Abstract

Nocturnal light pillar displays are not uncommon during conditions of very light snowfall when temperatures are well below freezing.¹ Like the related reflection phenomenon of the sun pillar,² light pillar occurrences demonstrate the presence of ice crystals oriented with their reflecting faces very nearly parallel to the ground. Assemblages of simple plate crystals, with their large basal faces, constitute the best scattering medium for these displays, provided that their sizes correspond to those which display stable fall attitudes.³ Sun and light pillars, however, have quite different structures as a result of the reflection geometries from light sources positioned essentially at infinity, and locally.⁴ Moreover, light pillars are perhaps unique among atmospheric optical displays in that they are anthropogenic and, since the observer is actually located within the cloud of crystals, it is a simple matter to study the nature of the particles causing the display.

© 1986 Optical Society of America