Sea ice is formed when ocean water freezes. It is often confused with icebergs which are chunks of ice shelves or glaciers that calve into the ocean.

Ocean water does not freeze as easy as fresh water because it contains salt. That is why at least -1.8 °C is needed to begin the process of freezing. When the water at the surface of the ocean cools, it becomes heavier and hence it sinks. Deeper water climbs to the surface and is cooled. So it takes a while before a thick layer of water is cold enough for the first ice crystals to appear. Sea ice is thus more easily formed in shallow water near shores or sand-banks.

Sea ice can appear in numerous shapes depending on the circumstances in which it was formed. The wind and currents for example can tear the ice apart or can just bring different patches of ice together.

The following types of sea ice can be distinguished.

• Slush: this is a slurry mixture of ice and liquid water. It’s like the pulp when snow begins to melt. The ice crystals are not or only slightly stuck to one another.

• Grease ice: this is the next stage of sea ice. The ice crystals begin to freeze together. Grease ice can be recognized as a thin, soupy layer, which makes the ocean surface resemble an oil slick.

• Shuga: shuga is composed of spongy, white lumps, a few centimetres across. It tends to form in agitated conditions by accumulation of slush or grease ice.
  

• Pancake ice: when the surface water sloshes, due to swell for example, and when the temperature is low enough, slush or grease ice may freeze into a shape that resembles pancakes. These pancakes are normally 50 to 100 cm in diameter and 10 cm in thickness, but they sometimes can grow to even 3 m in diameter. They typically have raised rims due to the pieces striking against one another.

• Young ice: young ices consist of patches of flat ice with a thickness up to 30 cm. It is often formed when pancakes are frozen to one another. In its early stages, when it is still transparent, it is called nilas. When it is less than 5 cm thick it is still transparent and therefore dark in colour: dark nilas. Light nilas is thicker and reflects proportionally more light than dark nilas.

• Old ice: this is sea ice that survived has the summer. It is harder than young ice and contains less salt.

• Hummocks: hummocks are rounded knolls of ice, rising above the general level of an ice-field. They are formed by the bumping of different ice fields against one another, which is often accompanied by strong pressures. Hummocks may in some cases rise up to 20 m above the general ice level.
  

• Lead: this is a river-like passage trough sea ice, which is navigable.
  

• Polynya: this is an area of open water surrounded by sea ice. It may be formed by two different mechanisms: it may either occur when the water never reaches the freezing level, for example due to an upwelling of warmer water; or it may be formed through the action of the wind or ocean currents, driving the ice away from a certain place. When a polynya is wide enough, it can be responsible for the formation of clouds, when the ice cold katabatic winds flow over it. Due to the higher temperatures above the polynya, the air starts to rise and consequently clouds can be formed.