Fall velocity and axial ratio of snowflakes

In this work, we describe a method for optical snowflake measurements. The measurement area of the optical instrument consists of two vertically offset measuring planes. Thus, side projections of the particles are taken. The instrument is capable to measure size, shape, and fall velocity of individual and ensembles of particles. The method is used in field campaigns to collect snowflake data of different crystal types and riming degrees and includes a statistical evaluation and presentation of the data. An example shows a clear dependence of particle fall speeds on the riming. The fall velocity distribution depends on the particle density which is increased due to the riming. The measurements include a specification of the speed of the measured fall spread, e.g. 50% of unrimed dendrites have a fall velocity, which is more than 0.22 m/s different from the median value in the corresponding size class. Fall velocity scatter increases vs. size in the diameter range of 0 mm to 3.2 mm. Another example shows a different and clear axis ratio behaviour of 1-dimensionally (e.g. needles) and 2-dimensionally (e.g. dendrites or plates) growing particles and aggregates. 2-dimensional growing particles have a constant axis ratio, whereas 1-dimensionally growing particles show a decreasing axis ratio vs. size. For both, the 1-dimensional and 2-dimensional particles, the scatter decreases vs. size. Finally, an effort is made to estimate the area Ar of the particles, which is exposed to the flow and relevant for the drag coefficient of the particle. Through a comparison of simulated and measured two dimensional side view patterns, a calculated distribution of areas Ar can be obtained, because the distributions of Ar can be estimated from the simulation.