OPOD - Circumscribed Halo

Last updated on September 16, 2023

OPOD - Circumscribed Halo: A Stunning Atmospheric Phenomenon

The circumscribed halo is a mesmerizing optical phenomenon that captivates observers with its unique shape and appearance. This breathtaking halo is often mistaken for the more common 22° halo due to its close proximity and similarity in appearance. However, upon closer examination, one can discern the distinct characteristics that set the circumscribed halo apart.

The shape of the circumscribed halo undergoes dramatic transformations as the position of the sun in the sky changes. When the sun is below 29°, the circumscribed halo ceases to exist, and instead, separate upper and lower tangent arcs become visible. As the sun ascends, the halo takes on the form of a large droopy oval, touching the circular 22° halo at both the top and bottom. The oval gradually contracts towards the 22° halo as the sun climbs higher, making it increasingly challenging to differentiate between the two. Consequently, many high sun circumscribed halos are often misidentified as 22° halos.

The unique appearance of the circumscribed halo is attributed to the refraction of light through ice crystals with 60° inclined faces. These ice crystals, known as singly oriented columns, are characterized by their almost horizontal long axis. This horizontal orientation is a result of aerodynamic drag as the crystal descends relative to local air currents. Although each individual crystal may not necessarily spin or rotate, it possesses the freedom to assume rotational positions about its long axis and any orientation about a vertical axis. The combination of these rotational freedoms gives rise to the peculiar shape exhibited by the circumscribed halo.

The intricate formation process of the circumscribed halo adds to its allure. When sunlight passes through these horizontally oriented ice crystals, it undergoes refraction and internal reflection before exiting the crystal. This complex interaction results in the bending and scattering of light, ultimately producing the captivating halo that graces the sky.

To fully appreciate the beauty of the circumscribed halo, one must witness it firsthand. The vibrant colors and ethereal glow that accompany this phenomenon make it a sight to behold. From its distinct shape to its ever-changing appearance with varying solar altitudes, the circumscribed halo offers a visual spectacle that leaves observers in awe.

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In conclusion, the circumscribed halo stands as a testament to the wonders of atmospheric optics. Its enchanting form, shaped by the refraction of light through inclined ice crystals, creates a breathtaking display in the sky. As the sun's position alters, so too does the shape of the halo, adding to its mystique. Observing the circumscribed halo firsthand is an experience that allows one to marvel at the beauty and complexity of nature's artistic creations.

Circumscribed Halo

A spectacular halo imaged at Tampere, Finland by Kari A Kuure. ©Kari A Kuure, shown with permission.

The circumscribed halo changes shape dramatically with solar altitude.

When the sun is below 29� it does not exist. There are separate upper and lower tangent arcs instead.

At 35� it is a large droopy oval touching the circular 22� halo at top and bottom. As the sun climbs the oval contracts towards the 22� halo and eventually is only distinguished from it with difficulty. Many high sun circumscribed halos are mistakenly identified as 22� halos.

The arc is produced by refraction through 60� inclined faces of horizontal column ice crystals (singly oriented columns).

The column long axis is forced to be almost horizontal by aerodynamic drag as the crystal drifts downwards relative to local air currents. The crystal can take rotational positions about its long axis and can also have any orientation about a vertical axis. The combination of the two rotational freedoms (and individual crystal does not necessarily spin or rotate) produces the peculiar halo shape.

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Reference Atmospheric Optics

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