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   Plate Orientation & Plate Halos


As plate crystals drift downwards, they become oriented in a maximum drag condition. Their large hexagonal end faces are nearly horizontal and the dotted axis (the crystal ‘c’ axis parallel to the side face edges) is almost vertical.

The configuration is dynamically stable in that small deviations produce correcting forces that restore the orientation.

All crystals wobble slightly but good halos result when the c axis stays within a degree of vertical. That halos are so frequent testify to the remarkably good orientations of cloud crystals.

The plates are free to take all rotational orientations about the c-axis. This does not mean that individual crystals 'spin' but rather that an ensemble of millions of crystals in a cloud has all possible rotational positions.

The resulting halos are called plate arcs or halos.

Rays passing between vertical side faces inclined with a wedge angle of 60° form sundogs. Rays passing between one of the large horizontal faces and a side face (wedge angle of 90°) form the circumzenithal and circumhorizon arcs.

Reflections off horizontal faces give us a subsun and sun pillars. Reflections from vertical faces contribute to the parhelic circle.


   parhelia  circumzenithal arc  circumhorizon arc  subsun - subparhelia  pillar

Images:  parhelia, Stan Richard    cza, Jane Wertenberger
cha, Mark Sorenson   subhorizon arcs, Don Davis
pillar,Dave Liquorice  Thumbnails lacking a
larger version will be updated soon.