Circumzenithal Arc, Johnston, Iowa

Circumzenithal Arc: A Delicate Atmospheric Phenomenon in Johnston, Iowa

The sky is a canvas of wonders, and one of its most delicate and mesmerizing displays is the Circumzenithal Arc. In Johnston, Iowa, on the 29th of May in 2001, an incredible sighting of this optical phenomenon occurred. Captured by Stan Richard, the image beautifully portrays the arc's delicacy as it adorned the sky at 18:50 local time, with the sun positioned 8 degrees high. Let us delve deeper into the enchanting world of the Circumzenithal Arc and explore its characteristics and formation.

What is a Circumzenithal Arc?

The Circumzenithal Arc, often referred to as the "upside-down rainbow," is a rare atmospheric optical phenomenon that occurs in the sky. It forms when sunlight interacts with ice crystals in the upper atmosphere, creating a stunning display of colors. Unlike a traditional rainbow that arcs across the sky, the Circumzenithal Arc appears as an arc positioned directly above the sun, with its convex side pointing downwards.

Characteristics and Appearance

The Circumzenithal Arc is known for its vibrant colors and distinct shape. It typically exhibits a vivid range of hues, including reds, oranges, yellows, and blues. The arc itself appears as a smooth, curving band that stretches across the sky. Due to its unique positioning above the sun, it may sometimes be mistaken for a fragment of a rainbow or a cloud formation. However, upon closer inspection, its inverted nature and vibrant colors distinguish it as a Circumzenithal Arc.

Formation Process

The formation of a Circumzenithal Arc relies on specific atmospheric conditions and the presence of ice crystals. When sunlight passes through these ice crystals, it undergoes refraction, which causes the light to bend. The bending of light within the ice crystals separates the sunlight into its component colors, resulting in the stunning array of hues seen in the arc. The angle at which the sunlight enters and exits the ice crystals determines the curvature of the arc.

Atmospheric Optics and Johnston, Iowa

Johnston, Iowa, provides an ideal location for observing atmospheric optics phenomena such as the Circumzenithal Arc. Its geographical position and atmospheric conditions contribute to the occurrence of these awe-inspiring displays. The presence of ice crystals in the upper atmosphere, coupled with the appropriate angle of sunlight, creates the perfect conditions for the formation of the Circumzenithal Arc.

Rare Occurrence and Viewing Tips

The Circumzenithal Arc is considered a relatively rare phenomenon due to the specific conditions required for its formation. Observing this spectacle requires a combination of clear skies, the presence of ice crystals, and the sun positioned at a specific angle. As a result, sightings of the Circumzenithal Arc are infrequent and often fleeting. To increase your chances of witnessing this enchanting display, keep the following tips in mind:

  • Look for the Circumzenithal Arc when the sun is relatively high in the sky, typically between 20 and 40 degrees above the horizon.
  • Scan the sky directly above the sun for an inverted rainbow-like arc.
  • Pay attention to atmospheric conditions. Clear skies with scattered clouds increase the likelihood of spotting this phenomenon.
  • Be patient and observant. The Circumzenithal Arc may appear and disappear quickly, so keen attention is necessary to catch this fleeting beauty.

The Beauty of Atmospheric Optics

The world of atmospheric optics is filled with captivating displays that remind us of the awe-inspiring beauty of nature. The Circumzenithal Arc in Johnston, Iowa, serves as a testament to this wonder. Its delicate formation, vibrant colors, and rarity make it a sight to behold. So, keep your eyes on the sky and embrace the enchantment that atmospheric optics has to offer. You never know when you might witness a breathtaking display like the Circumzenithal Arc.

Circumzenithal arc at Johnston, Iowa. This image by Stan Richard nicely portrays the arc's delicacy. Imaged on 29th May '01 at 18.50 local time, sun 8� high. �2001 Stan Richard, shown with permission.

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Reference Atmospheric Optics

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  • "Circumzenithal Arc, Johnston, Iowa". Atmospheric Optics. Accessed on March 29, 2024. https://atoptics.co.uk/blog/circumzenithal-arc-johnston-iowa/.

  • "Circumzenithal Arc, Johnston, Iowa". Atmospheric Optics, https://atoptics.co.uk/blog/circumzenithal-arc-johnston-iowa/. Accessed 29 March, 2024

  • Circumzenithal Arc, Johnston, Iowa. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/circumzenithal-arc-johnston-iowa/.