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   Infrequent Halos  

How infrequent are some halos?

The relative frequencies of halo sightings at right were calculated from 10 years of observations by the German Halo Research Group. In Europe a 22° halo occurs on about 100 days each year so dedicated sky watching might catch a Parry arc on average once a year.

Visibility also depends on location, season and how much of the sky can be seen. Whatever, always search for rare halos whenever there are cirrus clouds.


     Halo

         

  22°
  Parhelia
  Tangent arc
  Sun pillar 
  Circumzenithal
  Parhelic circle

  Supralateral arc/46° halo
  120° parhelia
  Parry arcs
  Pyramidal crystal halos 
  Subsun
  Circumhorizon
  All others
  		Relative
Frequency


100
 73
 27
 16
 13
  4

4.2
1.2
1.1
0.3
0.2
0.1
1.5
 

Why are some halos infrequent?   Why is the 46° halo seen much less often than the 22°?  
  
    Crystal quality.   Cloud crystals rarely attain gemlike perfection.   They have inclusions which impair halo clarity.  Sometimes the inclusions extend out to and damage crystal faces.  Sometimes the faces are imperfect anyway.   These faults tend to impair particular halos.   For example, column crystal end faces are most often imperfect thus blocking rays which would otherwise form the 46° halo, infralateral and supralateral arcs.

Alignment & crystal size.   Precise crystal alignment is needed for many halos. Large crystals (0.1mm or larger) are the best aligned but these are the most prone to imperfections. 

Large halos & dispersed colours. Some halos are so large and their colours so dispersed that they are inherently faint. Even if column crystals had perfect end faces the 46° halo would always be at least six times fainter than the 22°.

      
              

The effects of imperfect alignment.  At top right clear and distinct upper tangent and Parry arcs are formed by column crystals with tilts of only 0.1° std dev.  

As the crystal alignment becomes poorer the Parry arc becomes less and less distinct.   With tilts over 1° it is hardly visible and even the Upper Tangent Arc is mediocre.