Home
  OpticsPOD
  What's New 
  Rays & Shadows
  Water Droplets
  Rainbows
  Ice Halos
    Contents
    Crystals
     Crystals & Halos 
     Columns & Plates
     Orientations
        Plate
        Column
        Parry
        Lowitz     
        Random    
     Face numbers
     Real Crystals
     Diamond Dust
     Pyramidal
    Frequent Halos
    Infrequent Halos
    Multiple Displays
    Other Worlds
    Observing Halos
    HaloSim
  High Atmosphere
  Links & Resources
  Search - Index






 
123456789012345678


   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.