Orientations and aspect ratios of falling snow

Photographs of nearly 73,000 snowflakes in free fall are used to determine the aspect ratio and orientation of aggregates, moderately rimed particles, and graupel. Observations indicate that there can be a much broader range of orientation angles, with a larger median value, than has been indicated by previous observational and theoretical studies. The data show that aspect ratio depends on riming extent but that orientation is only weakly dependent on the degree of riming and on particle size. Instead, more vertical orientations for frozen particles become increasingly common with higher turbulence. The results suggest that distributions of size, fall speed, orientation, and aspect ratio may each need to be considered in order to optimize the accuracy of precipitation retrievals using microwave sensors.