The formation of ice particles in stationary ultrasonic fields (SUSFs) from ice aerosol was observed. Under suitable experimental conditions aerosol particles gather in stable ellipsoidal systems at temperatures down to <50 K below ambient. We call these systems cold gas traps. The aerosol concentrates in the pressure node areas of the SUSF and conglomerates to form larger particles and snowflakes with diameters up to 10 mm. The resulting shapes are shown. Most of them are similar to those occurring in nature. An attempt is made to explain the effect. Three different processes of ice particle formation are visualized and discussed. Here the existence of a quasiliquid layer (QLL) on the particle surfaces seems to be essential because both the formation of ice particles and the QLL occur only at temperatures higher than about -25 degrees C. The observed trapping and generation process can be used, for example, in atmospheric physics for the study of particle aggregation, precipitation formation, and water and radiation transport phenomena.