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
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
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
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.