Halos in cirrus clouds: why are classic displays so rare?

Upper tropospheric cirrus clouds consist of hexagonal ice crystals, which geometrical ray-tracingtheory predicts should regularly produce a variety of optical phenomena such as vivid 22(degree) and 46(degree) halos. Yet, cirrus inconsistently generate such optical displays, while a class of more exotic displays are reported, albeit rarely. I review current knowledge of the cirrus cloud microphysical factors that control ice crystal shape, and hence halo/arc formation, but also appeal to halo enthusiasts to help investigate the causes of unusually complex, brilliant, or rare optical displays. Currently, a wealth of meteorological information can be tapped from the Internet to help advance our knowledge of the basic meteorological factors leading to these rare events.