Drift ice

In the Danish and Swedish coastal waters of the Kattegat, ice forms in 30-50% of all winter seasons, with maximum values recorded in the harbors and relatively sheltered areas in the immediate vicinity of the outer coasts, and minimum values in the areas that are closest to the outer sea. In about 25% of all winter seasons, drift ice occurs in the outer sea area of the Kattegat, with thicker ice usually encountered in its southern part, decreasing toward the north. Most of the ice is formed in the Kattegat itself, beginning in its southwestern part and in... 

In strong winters a stable ice cover is formed in the protected and less navigated parts mostly already in the second decade of January and persists till March. The ice coverage in the sea areas has its highest value in strong winters in the last decade of February. In most inner fairways, especially in the entrances to Stralsund, fast ice predominates with level ice thickness up to 30 cm. In the eastern sea areas most frequently an ice thickness between 15 and 30 cm is measured and the very close to compact ice occurs somewhat often as the open drift ice. 

The frequency of ice occurrence in the outer sea is 10-20% from about 57 N northward, 25% from about 58 N, and 50% on the northern and northeastern margins of the area. The frequency of ice occurrence in the other, southerly part of the Gotland Sea is extraordinarily low. Only a combination of several unfavorable factors (particularly low salinity, particularly low heat storage, long-term inflow of extremely cold air masses from the east as early as December and through March, high radiation of heat) can lead to ice formation in this area. During the past century, there have been only three winters with major ice formation in the entire sea area (1939/1940, 1940/1941, 1946/1947), and within the past 50 years only two winters (1955/1956 and 1986/1987) with some drift ice occurrence, which is a frequency clearly below 5%. 

Phase 3 Drift ice floes freeze together into consolidated ice in the outer sea areas. 

Sea of Bothnia The southern half of the offshore area of the Sea of Bothnia remains ice-free in mild winters (about 34% of winters in the considered range) except for a very narrow strip of ice off the Swedish coastal fast ice and a wide belt of ice off the Finnish archipelagos. In all other winters, drift ice increasing in density toward the north is to be expected throughout the offshore waters. In severe winter seasons (about 16% of winters), the compact drift ice in the entire sea area freezes together, in normal winters only in the northern third. 

The specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C. Of common substances, only that of ammonia is higher. This property confers resistance to melting on the ice of glaciers and drift ice. Before and since the advent of mechanical refrigeration, ice was and still is in common use for retarding food spoilage. 

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