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

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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 μm and with a mean diameter of 1.5 μm. 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.

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