Circumzenithal Arc Queen of Halos

Last updated on September 16, 2023

Circumzenithal Arc Queen of Halos: A Mesmerizing Atmospheric Phenomenon

When it comes to atmospheric optics, the circumzenithal arc (CZA) reigns as the queen of halos. This captivating phenomenon, often referred to as an "upside down rainbow," is a surprising and ethereal sight that appears near the zenith. Unlike rainbows, which are formed by raindrops, the CZA is created by precisely shaped ice crystals suspended high in the atmosphere.

To catch a glimpse of the CZA, look straight up towards the zenith when the sun is relatively low in the sky, especially if sundogs are also visible. The arc's center always points towards the sun, with its outer edge adorned in vibrant shades of red. It is a spectacle that showcases the wonders of nature's artistic mastery.

The CZA is one of two spectrometers found in nature, the other being the circumhorizon arc. What makes the CZA truly unique is its ability to split the sun's almost parallel rays using 90° ice prisms. This splitting results in the display of an almost pure spectrum, unparalleled in its purity compared to rainbows, coronae, and glories.

The Science Behind the CZA

The mesmerizing display of the CZA is made possible by hexagonal plate crystals found in cirrus clouds. As parallel rays of sunlight enter the top face of these icy crystals, they exit through a side face, causing the light to split into a spectrum. This process mirrors the function of a laboratory prism spectrometer.

The formation of the CZA relies on specific conditions for ice crystal alignment and sunlight angle. The hexagonal shape of the ice crystals, along with their horizontal orientation, plays a crucial role in creating this optical phenomenon. When aligned properly, the crystals act as miniature prisms, bending and refracting sunlight to produce the stunning arc.

Capturing the CZA's Beauty

Photographing the CZA requires a keen eye and a bit of luck. Since it appears near the zenith, it can be challenging to capture the full extent of its beauty in a photograph. However, skilled photographers have managed to capture breathtaking images of this atmospheric marvel. Orion Elenzil, for example, captured a particularly demure CZA flickering in passing streaky cirrus clouds miles high over California.

The CZA and Weather Patterns

The presence of cirrus clouds, composed of ice crystals, is essential for the formation of the CZA. These clouds typically reside at high altitudes, often preceding an approaching warm front or an area of upper-level moisture. Observing the CZA can provide valuable insights into atmospheric conditions and serve as a visual indicator of changing weather patterns.

Cultural Significance

Throughout history, humans have been captivated by atmospheric phenomena. The CZA, with its enchanting display, has inspired awe and wonder across cultures. In some societies, it is considered a symbol of good luck or a sign of impending rain. Folklore and mythology often attribute mystical properties to such optical wonders, further adding to their allure.

Appreciating Nature's Spectacular Show

Witnessing the circumzenithal arc is a reminder of the awe-inspiring beauty and complexity of our natural world. Its ephemeral presence serves as a gentle reminder to slow down and appreciate the wonders that surround us. Whether you're lucky enough to witness this atmospheric queen firsthand or simply marvel at photographs, the CZA is a testament to the remarkable interplay between light, ice crystals, and the Earth's atmosphere.

In conclusion, the circumzenithal arc is a mesmerizing atmospheric phenomenon that showcases nature's artistic prowess. Its unique formation through precisely shaped ice crystals creates a breathtaking display of color and light near the zenith. Photographing this ethereal rainbow requires skill and luck, but the resulting images are truly awe-inspiring. Understanding the science behind the CZA allows us to appreciate the intricate processes that occur in our atmosphere. Whether it serves as a sign of changing weather patterns or a symbol of good fortune, the CZA continues to captivate and inspire us with its beauty. So, keep your eyes on the sky and be ready to witness the queen of halos in all her splendor.

The Queen of Halos

A particularly demure circumzenithal arc flickers in passing streaky cirrus miles high over California. Pictured by Orion Elenzil. Orion Elenzil

The circumzenithal arc, CZA, is the most beautiful of all the halos. The first sighting is always a surprise, an ethereal rainbow fled from its watery origins and wrapped improbably about the zenith. It is often described as an "upside down rainbow" although rainbow it is not. Exactly shaped ice crystals fashion it rather than mere raindrops.

Look straight up near to the zenith when the sun is fairly low and especially if sundogs are visible. The arc's centre points sunwards. Red is on its outside.

The CZA is one of Nature's two spectrometers (the related circumhorizon arc is the other). 90° ice prisms split the sun's almost parallel rays to show us an almost pure spectrum. Rainbows, coronae nor glories can ever match its purity.

Parallel sun rays enter the top face of icy hexagonal plate crystals composing the cirrus

cloud. They leave through a side face split into a spectrum.

A laboratory prism Spectrometer works in the same way.

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