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The 46-Degree Halo

© Keith C. Heidorn

Welcome to the first installment of "Science of the Sky." As I sat on my balcony scribbling the notes that would become this essay, I looked up into the southern sky to see floes of icy cirrus clouds drifting by. Beneath the sun, a brilliant spectrum of color interjected itself on the white and light blue background. What I was seeing was a 46-degree halo, or at least a portion of one since it did not completely encircle the sun on this occasion. Shielding my eyes from the sun's glare, I could also see a 22-degree halo ringing the solar orb. (These halos are named for the angle formed between the solar center and the edge of the halo).

In the photo to the left, you can just see the 22 degree halo at the top of the picture. An arc of the 46-degree halo is visible at the bottom.

The 46-degree halo is related to the more familiar 22-degree halo, known from weather lore as "Ring around the sun or moon, rain will be a-fallin' soon." The 46-degree halo forms outside the 22-degree halo and shows to same color sequence: red-orange on the inside, blue in the outer portion. But while the 22-degree halo generally forms a complete ring around the sun, the 46-degree halo is rarely seen as a complete encirclement.

The cause of these halos is the refraction and spectral splitting of sunlight as it passes through ice crystals that form the cirrus cloud deck above me. In these clouds, the tumbling crystals will randomly orient themselves in the proper position for the sun's light to refract toward my eye. The 46-degree halo forms from the same crystals as the 22-degree halo but arises from a different light path through the ice crystals.

Ice crystals found in the cirrus-family of clouds that form halos are usually hexagonally shaped columns, rather similar to the shape of a common lead pencil but much smaller (around 15-25 micrometres in diameter). The best viewing for the 22-degree halo occurs with a veil of cirrostratus overhead containing uniform-sized hexagonal ice crystals, much longer than wide.

Light rays which go in one side of the ice crystal and out another form the 22-degree halo. Those that go in one side and then out one of the ends form the 46-degree halo. Because the 46-degree halo is rarer than its 22-degree partner, some investigators believe the crystal must be "fatter than" normal (a form known as a "plate" crystal) to produce the 46-degree variety halo. They argue that if the crystal is long and thin like a pencil, there is too little area on the ends of the crystal for the light to emerge through an end rather than a side. In addition, the light forming this halo disperses over a wider stretch of sky and is thus generally more faint than the common 22-degree form.

22 and 46 degree halos

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