Congelation ice forms as water freezes on the bottom of the ice cover, and the latent heat of crystallization is conducted upward through the ice and snow to the atmosphere. Formation of sea ice on the bottom of an established sea ice cover in the form of platelets, which coalesces to form solid ice, is also represented by congelation ice. Its growth rate is proportional to the rate at which energy is transferred from the bottom surface of the ice layer to the air above. Congelation ice is often referred to as black ice because it has a high optical depth that permits significant light transmission to the underlying water. It forms as a stable ice sheet with a smooth bottom surface.