Sub-Suns and Low Reynolds Number Flow

Abstract

The phenomenon called the ``sub-Sun'' is the specular reflection of sunlight by horizontally oriented plates of ice. Although well-known in meteorological optics, the hydrodynamics of the orientation is not quantitatively understood. I review the theory of torques on objects at low Reynolds numbers, define coefficients $C_o$, $C_p$, and $C_ψ$ which describe the orienting torques on discs, rods, and hexagonal prisms, and report here the results of experiments to measure $C_o$ and $C_p$.