OPOD - Henley Circumzenithal Arc

Last updated on September 16, 2023

OPOD - Henley Circumzenithal Arc: A Spectacular Atmospheric Phenomenon

Have you ever looked up at the sky and been amazed by the vibrant colors and patterns? One such mesmerizing atmospheric phenomenon is the circumzenithal arc, which can be seen in all its glory near Henley-on-Thames, England. This optical display, captured by Gemma Worswick with a phone camera, showcases the incredible brightness and vivid colors that the circumzenithal arc can exhibit as it gracefully arches overhead.

Unlike many other atmospheric optical phenomena like the common 22° halo, sundogs, and rainbows, the colors of the circumzenithal arc are remarkably pure and spectrally distinct. This phenomenon occurs when sunlight enters the top faces of horizontally aligned plate crystals and undergoes refraction and dispersion, separating the light into its constituent spectral colors. As the rays of light leave a near-vertical ice face, they continue to disperse and are directed downwards, creating the stunning display of colors that we see.

The curvature of the circumzenithal arc is a result of the rotational positions of cloud crystals around their vertical axes. These crystals can take on various orientations, leading to the curve in the arc. However, it is important to note that the circumzenithal arc can never form a complete circle. That distinction belongs to the exceedingly rare Kern arc.

Now that we have explored the basics of the circumzenithal arc, let's delve deeper into this fascinating atmospheric phenomenon:

1. Formation of Plate Crystals: The circumzenithal arc owes its existence to plate crystals, which are flat, hexagonal ice crystals that form in high-altitude clouds. These crystals have a preferred orientation that allows them to act as tiny prisms, refracting and dispersing sunlight to create the vibrant colors of the arc.

2. Optical Characteristics: Unlike rainbows, which are formed by the refraction and reflection of sunlight within raindrops, the circumzenithal arc is formed by the refraction and dispersion of sunlight within ice crystals. This distinction accounts for the different optical properties and colors exhibited by these atmospheric phenomena.

3. Spectral Purity: One of the most remarkable features of the circumzenithal arc is its nearly spectrally pure colors. This means that the colors observed in the arc are highly saturated and free from any significant blending or mixing with adjacent colors. The purity of these colors adds to the awe-inspiring beauty of this optical phenomenon.

4. Angle of Observation: To witness a circumzenithal arc, one must look upward towards the zenith, which is directly overhead. The arc forms a graceful curve that extends from the zenith towards the horizon, creating a celestial spectacle that can captivate observers.

5. Meteorological Conditions: The presence of high-altitude clouds, typically composed of ice crystals, is necessary for the formation of a circumzenithal arc. These clouds often occur in cold regions or during specific weather patterns, such as frontal systems or the presence of an upper-level disturbance.

6. Variations in Appearance: While the circumzenithal arc typically appears as a single arc, there can be instances where multiple arcs are observed. These additional arcs are known as supralateral arcs and can form alongside the primary circumzenithal arc, adding to the complexity and beauty of the display.

7. Rare Atmospheric Sightings: Although the circumzenithal arc is relatively common compared to some other atmospheric optics phenomena, it is still considered a rare sight for many people. Its occurrence is dependent on specific conditions aligning perfectly, making each sighting a special and unique experience.

8. Photographing the Circumzenithal Arc: Capturing the full grandeur of the circumzenithal arc in a photograph can be challenging. The dynamic range of colors and the arc's position near the zenith require careful composition and exposure settings to do justice to its stunning beauty. Gemma Worswick's photograph near Henley-on-Thames is a testament to the captivating nature of this phenomenon.

9. Similar Optical Phenomena: While the circumzenithal arc stands out with its vibrant colors and distinctive curve, there are other optical phenomena that share similarities. The circumhorizontal arc, for example, forms a horizontal arc instead of an overhead one and is also caused by the refraction of sunlight through ice crystals.

10. Appreciating Nature's Splendor: The circumzenithal arc serves as a reminder of the astonishing beauty and complexity of our natural world. These rare displays of light and color inspire wonder and awe, prompting us to appreciate the intricate workings of our atmosphere and the incredible phenomena it can produce.

Next time you find yourself gazing at the sky, take a moment to look for the circumzenithal arc. Its breathtaking colors and graceful curvature are sure to leave you in awe of the wonders that can be found right above our heads. So keep your eyes peeled for this rare and captivating atmospheric phenomenon, and let yourself be enchanted by nature's artistry.

Circumzenithal Arc

This image taken with a 'phone camera by Gemma Worswick near Henley-on-Thames, England shows how very bright and colourful the circumzenithal arc can be as it rides overhead. ©Gemma Worswick.

Unlike the common 22° halo, sundogs, most other halos and even rainbows, the colours of the circumzenithal arc are almost spectrally pure.

Near parallel sunlight rays enter the top faces of horizontally aligned plate crystals where they are refracted and dispersed into spectral colours. The rays are further dispersed as they leave a near vertical ice face to be directed downwards.

The arc curves because cloud crystals have all rotational positions about their vertical axes. But the arc can never be a complete circle - that is the exceedingly rare Kern arc.

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Reference Atmospheric Optics

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