Upper: Schematic of ice crystal
Lower: Sunlight path through ice crystal
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I could observe a very rare atmospheric phenomenon called "Circumhorizontal Arc" about noon of April 22, 2001 in Tokyo.
The illusory event showed me for about a half hour, a bit arc dimmed light like a rainbow appeared in front of a thin cloud paralleling with the horizon.
The Circumhorizontal Arc looks like a rainbow, but the mechanism of it is quite different from that of rainbow.
The rainbow is observed when water spheres in sky reflect and refract sunlight.
While the Circumhorizontal Arc appears by fine ice particles in high altitude sky refracting sunlight.
Besides we can see the phenomenon only when ice particles form horizontal-plate crystals shown in right figure and they're turning same direction.
The ice crystal in sky brings about various interesting optical phenomena like Halo and Parhelia.
When sunlight enters the vertical prism face and exits at bottom basal face of the hexagonal horizontal-plate ice crystal, the sunlight refracts about 46 degrees (shown in right figure).
This refracted light causes the rare phenomenon, we can observe as a dimmed rainbow-light about 46 degrees lower sky from the sun.
This light starts with a sun elevation of 58 degrees.
And in case that sunlight refracts through reverse path, the halo appears about 46 degrees upper than sun,
this spectrum called "Circumzenith Arc", appreciated when solar elevation is low.
Because appearance conditions of both arcs are restricted by solar elevation,
we can never detect Circumhorizontal and Circumzenith Arcs simultaneously.
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