Quebec City Circumscribed Halo

November 15, 2005


Provided by: Manon Boily
Summary authors & editors: Jim Foster; Manon Boily

The above photo showing a portion of classic circumscribed halo was taken from Quebec City, Quebec on October 8, 2005. While the more common 22 degree halo encircles the Sun, the circumscribed halo has an ellipsoidal shape and surrounds (circumscribes) the 22 degree halo and is tangent to it at its upper and lower points. Circumscribed halos form in column-shaped ice crystals -- light enters through one of the side faces of a hexagonal crystal and exits through an alternate side face. The "c" axes (flat ends) of these columnar crystals must be more or less horizontally oriented, and the "a" axes (side faces) assume random orientations. With the 22 degree halo, the columnar crystals have no preferred orientation and don't "spin" as do the crystals forming circumscribed halos. In order to observe circumscribed halos, the Sun must be higher than about 30 degrees above the horizon.

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