Frazil ice is a collection of loose, randomly oriented, plate or discoid ice crystals formed in supercooled turbulent water. Its formation is common during the winter in rivers and lakes located in northern latitudes, and usually forms in open-water reaches of rivers where and when the heat exchange between the air and the water is such that the water temperature can drop below its freezing point (typically not lower than -0.1 °C in rivers). As a rule of thumb, such conditions may happen on cold and clear nights, when the air temperature is lower than . Frazil ice also forms in oceans, where it is often referred to as grease ice when floating on the surface.