A circumzenithal arc appears to stretch across the tops of trees on Cougar Mountain near Bellevue, Washington. Circumzenithal arcs appear as upside-down rainbows and are the brightest and most colorful of all the solar halos. They appear when the sun is relatively low on the horizon, less than 32 degrees above the horizon and ideally 20°, and cirrus clouds are overhead. Circumzenithal arcs are especially bright and vibrant because the ice crystals in the cirrus clouds are perfectly aligned, passing through almost parallel bands of light.
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