Circumzenithal arc over Bulgaria

Last updated on September 16, 2023

Circumzenithal Arc over Bulgaria: A Spectacular Atmospheric Phenomenon

When it comes to atmospheric optics, nature often puts on a mesmerizing show. One such captivating display is the Circumzenithal Arc (CZA), which recently graced the skies over Bulgaria. Captured in stunning images by Nikolay Stoilov from Cherni Vrah (Black peak) on Vitosha Mountain near Sofia, this rare optical phenomenon left spectators in awe.

The Beauty of Ice Crystals

The Circumzenithal Arc over Bulgaria was formed by two distinct types of ice crystals. Hexagonal plate crystals were responsible for generating the bright CZA, while the down-curving, much fainter arc touching the CZA was produced by sun rays passing between the side and end faces of horizontal column crystals. Together, these crystals created a breathtaking spectacle against the backdrop of the Bulgarian sky.

Understanding the Circumzenithal Arc

The Circumzenithal Arc is an atmospheric optical phenomenon that occurs when sunlight interacts with ice crystals in the atmosphere. It forms a vivid arc that appears above the sun, resembling an upside-down rainbow. The arc is known for its vibrant colors, ranging from red at the top to blue at the bottom. Its position in the sky is higher than that of a traditional rainbow, adding to its allure.

The Science Behind the Phenomenon

To understand how the Circumzenithal Arc is formed, we must delve into the intricate physics of light and ice crystals. As sunlight passes through hexagonal plate ice crystals, it undergoes a process called refraction. This bending of light causes the rays to separate into different colors, similar to what occurs in a prism. The resulting dispersion creates the beautiful spectrum of colors seen in the CZA.

Optimal Conditions for the Circumzenithal Arc

The formation of a Circumzenithal Arc requires specific atmospheric conditions. First and foremost, there must be an abundance of hexagonal plate ice crystals present in the atmosphere. These crystals typically form in high-level cirrus clouds, which are composed of ice particles. Additionally, the angle of the sun must be just right for the light to pass through the crystals at the correct angle to create the CZA.

Rare and Transient Sightings

Although the Circumzenithal Arc is a stunning phenomenon, it is relatively rare to witness. The specific combination of ice crystal shape and sunlight angle needed for its formation is not frequently encountered. Moreover, the arc itself is transient, often lasting for only a short period of time before dissipating. Therefore, capturing this ephemeral spectacle on camera, as Nikolay Stoilov did, is a remarkable achievement.

Appreciating Atmospheric Optics

The Circumzenithal Arc over Bulgaria serves as a reminder of the incredible beauty that can be found in the natural world. Atmospheric optics phenomena like this offer a glimpse into the wonders of light and physics. By taking the time to observe and appreciate these fleeting moments, we gain a deeper understanding and appreciation for the intricate workings of our atmosphere.

Conclusion

The Circumzenithal Arc over Bulgaria was a sight to behold. Its vibrant colors and ethereal appearance captivated onlookers and showcased the magic of atmospheric optics. Understanding the science behind this phenomenon adds another layer of appreciation for the intricacies of nature. While rare and transient, occurrences like the CZA remind us to look up and marvel at the wonders that can be found right above our heads.

Halos over Bulgaria imaged at Cherni Vrah (Black peak), Vitosha Mountain near Sofia by Nikolay Stoilov. ©Nikolay Stoilov, shown with permission.

Two types of ice crystal produced this display.

Hexagonal plate crystals generated the bright circumzenithal arc (CZA).

The down curving much fainter arc touching the CZA is a supralateral arc produced by sun rays passing between the side and end faces of horizontal column crystals.

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