Laboratory measurements of the sedimentation velocity of hexagonal planar ice crystals

Abstract

New experimental measurements of the falling velocity, size and orientation of individual hexagonal plate-like ice crystals are reported. The measurements were conducted at three different temperatures: −13, −16 and −20 °C. The diameter of the ice crystals measured in the experiments were between 50 μm and 250 μm, a size range which is in agreement with the size found in natural clouds. In this range, ice crystals show a random orientation during free fall and a falling velocity which increases with size. Results show that the fall velocity is insensitive to the temperature at which the ice crystals grow for the temperatures used in this study. An empirical power-law between the Best and Reynolds numbers is presented using the capacitance as characteristic length and an estimation of the ice crystal mass. Despite the dispersion of the experimental data, the Best–Reynolds relationship found seems to be similar to the relationship for falling spheres in Stokes flow using the capacitance as the hydrodynamic radius. The fall velocities of hexagonal and columnar ice crystals were compared. The columnar ice crystals show a velocity larger than that of hexagonal ice crystals with the same value of capacitance. However, both crystalline habits show a unique empirical Be–Re relationship.