A
tangent arc is an ice-based
atmospheric effect. At its most defined, it appears as an elegant
arc of white light resting its sharply defined, slightly red,
convex edge on the outside of the
22° halo around the
sun. Its diffuse,
concave edge fades into the blue sky beyond. At their least defined, they can appear to be bright bulges of the 22° halo. Tangent arcs can appear either directly above or directly below the sun.
Unlike the other common halos, the shape of the tangent arc changes significantly with slight differences in the elevation of the sun. When the sun is at the horizon, both the upper and lower tangent arcs look like "V"s with gently curving bases, or 1-minute-painter versions of seagulls, with a height around 10° and a wingspan of 30°.
The upper tangent arc unfolds:
As the sun rises in the sky, the upper arc slowly unfolds its lengthening wings downward until their tips are just above the solar elevation.
The lower tangent arc twists:
The lower tangent arc is hidden to ground-based observation by the horizon when the sun is lower than 22°, but it can be seen from a mountain peak or airplane. At 5° it begins to fold inwards, becoming pointed at 10°, twisting to a fish shape at 15° and becoming an inverted teardrop shape at 20°. At 25° the lower tangent arc looks similar to its 0° "V" shape.
When the sun is past 30° in the sky, the two arcs join together to form the circumscribed halo.
Tangent arcs occur when hexagonal needle-shaped ice crystals in cirrus clouds are distributed before the sun, with their long axes horizontal. Sunlight passes into one edge face of the crystal and diffracts out the next adjacent face plus one, making a 60° bend from sun to eye.
Similar conditions can produce parhelia, the 22° halo, and the Parry arc, depending on the orientation of the crystals.
Sources:
- http://www.sundog.clara.co.uk/halo/column.htm
- http://www.meteoros.de/arten/ee05e.htm
- http://www.meteoros.de/arten/ee06e.htm
- An interactive animation is available at
http://www.sundog.clara.co.uk/halo/colsolat.htm