Circumscribed Halo

Last updated on September 26, 2023

Circumscribed Halo: Exploring the Mysteries of Atmospheric Optics

When we gaze up at the sky, we are often captivated by the ethereal beauty of atmospheric phenomena. One such phenomenon that intrigues both scientists and casual observers alike is the circumscribed halo. This celestial spectacle manifests as a ring of light encircling the sun, creating a mesmerizing display of colors and shapes. In this article, we will delve deeper into the intricacies of the circumscribed halo, unraveling its changing shape with solar altitude and uncovering its unique characteristics.

The circumscribed halo reveals its enigmatic form when the sun's elevation reaches 29º. At this point, the upper and lower tangent arcs merge to form the initial structure of the halo. As the sun climbs higher in the sky, the halo takes on a distinctive drooping distorted oval shape. During this phase, it is common for only the brighter upper and lower sections of the halo to be visible to the naked eye.

As the sun continues its ascent, reaching approximately 50º in altitude, the circumscribed halo gradually approaches the appearance of the 22º halo. Its oval shape begins to transform into a more circular configuration, exhibiting a closer resemblance to its counterpart. However, it is essential to note that even at this stage, there are still discernible differences between the two halos.

The circumscribed halo truly comes into its full glory when the sun reaches its zenith, directly overhead. At this point, the halo achieves perfect circularity, mirroring the symmetry of the sun's position. It is during these moments that the circumscribed halo merges seamlessly with the 22º halo, making it challenging to differentiate between the two phenomena. Only keen observers with a discerning eye can detect subtle nuances that set them apart.

One fascinating aspect of the circumscribed halo lies in its colors. Compared to the 22º halo, the circumscribed halo exhibits somewhat brighter and purer hues. This phenomenon arises due to the orientation of the ice crystals responsible for producing each halo. The singly oriented columns that give birth to the circumscribed halo allow for less color overlap, resulting in a more vibrant and vivid display. In contrast, the less oriented crystals forming the 22º halo create a slight blending of colors, reducing their intensity.

As we explore the mysteries of the circumscribed halo, it is important to note that this article has been automatically converted from its original source. To view the intended version of this article, you can refer to the link provided.

Upper and lower tangent arcs and a faint 22º halo. HaloSim3 simulation. The region below the horizon is dark blue.

Changing shape of the circumscribed halo with solar altitude. Mouse slowly over the slider to see the changes.

The halo "starts" at a sun elevation of 29º as the joined form of the upper and lower tangent arcs.

It has a pronounced drooping distorted oval shape until the sun reaches ~50º high. Often only the brighter upper and lower sections are visible.

When the sun is higher, the halo approaches the 22º halo more and more closely and it becomes exactly circular when the sun is overhead.

At high sun it is not easy to tell the circumscribed and the 22º halos apart.

The circumscribed halo has somewhat brighter and purer colours because rays through the singly oriented columns producing it give less colour overlap than those through the much less oriented crystals making the 22º halo..

Note: this article has been automatically converted from the old site and may not appear as intended. You can find the original article here.

Reference Atmospheric Optics

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