Fall velocity of snowflakes of different riming degree and crystal types

The fall velocity of graupel and of snowflakes of different riming degree and consisting of different crystal types is investigated. The naturally falling solid precipitation particles are measured with an optical instrument which is capable to record the size, shape and fall velocity of each single particle. Measurements were performed during 2 winter seasons and results are based on approximately 40,000 particles. It can be shown that the fall velocity of snowflakes depends on both the riming degree and their crystal type composition. Whereas the fall velocity of snowflakes consisting of needles or plates is strongly dependent on the riming degree, it seems that the fall velocity of snowflakes consisting of dendrites or irregular ice crystals is not dependent on the riming degree. The average fall velocity of any type of snowflakes tends to remain constant between 1 and 2 m s− 1 after the snowflakes have reached a certain size. However, the average variation of the fall velocity for a given snowflake size amounts to ± 0.3–0.7 m s− 1.