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]
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
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
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
Wikimedia Commons has media related to Tangent arcs.
www.paraselene.de - Tangent Arcs (including several HaloSim simulations.)
Atmospheric Optics - Alaska Lower Tangent Arc - Great photo by .
Atmospheric Optics - Lower Tangent arc - A photo taken from an aeroplane.
Lower Tangent Arc Sep 25, 2005 - Photos of a lower tangent arc.
April 27, 2023
tangent, type, halo, atmospheric, optical, phenomenon, which, appears, above, below, observed, moon, tangent, halo, produce, these, arcs, shaped, hexagonal, crystals, need, have, their, long, axis, aligned, horizontally, upper, tangent, appearing, above, franc. 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 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 Contents 1 Description 1 1 Upper arc 1 2 Lower arc 2 Origin 3 See also 4 References 5 External linksDescription EditUpper 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 EditBoth 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 Circumhorizontal arc Circumzenithal arc Kern arc SubsunReferences Edit A magnificent halo NOAA 1980 12 21 Archived from the original on 2006 12 13 Retrieved 2007 04 14 a b Les Cowley Tangent Arcs Atmospheric Optics Retrieved 2015 04 07 Les Cowley Upper Tangent Arc Arbeitskreis Meteore e V Retrieved 2007 04 15 a b Les Cowley Lower Tangent Arc Arbeitskreis Meteore e V Retrieved 2015 04 07 External links Editwww paraselene de Tangent Arcs including several HaloSim simulations Atmospheric Optics Alaska Lower Tangent Arc Great photo by Ryan Skorecki Atmospheric Optics Lower Tangent arc A photo taken from an aeroplane Lower Tangent Arc Sep 25 2005 Photos of a lower tangent arc Retrieved from https en wikipedia org w index php title Tangent arc amp oldid 1150187418, wikipedia, wiki, book, books, library,
