Formation of Ice Crystals and Dissipation of Supercooled Fog by Artificial Nucleation, and Variations of Crystal Habit at Early Growth Stages

Abstract

The early stages of ice crystal formation in supercooled fogs were studied in detail by electron microscopy, and ice nucleation experiments using liquid propane seeding were conducted in a thermostatically controlled coldroom. Ice crystals, formed by rapid cooling created by the evaporation of liquid propane from a fine nozzle at temperatures from ?0.1 to ?40°C, were collected and replicated on filmed grids for electron microscope examinations. Most of the ice crystals formed immediately after the liquid propane seedings were spherical (although ?20% were hexagonal) with diameters ranging from 0.3 to 3 ?m and with a mean diameter of 1.5 ?m. Electron microscopy revealed a grain boundary in some of the ice crystals. The production rates of ice crystals per gram of liquid propane seeding were measured at temperatures from ?0.1 to ?20°C. The production rate increased exponentially at temperatures from ?0.1 to ?4°C, and remained at about 1011 ice crystals per gram of liquid propane seeding at temperatures below ?5°C. Experiments of supercooled fog dissipation by liquid propane seeding were performed in the coldroom. The results showed that supercooled fog dissipation becomes effective at temperatures colder than ?0.5°C. The habit of early stage ice crystals formed at temperatures from ?0.1 to ?40°C and ?90 to ?160°C was studied. Two basic types of hexagonal plates and columns were observed at temperatures from ?22 to ?40°C. A cold stage was used with the electron microscope to investigate the structure of the crystals formed at temperatures of ?90 to ?160°C. Plates were observed at ?100°C, and were analyzed as having a hexagonal form by their electron diffraction patterns. Cubic forms of ice crystals were observed below ?100°C.