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.
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Tangent arc in the context of Atmospheric phenomenon
Atmospheric optical phenomena include:
- Afterglow
- Airglow
- Alexander's band, the dark region between the two bows of a double rainbow.
- Alpenglow
- Anthelion
- Anticrepuscular rays
- Aurora (northern and southern lights, aurora borealis and aurora australis)
- Belt of Venus
- Brocken Spectre
- Circumhorizontal arc
- Circumzenithal arc
- Cloud iridescence
- Crepuscular rays
- Earth's shadow
- Earthquake lights
- Glories
- Green flash
- Halos, of Sun or Moon, including sun dogs
- Haze
- Heiligenschein or halo effect, partly caused by the opposition effect
- Ice blink
- Light pillar
- Lightning
- Mirages (including Fata Morgana)
- Monochrome Rainbow
- Moon dog
- Moonbow
- Nacreous cloud/Polar stratospheric cloud
- Rainbow
- Sprite (lightning)
- Subsun
- Sun dog
- Tangent arc
- Tyndall effect
- Upper-atmospheric lightning, including red sprites, Blue jets, and ELVES
- Water sky
Tangent arc in the context of Circumzenithal arc
The circumzenithal arc, also called the circumzenith arc (CZA), the upside-down rainbow, and the Bravais arc, is an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halos arising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratus clouds, rather than from raindrops. The arc is located a considerable distance (approximately 46°) above the observed Sun and at most forms a quarter of a circle centered on the zenith. It has been called "a smile in the sky", its first impression being that of an upside-down rainbow. The CZA is one of the brightest and most colorful members of the halo family. Its colors, ranging from violet on top to red at the bottom, are purer than those of a rainbow because there is much less overlap in their formation.
The intensity distribution along the circumzenithal arc requires consideration of several effects: Fresnel's reflection and transmission amplitudes, atmospheric attenuation, chromatic dispersion (i.e. the width of the arc), azimuthal angular dispersion (ray bundling), and geometrical constraints. In effect, the CZA is brightest when the Sun is observed at about 20°.
View the full Wikipedia page for Circumzenithal arc