9° Circular Halo

Last updated on September 16, 2023

The Mysterious 9° Circular Halo: A Closer Look at Atmospheric Optics

Atmospheric optics never fails to mesmerize us with its stunning displays of light and color. Among these captivating phenomena is the 9° circular halo, an intriguing optical effect that leaves us in awe. In this article, we will delve into the depths of this phenomenon and explore the science behind its formation.

The 9° circular halo, as the name suggests, manifests as a circular ring around the Sun or Moon. It is often accompanied by a fainter 18° halo within the outer ring, adding to its allure. This captivating sight has been beautifully captured by Harald Edens in the Netherlands, showcasing the delicate interplay of light and ice crystals in the atmosphere.

The formation of the innermost odd radius halo is closely tied to the unique structure of pyramidal ice crystals. These crystals possess a central prism section, through which sunlight passes. Specifically, the light traverses between a central face and an opposite pyramidal face, which are inclined at an angle of approximately 28°. As a result, the rays passing through these faces experience a minimum deviation of 9°. However, due to the random alignment of these crystals, the circular halo forms instead of a precise point of light.

To understand this phenomenon more comprehensively, let's take a closer look at the intricate process behind the formation of the 9° circular halo:

1. Ice Crystal Alignment: Pyramidal ice crystals are often poorly aligned in the atmosphere, leading to a diverse array of orientations. This lack of alignment contributes to the scattered and diffused nature of the circular halo.

2. Sunlight Interaction: As sunlight enters the crystal, it undergoes multiple reflections and refractions within its intricate structure. These interactions play a vital role in shaping the appearance of the circular halo.

3. Scattering and Diffraction: The scattering of light by the ice crystals causes it to disperse in different directions, creating the circular ring shape. Additionally, the diffraction of light further enhances the halo's visibility and intensity.

4. Colorful Display: The circular halo often exhibits a range of colors, including reds, blues, and greens. These colors arise due to the dispersion of light within the ice crystals, similar to the formation of a rainbow.

5. Atmospheric Conditions: The presence of high-altitude cirrus clouds, composed of ice crystals, is crucial for the formation of the 9° circular halo. These clouds act as a canvas on which the halo is painted, enhancing its visibility against the backdrop of the sky.

6. Variations and Subtle Features: The appearance of the 9° circular halo can vary depending on atmospheric conditions and crystal characteristics. Occasionally, subtle features such as a tangent arc or a parhelic circle may accompany the main halo, adding further complexity to the optical display.

The study of atmospheric optics continues to unravel the mysteries behind these mesmerizing phenomena. By unraveling the science behind the formation of the 9° circular halo, we gain a deeper appreciation for the wonders that occur in our atmosphere. So, next time you catch a glimpse of this ethereal ring encircling the Sun or Moon, take a moment to marvel at the intricate interplay of light and ice crystals that creates this captivating spectacle.

A fine 9� halo imaged by Harald Edens in the Netherlands. A faint 18� is also visible inside the bright outer halo.

�Harald Edens, reproduced with permission.

The innermost of the odd radius halos forms when pyramidal crystals have a central prism section. Sunlight passes between a central face and an opposite pyramidal face. These faces are inclined at only 28� and the corresponding minimum deviation for rays passing through them is 9�. When the crystals are poorly aligned - as they usually are - then a circular halo forms.

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Reference Atmospheric Optics

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