Aggregation of needle snow crystals

For the purpose of studying the early process of snowflake growth, characteristics of the Ž . aggregation collision–adhesion process of needle snow crystals were investigated by examining photomicrographs of snowflakes composed of two crystals. The adhesion type of the snowflakes was classified into crossed adhesion and point adhesion types. When the sizes of the two crystals were similar, which is indicative of nearly equal fall velocity, there was an increased number of the crossed adhesion type, whereas the point adhesion type was predominant when there was a large difference in size, thus fall velocity. In the case of the crossed adhesion type, the angle between the c-axes of the two crystals was usually greater than 708, whereas the angle in the case of the point adhesion type showed no such distinct feature. Our findings suggest that the crossed adhesion type snowflakes are formed mainly through the Ž . interaction of the flow fields around two crystals the collision due to relative horizontal motions and the point adhesion snowflakes were formed through the inertial collision between smaller Ž . crystals and larger ones the collision due to the velocity difference . q 2000 Elsevier Science B.V. All rights reserved.