Circumhorizon Arc

Last updated on September 16, 2023

Circumhorizon Arc: A Spectacular Atmospheric Phenomenon

The natural world is full of breathtaking wonders, and one such marvel that often captivates observers is the circumhorizon arc. This atmospheric optical phenomenon is a stunning display of vibrant colors that can be seen in the sky, adding a touch of magic to the natural scenery. Also known as the "fire rainbow," the circumhorizon arc is a sight to behold and leaves spectators in awe of its beauty. In this article, we will delve into the intricacies of this remarkable phenomenon and explore its formation, characteristics, and where it can be observed.

What is a Circumhorizon Arc?

A circumhorizon arc is an optical phenomenon that occurs when sunlight passes through ice crystals in cirrus clouds, creating a stunning display of vibrant colors. It appears as a horizontal band of vivid hues, resembling a rainbow lying flat on the horizon. Unlike a traditional rainbow that forms from raindrops, the circumhorizon arc is formed by the refraction and dispersion of sunlight through hexagonal ice crystals suspended in the atmosphere.

Formation of a Circumhorizon Arc

To understand how a circumhorizon arc forms, it's important to grasp the role of ice crystals in its creation. Cirrus clouds, which are thin and wispy, are composed of tiny ice crystals that are shaped like hexagonal prisms. When sunlight enters these crystals at a specific angle, it undergoes refraction, causing the light to bend as it passes through. The different colors of light are then dispersed, creating a spectrum of colors that form the distinct arc shape.

The angle at which the sunlight enters the ice crystals is crucial for the formation of a circumhorizon arc. It must be at a specific range between 58 and 68 degrees from the observer's perspective. This means that circumhorizon arcs are only visible in certain latitudes where the sun reaches the required altitude. Consequently, they are most commonly observed in regions between 55 degrees north and 55 degrees south.

Characteristics of a Circumhorizon Arc

The circumhorizon arc is characterized by its vibrant colors and distinct shape. It typically appears as a long, horizontal band of colors that stretches across the sky parallel to the horizon. The colors are similar to those of a traditional rainbow, ranging from red on the top to violet on the bottom. However, the circumhorizon arc often displays more intense and saturated colors due to the unique atmospheric conditions required for its formation.

One of the most striking features of a circumhorizon arc is its size. These arcs can span up to 46 degrees in length, making them an impressive sight to behold. Their vast size is a result of the specific angle at which sunlight must enter the ice crystals in order to create the refraction and dispersion necessary for the phenomenon to occur.

Where to Observe a Circumhorizon Arc

While circumhorizon arcs can occur in various locations around the world, their visibility is influenced by specific geographical factors. As mentioned earlier, they are typically observed between latitudes of 55 degrees north and 55 degrees south. This means that countries such as the United States, Canada, United Kingdom, Australia, New Zealand, and parts of Europe and Asia fall within the potential viewing range.

To spot a circumhorizon arc, certain conditions must be met. Firstly, there must be cirrus clouds present in the sky. These clouds are often high-altitude clouds that appear thin and wispy. Secondly, the sun must be at a specific altitude, typically around 58 to 68 degrees above the horizon. Lastly, the observer must have an unobstructed view of the sky without any lower clouds obscuring the phenomenon.

Conclusion

The circumhorizon arc is a truly magnificent atmospheric phenomenon that enchants and mesmerizes those lucky enough to witness it. Its vibrant colors, vast size, and distinct shape make it a spectacle that leaves observers in awe of the wonders of nature. Understanding the science behind its formation adds to the appreciation of this captivating display in the sky. So, the next time you find yourself gazing at the heavens, keep an eye out for the elusive circumhorizon arc and prepare to be amazed by its beauty.

Circumhorizon arcs must often be sought between lower obscuring clouds. Mitsy Marx of Redding, CA saw this fine example in June 2004. Image ©Mitsy Krzywicki, shown with permission.

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