On the symmetry of snow dendrites

Abstract

Natural snow dendrites sometimes display symmetry of side branches along each arm, whereas on other occasions such symmetry does not occur. Laboratory studies under simulated atmospheric conditions show that a fluctuation of local airflow, equivalent to changing supersaturation, for a period of only a few seconds leads to symmetrical initiation of arms. Growth under uniform conditions does not give symmetrical arms, neither does growth in uniformly supercooled water. It is hypothesized that each side branch is initialized by a local short term fluctuation, in temperature or supersaturation; symmetrical branches form from longer period fluctuations that affect each side of an arm simultaneously. Asymmetrical arms result from local fluctuations such as the close approach of a cloud drop. Symmetry of side branches therefore suggests that the crystals have grown in an environment with a turbulence scale of several meters.