Abstract
Ice crystals were grown in air at pressures lo\ver than 10-1 Torr in a cryostat. It was found that at pressures lower than 10-1 Torr ice crystals grew in spherically symmetric form with facets of higher indices and then the facets of lower indices developed. These ice crystals were transparent and facets were specular. Even at high supersaturation, neither skeletal, nor edge growth were observed. The measured growth rates indicate that the growth of the ice crystals was not controlled by diffusion of water vapor in air, but by the free molecular influx onto surfaces of the ice crystals.
The sublimation coefficient of ice was determined at temperatures between -50°C and -ll0°C; Its values were about 0.07 at temperatures between -50°C and -80°C and below -83°C increased with decreasing temperature to 0.4 at -110°C, which is nearly equal to the temperature at the level where noctilucent clouds are often observed.
