Optical Phenomena
Cirrus clouds produce several optical effects, including glories. A glory is a set of concentric, faintly-colored glowing rings that appear around the shadow of the observer. These clouds only form glories when the constituent ice crystals are aspherical, and researchers suggest that the ice crystals must be between 0.009 millimeters and 0.015 millimeters in length. Cirrus mixed with cirrostratus can produce halos around the sun and sundogs, which are arcs of brightness near the sun.
The cirrus/cirrostratus combination can also produce colorful arcs such as the circumzenithal and circumhorizontal arcs. The top color of a circumhorizontal arc is red, followed by orange, then running through all the colors of the rainbow to violet on the bottom. The ice crystals required to produce one of these arcs must be shaped like plates, and the crystals must be oriented horizontally. Light comes from the sun and passes through a cirrus/cirrostratus cloud. The sun must be either below 32° or more than 58° above the horizon to produce a circumzenithal or circumhorizontal arc, respectively. The sunlight enters one face of a crystal and refracts through it, exiting with its colors spread in a rainbow-like pattern.
Read more about this topic: Cirrus Cloud
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