The circumzenithal arc (CZA) is considered one of the most beautiful atmospheric optical phenomena and is often described as an "upside-down rainbow." It appears as a brilliant arc of spectral colours situated approximately 46° above the sun, with its convexity pointing upward (away from the sun), opposite to a conventional rainbow. Its colours are exceptionally pure and saturated—more so than any other halo phenomenon. It forms by refraction of sunlight entering the flat upper face of hexagonal ice crystals (plate type) and exiting through a side face, with crystals oriented horizontally in high cirrus clouds. This specific geometry produces an angular dispersion of approximately 46° with exceptional chromatic separation. The CZA is only visible when the sun is below 32° elevation; at higher solar altitudes, the refraction angle prevents its formation. Despite its spectacular appearance, the circumzenithal arc frequently goes unnoticed because it forms very high in the sky where few people tend to look. Its presence indicates cirrus clouds with well-formed and oriented ice crystals, and it frequently accompanies sundogs and 22° solar halos. At European mid-latitudes, it can be observed several dozen times per year with cirrus skies, especially during transitional seasons (spring and autumn).