How can the sea water freeze in winter?

The freezing conditions of seawater and fresh water are different. Everyone who lives by the sea has this experience. Whenever cold air strikes in early winter, shallow ponds on land quickly freeze into a thin layer of ice. In the middle of winter, the rivers are frozen, but the sea is still choppy and choppy. Only when cold waves break out frequently and the air is low for a long time will the sea water freeze. There are three conditions for seawater to freeze: ① the temperature is lower than the water temperature, and a lot of heat is lost in the water; (2) Compared with the temperature (freezing point) when water begins to freeze, there has been a little supercooling; (3) There are impurity condensation nuclei such as suspended particles and snowflakes in the water. The density of fresh water is the highest at around 4℃, and it can freeze when the water temperature drops below 0℃. Sea water contains a lot of salt. Because of the high salinity, the freezing temperature is lower than that of fresh water, and the water temperature is lower than 4℃ at the maximum density. With the increase of salinity, the freezing point and the temperature at which the seawater density is maximum gradually decrease. When sea ice is born, it is a needle-like or flaky ice crystal; Then forming paste or sponge; After further freezing, it becomes ice skin or ice cake floating on the sea, also called lotus leaf ice; After the sea surface is covered by this ice, it extends to the thickness direction, forming gray ice and Bai Bing covering the sea surface. Sea ice is a mixture of fresh water ice crystals, "brine" and salty bubbles. According to the development stage, it can be divided into six categories: primary ice, Nile ice, cake ice, primary ice, one-year ice and old ice. According to the motion state, it can be divided into two categories: fixed ice and flowing ice. Fixed ice freezes with the coast, seabed or island, can rise and fall with the sea surface, and can extend from the sea surface for several meters or hundreds of kilometers. Ice floes float on the sea surface and move around with the wind direction and seawater flow direction.