The observer is always amazed by the wide variety of shapes of natural ice particles, be they ice crystals, snow flakes, graupel or hailstones. They can be photographed, measured and categorized, but to understand the physics of their growth is a different story. However, one aspect is basic: they all have a non-negligible fall speed and can exhibit intricate fall patterns. The aerodynamic behavior determines the molecular/convective heat and mass transfer [HMT]. Both aerodynamics and HTM determine the capture and partial or complete accretion of cloud droplets and other particles, i.e. their growth, as well as their overall HMT.

Establishing the “physics” often implies simulation concepts such as the Reynolds number, the characteristic quantity describing flow and forming the entry into HMT. An example will be given for the aerodynamics and HTM of pristine shapes of ice crystals whereby ice crystal fall was simulated in a water tank and the HTM by electrolysis in a liquid tunnel (mass transfer equivalent to diffusion of ions).

With hailstones, secondary motions such as rotations and gyrations come into play, HTM becomes more complex and measuring methods are more sophisticated involving surface-scanning radiometric instruments. How the information can be accessed and assembled is going to be shown in a flow diagram.