Not so mundane 22° halo, Cambridge, England - OPOD

Last updated on September 16, 2023

Not so Mundane 22° Halo, Cambridge, England - OPOD

The 22° halo, although often considered commonplace, holds a mysterious allure that surpasses even the Kern arc. Conor McManmon captured a stunning example of this atmospheric phenomenon in Cambridge, England on the morning of May 1st, 2015. What makes this particular halo intriguing is the presence of a dark 'hole' within it, where crystals fail to glint. The halo itself exhibits a red rim and gradually tapers through shades of straws and pastel blues until it reaches a pure white. Additionally, the halo's light extends over a significant distance. To complete the captivating scene, a faint sundog and a hint of a parhelic circle make their appearance.

The conventional explanation for the 22° halo involves the refraction of light rays passing through hexagonal ice prisms with 60° inclined side faces. This refraction is responsible for the formation of the halo. However, a closer examination reveals that hexagonal ice prisms do not tumble as previously thought. Instead, short prisms, known as plates, drift with their large hexagon faces horizontal, often with remarkable precision. On the other hand, long prisms, referred to as columns, align themselves with their long axes horizontal due to aerodynamic drag forces. So, if neither short nor long prisms tumble significantly, what generates the 22° halo?

The scarcity of stubby crystals in crystal samples from displays featuring a 22° halo suggests that something other than tumbling hexagonal ice prisms may be responsible for its formation. One possibility is the involvement of cluster crystals or very large imperfect crystals that are more prone to tumbling. These alternative crystal structures could potentially explain the enigmatic nature of the 22° halo.

To gain a deeper understanding of this atmospheric phenomenon, it is crucial to consider the intricate interplay between light and ice crystals. When sunlight passes through these hexagonal ice prisms, it undergoes refraction, bending at an angle determined by the specific properties of the ice crystals. This refraction causes the light to spread out and form a circle with a radius of approximately 22° around the Sun or Moon. The resulting halo appears as a luminous ring encircling the celestial body.

The presence of the dark 'hole' within the halo is an intriguing aspect that demands further exploration. This absence of glinting crystals suggests a unique interaction between light and ice crystals in this region. It is possible that certain crystal orientations or crystal structures within this dark area prevent the reflection of light, leading to its peculiar appearance.

Another noteworthy feature of the 22° halo captured in Cambridge, England is its gradual transition of colors. Starting with a red rim, the halo then progresses through shades of straws and pastel blues until it reaches a brilliant white. This color gradient is a result of the dispersion of light as it passes through the ice crystals. The different wavelengths of light refract at slightly different angles, causing them to separate and create a spectrum of colors within the halo.

The extended reach of the halo's light is also worth mentioning. The halo's influence can be observed over a considerable distance, enhancing the ethereal beauty of the scene. Additionally, the presence of a faint sundog and a hint of a parhelic circle adds further complexity to the overall atmospheric display.

In conclusion, the 22° halo may appear mundane at first glance, but its intricacies and mysteries make it a captivating atmospheric phenomenon. The formation of this halo involves the refraction of light through hexagonal ice prisms, but the exact mechanisms behind its creation remain elusive. The presence of a dark 'hole' within the halo, the gradual transition of colors, and the extended reach of its light all contribute to the enigmatic nature of this optical marvel. Further research and exploration are needed to unravel the secrets hidden within the 22° halo and deepen our understanding of its fascinating properties.

Not So Mundane Halo?

After the Kern arc of the previous OPOD, a ‘commonplace’ 22° halo might seen mundane. But in some ways the 22° halo is more mysterious than the Kern.

Conor McManmon captured this fine example from Cambridge, England on the morning of 1st May, ’15.

Notice the dark 'hole' inside the halo where crystals do not glint. The halo is red rimmed and outside it tapers through straws and pastel blues to white. The halo's light extends a considerable distance. A faint sundog and hint of parhelic circle complete the picture.

©Conor McManmon, shown with permission

The usual explanation for the 22° halo is that refraction of rays passing through 60° inclined side faces of tumbling hexagonal ice prisms forms it. Tumbling confers an average random orientation and thus leads to a circular halo rather than a sundog or an upper tangent arc. These latter arcs have the same ray path.

But hexagonal ice prisms do not tumble. Short prisms - plates – drift with their large hexagon faces horizontal to within a few degrees and usually are even more precisely aligned. Long prisms – columns – are well aligned with their long axes horizontal. Aerodynamic drag forces do this.

And stubby crystals of intermediate length? Wouldn’t they tumble and generate the 22° halo? Perhaps. But crystal samples from displays with a 22° halo have precious few stubby crystals!

Something else is in most case generating the halo. Perhaps cluster crystals or sometimes very large imperfect crystals more prone to tumbling?

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