Upper tropospheric cirrus clouds consist of hexagonal ice crystals, which geometrical ray-tracing-theory predicts should regularly produce a variety of optical phenomena such as vivid 22° and 46° 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.
© 2005 Optical Society of America
Original Manuscript: January 11, 2005
Revised Manuscript: April 5, 2005
Manuscript Accepted: April 5, 2005
Published: September 20, 2005
Kenneth Sassen, "Halos in cirrus clouds: why are classic displays so rare?," Appl. Opt. 44, 5684-5687 (2005)