The properties of snowflakes such as their size distribution, fall velocity, axial ratio or their orientation when falling are important in modelling precipitation processes or in interpreting returned radar power, especially of polarized radar. Except for some in situ techniques, where snowflakes are captured and examined, optical techniques are the choice when snowflakes or ice crystals are to be investigated.

The Institute of Atmospheric and Climate Science at the ETH has developed an optical instrument which is capable to measure reliably, with high accuracy and automatically the properties of each single snowflake falling through a 77 square cm area. The instrument works with two light beams which are slightly offset. Thus, two images are recorded of each particle which can be used to calculate, in addition to the other properties, the fall velocity of the snowflakes.

A large data set of several winter seasons is available. One hour of moderate precipitation can yield data of about 30'000 particles in a size range of 0.15 mm to 70 mm. A detailed study is shown with the fall velocity and axial ratio of snowflakes of different riming degree and of different ice crystal types. In addition, the natural variability of the fall velocity of snowflakes of a given size is provided.