Clear ice refers to a solid precipitation which forms when air temperature is between 0 °C (32 °F) and -3 °C (27 °F) and there are supercooled, relatively large drops of water (from freezing fog).
"The opacity of ice formed from water containing dissolved air is due to the presence of bubbles of air in the ice. Both bubble concentration and sizes were found to depend on the rate of freezing. Bulk water saturated with air at 0°C was found to freeze into ice containing about six bubbles per mm3 when freezing proceeded at 0.5 mm min-1 and 300 per mm3 at a rate of 5 mm min-1. Bubbles were formed at the ice-water boundary when the concentration of dissolved air reached a critical value which, for rates of freezing greater than 2 mm min-1, corresponded to a supersaturation ratio of 30. Agitation of the water could prevent the critical concentration from being attained and clear ice then formed."
"Other factors which influenced bubble concentrations and sizes were the amount of dissolved air, pressure, thickness of the layer of water ahead of the growing ice and escape of bubbles by buoyancy."
"Bubbles in ice were found to change shape with time particularly when under the influence of a temperature gradient."