Ice particles in the atmosphere range in size from ice fog particles a few pm diameter to hailstones some 10 cm across. Many smaller particles tend to have singular crystalline facets and grow in the form of either solid or hollow hexagonal columns or needles, hexagonal plates, sector plates or dendrites. Size and shape determine fall speed and orientation. Beyond a size of a few hundred microns, aggregation to snowflakes takes place and particles may rime, to form soft hail particles, and ultimately higher density hailstones; aggregation to snowflakes takes place. Particles often depart drastically from the idealized shapes and dimensions sometimes used in modelling radiation interactions. Particles may be characterized by scales downward from their maximum dimension, to surface steps and bubbles of dimension < μm. Size and fall velocity distributions are intimately related to generation and velocity sorting mechanisms, and may be quite narrow or very broad depending on circumstances.
John Hallett, John Hallett,
"On The Characterization Of Ice Particles In The Atmosphere", Proc. SPIE 0414, Optical Engineering for Cold Environments, (22 September 1983); doi: 10.1117/12.935868; https://doi.org/10.1117/12.935868