Ice Navigation in Theory (Part 1)

Ice Navigation

The Baltic Sea freezes anually.
The two most heavily marine operated areas in the world where seasonal sea ice plays an important role in navigation are the Gulf of St. Lawrence in Canada and the Baltic Sea in Europe. In the Baltic Sea aproximetely 40 percent of the total amount of cargo turnover, about 700 milion tons, occurs during winter months. Tha Baltic Sea freezes anually and in some parts the ice season lasts up to 7 months, from November to May. For example, in the Gulf of Finland the average length of the ice season is 120 days outside St. Petersburg, and 30 days at the entrance of the gulf.

The ice conditions are mostly affected by two factors: the number of sub-zero days and the prevailing winds. The sum of sub-zero degree-days controls the ice growth and the amount of ice. The prevailing winds control drifting and ridging of an ice field.

Forming of an ice cover

An ice cover starts to form on water when the surfer temperature reaches freezing point. Fresh water freezes at 0 C and in sea water freezing point decreses with increasingly salinity. Thus freezing point in ocean water is about -1.8C, but in the brackish water of the Baltic Sea it is about -0.4C.

In the Gulf of Finland ice thickness is greatest in the eastern parts of the gulf and is about 50 cm in an average winter. The biggest obstacles to winter navigation are ridges which are normally thicker than the level ice and are difficult to penetrate. Channels with thick side ridges and thick brash ice in the middle are formed when the ice cover in the fairway is repeatedly broken and frozen. The side ridges make passing of other vessels very difficult. The keel heights of the ridges are normally a lot bigger than the sail heights. The side ridges may grow several metres thick and the brash ice layer in between may become up to one metre thick. Ridges also form when winds push ice together.

In the Baltic Sea area ice conditions are monitored on a daily basis. The Finnish Ice Service of the Institute of Marine Research issues ice charts and ice reports and produces ice drift forecasts. The daily ice chart and ice report include a description of current ice conditions and information about the icebreakers operational areas.

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