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   Column Orientation and Column Arcs





 

A high drag orientation for column crystals is with their long ‘c’ axis horizontal and so they drift. The c axis is parallel to the edges of the prism faces and perpendicular to the end faces.

Column oriented crystals have two degrees of freedom. They can take all rotational orientations about the ‘c’ axis and also about a vertical axis perpendicular to the c axis. They do not individually spin but rather, in a large collection of them some will be found in all these rotational positions.

The halos produced are called column arcs or halos.

Good halos need the c axis to be within less than 1° of horizontal.

Rays passing through prism side faces with a wedge angle form tangent arcs (the equivalent rays through plates give sundogs).

Rays passing between a side face and end (basal) face, wedge angle 90°, form supra- and infralateral arcs (plates give circumzenithal and circumhorizon arcs).

Reflections off side faces (surprisingly) form sun pillars. Reflections from the vertical basal faces contribute to the parhelic circle. More complicated ray paths give many rare arcs.

  
 

   upper tangent arc  lower tangent  supralateral arc  infralateral  pillar
 


Images:  upper tangent , Brian Hartmann     lower tangent, Franck Schwitter
supralateral, Jim Grossman    infralateral, Alastair Adams
column pillar ,Erik Brenna   Thumbnails lacking a
larger version will be updated soon.