

      SIMULATING COMPLEX DISPLAYS

      Complex halo displays when crystals with a variety of 
      c/a ratios and orientations are present in the sky.

      HALOSIM can simultaneously simulate up to 12 different
      combinations of crystal shapes and orientations.

      These instructions show how to set up the simulation
      parameters for complex displays.


      A CIRCULAR HALO, PARHELIA and TANGENT ARCS.

      Press 'Reset'.  Clear channels 2 and 3 by clicking
      the channel up-down buttons and then the 
      'Clr <channel>' button.

      Each shape/orientation is stored under a different 
      "Crystal type number".

      Select Type number 1 using the left hand up-down 
      control on the Control Panel.  Select "Hexagonal 
      Ice Prism" for the crystal shape.   Set the c/a 
      ratio to 0.8.   Choose "Random" for the crystal
      orientation.  This simulates the circular halo 
      component of the display.

       - Set %tot, the crystal population, to 100.
       - Set the solar altitude to 22 degrees.
       - Select a "Sun centered Plan" view and "Grey 
         shades on white", 4 levels.
       - Select 30,000 rays.

      Run the simulation.   A 22 degree halo and much 
      fainter 46 degree halo will be seen.   

      Do not erase the settings!

      Now let's add the parhelia or sun dogs.   

          All the halos for this display use "Hexagonal 
          Ice Prism" and the file could be selected 
          separately for each crystal type number.  A 
          faster way is to select the same shape file 
          into all 12 channels by clicking the 'Copy 
          shape to all' button.   [Go to 'Tools' 
          'Options' if it is not visible.]  

          The Crystal Selection Panel show what happens.
          Its visibility can be toggled by the 'Crystal 
          report' button or via Menu 'View' 'Crystal 
          report'.

      Select the orientation files for parhelia by 
      clicking the channel up-down buttons until '2' shows.
      Then click 'Orientation' and select 'plate 1deg 
      disp.xng'.

      Set the %tot for channel 2 to 10 (i.e. 10% of the
      crystals will be plates).   To keep the total 
      population at 100% go back to channel 1 and change 
      its population to 90%

      Go to channel 2 and set the crystal c/a ratio to 
      0.2 to simulate thin plates.

      Click run to see circular halos and sun dogs.  Note
      that the plates also form the circumzenithal arc 
      that is tangential to the 46 degree halo.

      Halos are visible 'beneath' the horizon - these 
      are the subsun and subparhelia.


      Now add the upper and lower tangent arcs. 

      These are formed by singly oriented columns, prisms 
      with their long axes nearly horizontal.

      Select crystal channel 3.   Click 'Shape' and select
      'Horiz column 1 deg disp.xng'.  Set the %tot 
      population to 50% and reduce the population in 
      channel 1 to 40%.   Set the c/a ratio for the 
      prisms to 2.

      Click 'Start' to generate the complete complex 
      display.

           What are the bright arcs that almost overlay 
           the 46 degree halo?   Which crystals and 
           orientations cause them?

      Finally, improve the appearance once the 
      simulation parameters are adjusted to satisfaction.
      Change the grey levels from 4 to 32.  10 times as 
      many rays will be needed so select 300,000 rays 
      (enter 200 because they are counted in thousands.
      Then try 256 grey levels and 2 million rays.

           Change the projection to a sun centered 
           camera view.  After storing the simulation (see 
           Help) compare it with the stored simulation of 
           Parry's display observed in the Arctic around
           1820. 


      

