congelation ice

Ice formation, or congelation ice, occurs when the surface of lakes and rivers drops below freezing temperatures (0° C or 32° F). Before ice can form, water must supercool, and ice crystals need to nucleate. While homogeneous nucleation (without foreign particles) happens far below freezing, heterogeneous nucleation (with foreign particles) depends on the particle type.12

In natural water bodies, significant supercooling is uncommon. It's believed that ice particles from above the water surface initiate the first ice formation. Once ice is present, its growth rate determines further formation. On cold, still nights, ice crystals can spread rapidly across the surface, with their vertical c-axes oriented upwards. These initial crystals can be large and transparent.2