The Circumzenithal Arc (CZA), often referred to as the “Smile in the Sky”, is one of the most spectacular natural light phenomena in the sky. Although it is relatively rare, whenever it appears, it captivates observers with its vivid colors and the characteristic “rainbow smile” high above. Among halo phenomena, the CZA stands out for its brightness and intensity, often surpassing other types of halos.
The CZA occurs when sunlight is refracted through horizontally oriented, plate-shaped ice crystals suspended in high-altitude clouds, such as Cirrus or Cirrostratus clouds. When these crystals are positioned just right, sunlight entering at a specific angle is bent upward, producing a colorful arc that curves overhead. From the ground, it appears like an “upside-down rainbow,” forming only about a quarter of a full circle rather than a complete arc.
Although it resembles a rainbow, the CZA differs fundamentally from a traditional rainbow, which forms when sunlight is reflected and refracted in water droplets after rainfall. In contrast, the CZA results from sunlight passing through ice crystals and is classified as a halo phenomenon. Typically, it appears in clear skies without rain, and its vivid colors are strikingly more intense than those of other halos.
The arc of a CZA is located on the same side as the sun but high above, near the zenith, requiring observers to tilt their heads upward to see it. Its color arrangement is also reversed compared to a rainbow: blue is on the inner edge (closer to the zenith), while red is on the outer edge (closer to the horizon).
Several precise conditions must be met for a CZA to appear:
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The sun must be lower than 32 degrees above the horizon.
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High-altitude clouds must contain an adequate number of plate-shaped ice crystals.
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The crystals must be oriented horizontally and remain stable.
Because these conditions are rare, the CZA does not appear frequently, making each sighting a special and memorable event for sky watchers and nature enthusiasts.
In summary, the Circumzenithal Arc is a beautiful, simple phenomenon that showcases the wonder of natural physics. It is truly a “smile in the sky,” formed by the perfect alignment of sunlight, ice crystals, and atmospheric conditions.
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