How big should hexagonal ice crystals be to produce halos?

It has been hypothesized that the frequent lack of halos in observations of cirrus and contrails and laboratory measurements is caused by small ice cr ystal sizes that put the particles outside the geometrical optics domain of size parameters. We test this hypothesis by exploiting a strong similarity of ray tracing phase functions for finite hexagonal and circular ice cylin ders and using T-matrix computations of electromagnetic scattering by circu lar cylinders with size parameters up to 180 in the visible. We conclude th at well-defined halos should be observable for ice crystal size parameters of the order of 100 and larger and discuss remote-sensing implications of t his result. (C) 1999 Optical Society of America.