Halo Complex

November 20, 2002


Referred by: Michael Ellestad
Summary authors & editors: Jim Foster; Michael Ellestad

The photo above shows a portion of an amazing halo complex that began over southern Ohio around noon on October 14, 2002. The halo that's visible over the corner of the building is the 22 degree halo, produced from randomly oriented hexagonal ice crystals. Note also the upper tangent arc (tangent to the 22 degree halo), which results from singly oriented hexagonal column (pencil) crystals. In addition, the upper Parry arc can be seen lying just above the upper tangent arc. This arc is not commonly observed and is created when sunlight passes through the alternate side faces of horizontally oriented pencil crystals. The parhelia or sundog (at bottom center) and circumzenith arc (upper right) are produced when sunlight enters hexagonal plate crystals. In the case of parahelia, light passes through the vertical faces of horizontally aligned plate crystals, whereas with circumzenith arcs, light passes through the upper plate face and exits through one of the vertical sides. One other phenomenon to note is a portion of the parhelic circle (colorless line just above the roof), which occurs at the same elevation of the Sun. For the parhelic circle to be observed, light must either pass through the vertical side faces of horizontally oriented plate crystals or through the vertical end faces of horizontally oriented pencil crystals. See also the Earth Science Picture of the Day for October 9, 2002.

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