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


   Parry Orientation & Parry Arcs




 

If the near perfect orientation of column crystals sounds improbable, that of Parry oriented crystals must seem wildly so. The long crystal c axis is horizontal AND so are two of the prism side faces. The only degree of freedom is the ability of the crystal to take all rotational positions about a vertical axis passing through the crystal centre and perpendicular to the two horizontal faces.

The resulting halos are called Parry arcs. They are named after the explorer William Edward Parry who first observed one of them. Some Parry arcs are not particularly rare and with careful observation will be seen a few times a year .

Rays passing through two prism side faces with a wedge of 60° produce sunvex and suncave Parry arcs (column orientations make tangent arcs). These are the most common Parry arcs.

Rays travelling between a side face and a vertical end face - wedge of 90° - make the exceeding rare Parry supralateral (Tape) and infralateral arcs. Rays passing between the horizontal and vertical faces make the circumzenithal and circumhorizon arcs much more commonly produced by plate crystals.

Reflection from a sloping prism face gives a very rare helic (heliac) arc until recently only seen in the Antarctic.
    

   suncave Parry and upper tangent
 arcs
 sunvex and suncave
  Parry arcs
 Parry supralateral (Tape) arc  Parry infralateral  helic arc
 


Images:  suncave Parry, Alastair Adams    double Parry , Loren Hall
Tape arc, Max Emerson   Parry infralateral, Marko Riikonen
helic ,Alastair Adams   Thumbnails lacking a
larger version will be updated soon.