Aggregation of needle snow crystals

For the purpose of studying the early process of snowflake growth, characte ristics of the aggregation (collision-adhesion process) of needle snow crys tals were investigated by examining photomicrographs of snowflakes composed of two crystals. The adhesion type of the snowflakes was classified into crossed adhesion an d point adhesion types. When the sizes of the two crystals were similar, wh ich is indicative of nearly equal fall velocity, there was an increased num ber of the crossed adhesion type, whereas the point adhesion type was predo minant when there was a large difference in size, thus fall velocity. In th e case of the crossed adhesion type, the angle between the c-axes of the tw o crystals was usually greater than 70 degrees, whereas the angle in the ca se of the point adhesion type showed no such distinct feature. Our findings suggest that the crossed adhesion type snowflakes are formed m ainly through the interaction of the flow fields around two crystals (the c ollision due to relative horizontal motions) and the point adhesion snowfla kes were formed through the inertial collision between smaller crystals and larger ones (the collision due to the velocity difference). (C) 2000 Elsev ier Science B.V. All rights reserved.