Circumscribed Halo

Last updated on September 16, 2023

Circumscribed Halo: A Fascinating Atmospheric Optics Phenomenon

When it comes to atmospheric optics, there are a myriad of stunning and captivating phenomena to behold. One such phenomenon is the circumscribed halo, which can be observed under specific conditions. In this article, we will delve into the intricacies of the circumscribed halo, exploring its characteristics, formation, and potential explanations.

A circumscribed halo is a large ring that appears almost circular when the sun is positioned at a high altitude. It is imperceptibly different from a perfect circle and often occurs alongside other optical phenomena such as the parhelic circle. The parhelic circle, smaller in size, intersects with the larger ring and touches the sun. Both these halos can be observed when the sun is positioned at an altitude of approximately 79º.

The formation of the circumscribed halo is attributed to a specific type of ice crystal known as horizontal columns. These crystals play a crucial role in creating the unique optical effects associated with this phenomenon. As light passes through the side faces of these ice crystals, it leads to the formation of the circumscribed halo. Furthermore, the nearly perpendicular end faces of these crystals reflect light efficiently, resulting in the creation of the parhelic circle.

However, there is more to the circumscribed halo than meets the eye. Observers have noted the presence of a faint halo located outside the circumscribed halo, touching it at the top and bottom. This additional halo is believed to be a high solar altitude form of the Wegener arc. The Wegener arc follows similar ray paths as the circumscribed halo but differs in that light is internally reflected from a crystal end face. Although Hutsemekers' image provides some hints of this arc, further investigation is required to confirm its presence.

Scientists have proposed an alternative explanation for the observed halos. It suggests that the halos could be a result of a 22º halo caused by poorly oriented crystals, in combination with a parhelic circle formed by plate crystals. However, at present, there is insufficient evidence to definitively determine which explanation holds true.

In conclusion, the circumscribed halo is an enthralling atmospheric optics phenomenon that showcases the beauty and complexity of nature. Its formation involves the interaction of specific ice crystals, resulting in the creation of a large ring resembling a circle. The presence of additional halos, such as the parhelic circle and the Wegener arc, adds further intrigue to this captivating display. While scientists continue to explore and analyze these optical phenomena, the circumscribed halo remains a fascinating sight for observers lucky enough to witness its splendor.

Halos at the European Southern Observatory, La Silla Chile. Image ©Damien Hutsemekers, shown with permission.

The larger ring is probably a circumscribed halo, imperceptibly differing from a circle when, as here, the sun is very high. The smaller ring through the sun and touching the larger is the parhelic circle still centered about the zenith but shrunk down to a mere 11º radius. The photographer, Damien Hutsemekers, caught the sun at the just the required altitude of 79º in January 1990 for the halos to touch.

A single variety of ice crystals, horizontal columns, could have produced the display as in the lower HaloSim simulation. Light passing through side faces makes a circumscribed halo. Light reflecting (efficiently because of the near glancing incidence) off their nearly perpendicular end faces produces a parhelic circle.

But what is the faint halo outside the circumscribed halo and touching it at top and bottom? This is the high solar altitude form of the Wegener arc which has the same ray paths as the circumscribed halo except that light is internally reflected from a crystal end face. Very strong enhancement of Hutsemekers' image perhaps shows hints of the arc but it is hard to be sure.

Another explanation is that the halos are a 22º halo produced by poorly oriented crystals and a parhelic circle from plate crystals. There is not enough evidence to decide between these alternatives.

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