The relative frequencies of halo sightings at right were calculated from 10 years of observations by the German Halo Research Group .
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. 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.