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One of a range of interesting and beautiful kinds of aerial spectra (rainbows, in a loose sense) that most people have never heard of and will probably never see because they don't look for them, a circumzenith arc is an arc partway around the zenith of the sky (ie. directly up), on the same side as the sun. It can be as striking and saturated as your basic rainy rainbow.

Along with sun dogs, the circumzenith arc is well worth looking out for when there are a lot of cirrus clouds about, and the sun is not too high in the sky (specifically, it is brightest when the sun is at an elevation of 22.1°, and doesn't appear at all if it is higher than 32.3°). Like sun dogs, a circumzenith arc is caused by refraction through flattish, horizontally aligned hexagonal ice crystals; being caused by the exact same kind of cloud, the two kinds of spectra are often seen at the same time or within a few hours of each other. Circumzenith arcs are said to be visible on about twenty to thirty days of the year.

Some extra information on the circumzenithal arc (CZA) follows. It is the third most common halo. It works like this: Sunlight enters the topmost face of the hexagonal ice crystal at a low enough angle to exit downward through an edge face. Chromatic aberration spreads the sunlight into a spectrum, with the red edge facing the sun and the violet edge towards you. The color they produce is purer than that of rainbows.

They circum your zenith with a diameter that gets larger and a thickness that gets thinner as the sun gets lower. When the sun is near the horizon they're about 32° in diameter and 1° thick. At 22° they are 22° in diameter and 3° thick. At 31° their red edge is around 10° in diameter, and they're almost 5° thick, but it's tough to say since they're so fuzzy. If the sun is higher than 32.3° above the horizon, the light entering the top face is simply reflected out the bottom face rather than an edge, and the chromatic aberration is largely negated. Halos that form this way are parhelic circles.

CZAs are blurry and less intense when they get close to the edges of a 32.3° and 5° sun elevation. Past that they don't exist. As Oolong notes, they are at their sharpest and most beautiful with the sun at about 22° high. When the sun rises above 58°, these same conditions may form a circumhorizontal arc.

In extremely rare cases where double-plate ice crystals (that look something like lopsided dumbbells) float in a vertical orientation, the abberated sunlight coming out of the top plate can reflect off of an edge face in the lower plate, and the reflected spectrum completes the circle, an effect dubbed the Kern arc. It is very rarely seen and has yet to be photographed. In similarly rare cases when these double-plate crystals float heavy-side up, the arc can be inverted. This means the sky is frowing at you, and noders given to augury best run for cover, or at the very least turn around so it's smiling again like a big, prismatic Cheshire Cat.

The first time I saw one of these I just happened to glance up through the sun roof on my car and almost wrecked into the car in front of me I was so absorbed in the strangeness of it (I didn't have any of this background knowledge then) and the colors more intense and pure than anything I had ever seen in my life. Like crack for your retinal cones. I pulled over and just watched it for awhile with my neck craned up. A couple of people walking by on the sidewalk cautiously looked up too, and we sat there like Deer in the Headlights of Beauty until it slowly faded and returned us to the world.


Sources
  • http://www.sundog.clara.co.uk/halo/cza.htm
  • http://www.meteoros.de/arten/ee11e.htm
  • http://amsglossary.allenpress.com/glossary/search?p=1&query=circumzenithal+arc
  • http://www.atmos.ucla.edu/~liougst/Group_Papers/Takano_AO_36_1997.pdf

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