Yosemite Circumhorizon Arc - OPOD

Last updated on September 16, 2023

Yosemite Circumhorizon Arc: A Spectacular Atmospheric Phenomenon

The Yosemite Circumhorizon Arc (CHA) is a mesmerizing atmospheric optical phenomenon that graces the skies above Yosemite National Park in California. This celestial display is a delicate and colorful halo that captures the imagination of those lucky enough to witness it. The CHA is formed by the interaction of sunlight with hexagonal ice crystals suspended in the atmosphere, creating a breathtaking spectacle that adds to the natural beauty of this iconic location.

To understand the formation of the CHA, we must first delve into the intricate workings of these ice crystals. Hexagonal plate-like ice crystals are responsible for this phenomenon, and their orientation plays a crucial role in the appearance of the arc. When sunlight enters the side faces of these crystals and exits through their lowest face, a CHA is born. The shape and orientation of the crystals allow the sunlight to refract and create a stunning display of colors against the backdrop of the sky.

The height of the sun in the sky also plays a vital role in the formation of the CHA. When the sun is above 58°, the conditions are ideal for a colorful CHA to manifest itself. As the sun's angle decreases, the appearance of the CHA changes. When the sun is lower, the CHA ray barely manages to escape from the lower face of the ice crystal. Finally, when the sun is even lower, the ray is completely internally reflected, resulting in the absence of a CHA.

The CHA's vibrant colors are a result of the refraction and dispersion of sunlight as it passes through the ice crystals. This phenomenon splits the sunlight into its constituent colors, similar to how a prism separates white light into a rainbow spectrum. The resulting colors can range from vivid reds and oranges to brilliant blues and greens, creating a captivating display in the sky.

It is important to note that witnessing a CHA requires specific conditions. First and foremost, a high sun angle is necessary for the formation of this atmospheric phenomenon. Additionally, the presence of hexagonal plate-like ice crystals in the atmosphere is crucial. These conditions are often found in regions with cold climates, such as Yosemite National Park, where the CHA is frequently observed.

The beauty of the Yosemite Circumhorizon Arc lies in its fleeting nature. This optical phenomenon is not a regular occurrence and requires a combination of precise conditions to manifest itself. Those fortunate enough to witness a CHA can consider themselves truly lucky, as it is a breathtaking spectacle that adds an extra layer of wonder to the already stunning natural landscapes of Yosemite National Park.

In conclusion, the Yosemite Circumhorizon Arc is a captivating atmospheric optical phenomenon that enchants viewers with its delicate appearance and vibrant colors. Formed by the interaction of sunlight with hexagonal ice crystals, this celestial display adds an ethereal touch to the skies above Yosemite National Park. While rare and elusive, witnessing a CHA is a truly remarkable experience that showcases the wonders of nature and the awe-inspiring beauty of our planet.

Yosemite Circumhorizon Arc ~ A delicate appearance of this huge and colourful halo imaged by Marc Salomon (photography) at Washburn Lake. ©Marc Salomon

CHA's need a high sun.

Circumhorizon arcs form by sun rays entering the side faces of plate oriented hexagonal ice crystals and leaving through their lowest face. A crystal is shown at right as a side cross-section.

In 'A' the sun is higher than 58° and a colourful CHA has formed.

In 'B' the sun is lower and the CHA ray only just manages to escape from the lower face'.

In 'C' the sun is lower still and the ray is totally internally reflected rather than leaving to form a CHA.

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Reference Atmospheric Optics

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