Subsuns and low Reynolds number flow

The phenomenon called the subsun 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. The theory of torques on objects at low Reynolds numbers is reviewed; coefficients C_v, C_p, and C_q that describe the orienting torques on discs, rods, and hexagonal prisms are defined; and the results of experiments to measure C_v, and C_p, are reported.