Tangent arc
Photo: Cindy McFee, NOAA, December 1980.[1]
An upper tangent arc is a halo, an atmospherical optical phenomenon which appears over and tangent to the 22° halo centred around the sun.
The shape of a 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 lowers towards the 22° halo while gradually becoming longer. As the sun rises over 29-32°, the upper tangent arc unties with the lower tangent arc to form the circumscribed halo.[2]
Both the upper and lower tangent arc form as 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 with many other halos, upper tangent arcs have a red inner edge gradually becoming bluish outwards because of red light is refracted more strongly than blue light.[3]
See also
References
- ^ "A magnificent halo". NOAA. 1980-12-21. 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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