Useful notes describing the process on the formation of Sea Ice

In the atmosphere, as we know, the condensation takes place on the hygroscopic nuclei, the microscopically small particles of dust. In the same way, in sea water ice crystals are formed on crystallization nuclei comprising the smallest possible particles of organic or inorganic origin.

Crystallization may also occur at an aggregation of a number of molecules which are grouped together by chance. These molecules also play a significant role in the formation of ice – crystals.

In addition to crystallization nuclei, the sea water must necessarily be super cooled. If the purity of water is of greater degree and it is totally undisturbed, then it must be super cooled to a greater degree.

Since sea water is in constant motion, and the crystallization nuclei are present in adequate quantity, only a very small degree of super cooling is sufficient to initiate the ice formation.

Another important point to keep in mind is the fact that formation of ice releases latent heat of condensation. For a change of water from the liquid to the solid state heat must be continually removed that should be greater in amount than the latent heat of condensation.

In very calm and cold water the ice crystals so formed are extremely small with a needle – like structure. On the contrary, if the water is in constant motion the needlelike structure is replaced by such crystals which look like flat plates with irregular rounded edges.

It is a well-known fact that the freezing point of sea water depends on salinity. At 35%. Salinity, the freezing point is -1.9°C.

As water freezes at the sea surface, the dissolved solids present in sea water do not fit into the crystalline structure of the ice, and thus, part of the salt content of the water is left behind during the formation of ice, and as a more or less concentrated salt solution, fills the various separating layers between the ice crystals.

Remember that the ice crystals consist of pure water. Since ice crystals are separated by the aforesaid salt solution, in the beginning ice is characterized by little solidity.

It is, however, interesting to know that the dissolved solids being left behind increase the salinity of the surrounding water. This greater salinity lowers the freezing point of the remaining water that is yet to freeze.

The low temperature, high density water that is excluded from the ice has a tendency to sink. This high density sinking cold water is replaced by the warmer and less dense water at the surface.

This diffusion process beneath a forming ice layer is perhaps the reason why the crystal plates in sea water are oriented perpendicular to the freezing surface.

The type of circulation as mentioned above enhances the formation of sea ice. In this process freezing is aided by low salinity and calm water where there is no mixing by wind or currents.

As stated earlier, in the beginning of the formation of sea ice, the ice crystals are needlelike. They are hexagonal in shape. Later on, the ice crystals grow in such a large number that a slush condition is produced.

When the slush begins to form a thin sheet of ice on the sea surface, it is broken by the wind stress (friction caused by blowing wind) and wave action. Now, the thin ice sheet is converted into small disc-shaped pancakes (a plate-shape form with upward bulging edges).

As the freezing proceeds further, the pancakes converge and coalesce to form what is called ‘floes’. The individual plates have a diameter of 0.5 – 1 m. with a maximum of about 3 m.

In calm weather the pancake ice and ice slush coalesce to form a solid layer of the so-called ‘young ice’ having a greenish blue colour, and a thickness of between 5 and 20 cm.

Even though the rate of ice formation is closely linked with temperature conditions, it becomes slow as the thickness of ice increases due to the poor heat conduction of ice. There is a significant quantity of brine that is trapped during the freezing process.

The newly formed ice may have salinity ranging from 4 to 5%o. Actually salinity of the newly formed sea ice depends on the rate of freezing. If the ice forms more rapidly, larger amount of brine will be captured resulting in higher salinity.

However, after some time the brine will percolate out through the coarse structure of the sea ice and the salinity of the sea ice will decrease accordingly. During the course of a year or so, sea ice becomes nearly pure.