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Any form of ice found at sea which has originated from the freezing of sea water.
sea ice less than 30 cm thick
The first indication of ice formation is the appearence of small ice spicules or plates in the top few centimetres of the water. These spicules, known as Frazil ice, form in large quantities and give the sea an oily appearance. As cooling continues the frazil ice coalesces to form Grease ice, which has a matt appearance. Under near-freezing but as yet ice-free conditions, snow falling on the surface may result in the sea surface becoming covered by a layer of Slush. These forms may be regrouped by the action of wind and waves to form Shuga and all are classified as New ice. With further cooling, sheets of Ice rind or Nilas are formed, depending on the rate of cooling and on the salinity of the water. Ice rind if formed when water of low salinity freezes into a thin layer of brittle ice which is almost free of slat, whereas when water of high salinity freezes, especially if the process is rapid and wind is very light, the ice has an elastic property which is characteristic of nilas. The latter form of ice is subdivided, according to its thickness, into dark and Light nilas; the second, more advanced form reaches a maximum thickness of ten centimetres.
The action of wind and waves may break up ice rind or nilas into Pancake ice, which can later freeze and thicken into Grey ice and Grey white ice, the latter attaining thicknesses up to 30 cm. These forms of ice are referred to collectively as Young ice. Rough weather may break this ice up into ice cakes or Floe s of varying size.
sea ice 30 cm to 2 m thick
The next stage of development (first stage see: "sea ice less than 30 cm thick") is known as First year ice and is subdivided into thin, medium and thick categories. Thin first year ice has a thickness of 30-70 cm. Medium first year ice has a range of thickness from 70 to 120 cm while in polar areas Thick first year ice ( > 120 cm thick) may attain a thickness of approximately two metres at the end of the winter.