Look for the rare one

Last updated on September 16, 2023

Look for the Rare One: Exploring Atmospheric Optics

Have you ever looked up at the sky and been mesmerized by the beautiful display of colors and halos that seem to appear out of nowhere? Atmospheric optics is a fascinating field that studies these optical phenomena and seeks to unravel the mysteries behind them. In this article, we will delve into one specific atmospheric optics topic: the rare halos known as circumzenithal arcs and supralateral arcs.

The Circumzenithal Arc: A Misinterpreted Rainbow

The upper bright ice halo that is often mistaken for a rainbow is actually a circumzenithal arc (CZA). This optical phenomenon is created by horizontal hexagonal ice plates in the atmosphere. When sunlight enters the uppermost face of these plates and exits through a side face, refraction occurs between faces at right angles. This refraction, combined with the near-parallel sun rays, results in pure and well-separated colors.

Unveiling the Supralateral Arc: Elusive and Unique

While the circumzenithal arc may be a familiar sight to some, it is often accompanied by a rarer halo known as the supralateral arc. This downward curving halo touches the CZA, but it is much fainter and harder to spot. The supralateral arc is formed by column crystals with their long axes positioned horizontally. However, its rarity stems from the fact that these column crystals rarely have optically perfect end faces, making it more challenging to observe.

Exploring the Origins of Rare Halos

To fully understand the formation of these rare halos, it is essential to explore the unique properties of ice crystals in the atmosphere. The hexagonal shape of the ice plates responsible for the CZA and the columnar structure of the crystals forming the supralateral arc play a crucial role in their appearance. The interaction between sunlight, these ice crystals, and the atmospheric conditions results in the captivating optical displays we observe.

The Hunt for Rare Halos

Now that we have a better understanding of circumzenithal arcs and supralateral arcs, how can we spot these rare halos in the sky? Here are some tips to help you in your quest:

• Whenever you see a circumzenithal arc, be sure to scan the sky for a fainter downward curving halo that touches it. This could be the elusive supralateral arc.
• Look for these halos when the sun is relatively high in the sky, typically between 22 and 46 degrees above the horizon.
• Pay attention to atmospheric conditions. These rare halos are more likely to appear in cold and icy environments, such as polar regions or during winter months.

Capturing the Beauty: Photography Tips

If you're lucky enough to witness these rare halos, why not try capturing their beauty through photography? Here are some tips to help you capture these atmospheric optical phenomena:

• Use a wide-angle lens to capture a larger portion of the sky and include any surrounding landscape for context.
• Experiment with different exposure settings to bring out the vibrant colors and details of the halos.
• Consider using a polarizing filter to reduce glare and enhance the contrast of the halos against the sky.

The Fascination Continues

Atmospheric optics never fails to amaze us with its intricate displays of light and color. The rare halos, such as circumzenithal arcs and supralateral arcs, add an extra layer of enchantment to our skies. As we continue to explore and study these optical phenomena, we deepen our understanding of the complex interactions between sunlight, ice crystals, and atmospheric conditions. So next time you find yourself gazing at the sky, keep an eye out for the rare one – it might just leave you in awe of nature's wonders.

Look for the rare one

Charles Uibel (GreatSaltLakephotos.com) caught this combination of rare and not so rare halos. Images ©Charles Uibel shown with permission.

The upper bright ice halo, very often mistaken for a rainbow, is a circumzenithal arc. Horizontal hexagonal plates form it. Sunlight enters the uppermost face and leaves via a side face. The combination of refraction between faces at right angles and near parallel sun rays gives pure, well separated, colours.

Whenever a CZA is seen, look for a fainter downward curving halo that just touches it. This is the rarer supralateral arc. Column crystals with their long axes horizontal form it. Its rarity stems in part from the ray path. Rays exit through a hexagonal column end face. these are rarely optically perfect.

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Reference Atmospheric Optics

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