Study of the Formation of Precipitation by the Aggregation of Snow Particles and the Accretion of Cloud Droplets on Snowflakes

Abstract

Taking stereographical photographs of falling snow particles with a stroboflash, the author has investigated statistically the motion of snow particles in a population in order to determine the collisionrate of snow particles, and then proceeded to the study of the formation of snowflakes based on the collision-rate thus obtained.
Part I: The motions of individual snow particles falling through the still air were studied using a stereographical method. It was found that the horizontal meandering motion of the snow particle was related with the aerodynamic lift force on it.
Part II: The statistical features of motions of snow particles were investigated. On this basis the collision-rates of snow particles are calculated.
Part III: The mechanisms of snowflake formation are discussed based on SMOLUCHOWSKI's equations. It is concluded that i) the stage of the snowflake formation is divided into two types. The first-type collision is one due to the random motion of snow particles, and the second-type collision due to difference in the mean velocity of individual snow particles. ii) it is necessary for the formation of further larger snowflakes to keep up the supply of individual snow crystals for a suitable time.
Part IV: In order to investigate the riming process of snowflakes, models of snowflake, the surfaces of which were coated with MgO, were set in the air flow in a vertical wind tunnel. The trajectories of water droplets sprayed into the air flow were determined from the photographs of the moving droplets. The features of capture of droplets were discussed.