OPOD - Infralateral Arc

Last updated on September 16, 2023

OPOD - Infralateral Arc: A Rare and Magnificent Atmospheric Phenomenon

Have you ever witnessed a breathtaking atmospheric display that left you in awe? One such phenomenon is the infralateral arc, a rare and captivating optical effect that can be observed in certain atmospheric conditions. In this article, we will delve into the intricacies of the infralateral arc and explore its unique characteristics.

What is an Infralateral Arc?

An infralateral arc is a stunning atmospheric phenomenon that appears as a faint, colorful band of light positioned below a 22° halo. Unlike the more common halo, which forms a complete circle around the sun or moon, the infralateral arc is a partial arc that hangs above the horizon. Its position in the sky, combined with its vibrant hues, makes it a captivating sight for skywatchers and photographers alike.

The Origin of Infralateral Arcs

Infralateral arcs, along with their counterparts known as supralateral arcs, are created by sunlight passing through hexagonal ice crystals present in the atmosphere. These ice crystals, shaped like columns, become aligned in the air due to aerodynamic forces. When sunlight enters these crystals at specific angles, it undergoes refraction and dispersion, resulting in the formation of these beautiful arcs.

The Formation Process

To better understand how infralateral arcs form, let's take a closer look at their unique characteristics. The rays of sunlight that contribute to the creation of an infralateral arc enter a near-vertical face of a hexagonal ice crystal and exit through one of its side faces. This particular path of light rays leads to the distinctive shape and positioning of the infralateral arc in the sky.

The Splendor of Color

One of the most captivating aspects of infralateral arcs is their vibrant and widely spread colors. As sunlight passes through the ice crystals, the refraction and dispersion of light cause the colors to separate, similar to the way a prism disperses white light into a spectrum of colors. This phenomenon results in a stunning display of hues that can range from pale pastels to intense shades, enhancing the visual allure of the infralateral arc.

The Enigmatic Supralateral Arc

Accompanying the infralateral arc, you may sometimes spot a faint section of a supralateral arc. While infralateral arcs form when light enters through a near-vertical face and exits through a side face of an ice crystal, supralateral arcs exhibit the reverse ray path. The rays of sunlight enter through a side face and exit through a near-vertical face of the hexagonal ice crystal, resulting in a supralateral arc positioned above the 22° halo.

Rare and Elusive Sightings

Due to specific atmospheric conditions required for their formation, infralateral arcs are relatively rare and elusive phenomena. These atmospheric conditions include the presence of hexagonal ice crystals with specific orientations and adequate sunlight to create the necessary refraction and dispersion. Consequently, witnessing an infralateral arc is a true testament to the beauty and unpredictability of nature.

Capturing the Magic

Photographers and sky enthusiasts fortunate enough to witness an infralateral arc often aim to capture its mesmerizing beauty. Through careful observation and skilled photography techniques, these individuals can immortalize this rare atmospheric phenomenon. By capturing the colors, position, and intricacies of the infralateral arc, they allow others to marvel at its magnificence long after it fades from the sky.

Conclusion

The infralateral arc is undoubtedly a remarkable spectacle that showcases the wonders of atmospheric optics. Its position below the 22° halo, vibrant colors, and unique formation process make it a sight to behold. While rare and elusive, those fortunate enough to witness an infralateral arc are treated to a visual feast that exemplifies the enchanting nature of our atmosphere. So keep your eyes on the sky, and who knows what extraordinary phenomena you may encounter on your next skywatching adventure!

Infralateral Arc Plus

A magnificent and rare infralateral arc seemingly hangs above an alpine valley. Willem Koppenol imaged this as part of a wider diamond dust halo display pictured in an earlier OPOD.

Look closely to its left to see a faint colouration. Peter Paul Hattinga Verschure spotted this and identified it as a supralateral arc. A bright example of the pair is here.

©Willem Koppenol, shown with permission.

Infralateral arcs and their cousins, supralateral arcs, glint their colours twice as far from the sun as the more commonplace 22° halo.

Both are produced by sun rays passing through hexagonal column ice crystals oriented by aerodynamic drag to have their long axes nearly horizontal.

The infralateral arc results from rays that enter a near vertical hexagonal end face and leave through a side face.

The arc is strongly coloured with the hues widely spread by the refraction that is in effect through a 90° prism.

Supralateral arc - a very faint section of one shows on the images to the left of the infralateral arc - have the reverse ray path. Rays enter a side face and leave through a near vertical one.

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Reference Atmospheric Optics

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