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.
Atmospheric
Optics

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