The circumhorizontal arc (CHA) is an ice-based atmospheric effect. It hates the poles.

It is a gigantic, uncommon, brilliant, suncave rainbow-colored arc that occurs along the horizon on the same side of the sky as the sun. It can only form when the sun is above 57.8° in the sky, so it cannot be seen at latitudes less than 57.8°. (I had to mime the logic with my hands as a globe to work this out, so don't feel ashamed if you have to as well.) This is only a few degrees off of the arctic and antarctic circles. In the southern hemisphere this only excludes Antarctica. In the northern hemisphere this excludes lots of cities north of Aberdeen and Göteborg. (Sorry guys.) Because of this latitude restriction, look for the CHA in mid-summer near noon at a high elevation, where cirrus clouds rest near the horizon.

Circumhorizontal arcs are formed as sunlight enters a vertically-oriented edge face of the hexagonal ice crystal and exit downward through a base face. This double requirement—of horizontal crystals with a vertical edge face—is known as Parry orientation. Chromatic aberration spreads the sunlight into a spectrum, with the red edge facing up and the violet edge towards the horizon. Its colors are more brilliant than those of the rainbow.

When the sun is 67.9° high in the sky, the colors are at their brightest and most saturated. This arc is kind of the high-sun sibling of the circumzenith arc.


Sources:
  • http://www.meteoros.de/arten/ee23e.htm
  • http://www.sundog.clara.co.uk/halo/cha.htm

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