How does sea ice form from freezing seawater?

Sea ice forms when seawater reaches its freezing point, a process driven by the extraction of latent heat. Unlike fresh water, which reaches its maximum density at 4 °C, seawater continues to increase in density as it cools until it reaches the freezing point. Because salt is largely excluded from the ice as it forms, the remaining water beneath the ice becomes slightly more saline and dense, triggering a continuous convective overturn that influences the rate of ice growth.1

The formation process varies based on environmental conditions:

• Calm conditions: Ice crystals form on the surface and freeze together into a continuous sheet of new ice known as nilas. As this sheet thickens through bottom-freezing, it becomes young ice.2
• Turbulent conditions: Wind and waves cause ice crystals to agglomerate into discs called pancakes. These pancakes eventually solidify and thicken by rafting on top of one another to form larger floes.2
• Initial stages: Regardless of conditions, the process often begins with the formation of a soupy suspension of ice crystals known as frazil or grease ice.2

As ice forms, it traps small pockets of liquid brine and air within a matrix of pure ice crystals. Over time, this brine is gradually lost through gravity drainage, reducing the ice's salinity.3