue light.<ref name="atoptics-tangent-arcs"/>


== See also ==
== See also ==
{{commons|Category:Tangent arcs}}
{{Commons category|Tangent arcs}}
⚫
* [[Circumzenithal arc]]
* [[Circumhorizontal arc]]
* [[Circumhorizontal arc]]
⚫
* [[Circumzenithal arc]]
* [[Kern arc]]
* [[Subsun]]
* [[Subsun]]


Line 37: Line 41:
* [http://www.atoptics.co.uk/halo/tanim2.htm Atmospheric Optics - Lower Tangent arc] - A photo taken from an aeroplane.
* [http://www.atoptics.co.uk/halo/tanim2.htm Atmospheric Optics - Lower Tangent arc] - A photo taken from an aeroplane.
* [http://www.paraselene.de?uk:113972 Lower Tangent Arc Sep 25, 2005] - Photos of a lower tangent arc.
* [http://www.paraselene.de?uk:113972 Lower Tangent Arc Sep 25, 2005] - Photos of a lower tangent arc.

{{optics-stub}}
{{climate-stub}}


[[Category:Atmospheric optical phenomena]]
[[Category:Atmospheric optical phenomena]]

Latest revision as of 19:43, 16 April 2023

An upper tangent arc appearing above San Francisco Bay and Marin County in California, at sunset on April 9, 2023.
An upper tangent arc with the Sun at an observed low altitude.
An upper tangent arc seen over the setting Sun at Santa Rosa Beach, Florida.
A 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.
Photo: Cindy McFee, NOAA, December 1980.[1]

Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.[2]

Description

[edit]

Upper arc

[edit]

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 observed Sun forming a sharp angle. As the Sun is seen to rise above the Earth's horizon, 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.[3]

Lower arc

[edit]

The lower tangent arc is rarely observable, appearing under and tangent to a 22° halo centred on the Sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the Sun. As the Sun is observed slipping over Earth's horizon the lower tangent arc forms a sharp, wing-shaped angle below the Sun. As the Sun rises over Earth's horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the Sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo.[4]

Since by definition, the Sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.[4]

Origin

[edit]

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 all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is refracted less strongly than blue light.[2]

See also

[edit]
Wikimedia Commons has media related to Tangent arcs.

References

[edit]
  1. ^ "A magnificent halo". NOAA. 1980-12-21. Archived from the original on 2006-12-13. Retrieved 2007-04-14.
  2. ^ a b Les Cowley (?). "Tangent Arcs". Atmospheric Optics. Retrieved 2015-04-07.
  3. ^ Les Cowley. "Upper Tangent Arc". Arbeitskreis Meteore e.V. Retrieved 2007-04-15.
  4. ^ a b Les Cowley. "Lower Tangent Arc". Arbeitskreis Meteore e.V. Retrieved 2015-04-07.
[edit]
Retrieved from "https://en.wikipedia.org/w/index.php?title=Tangent_arc&oldid=1150187418"