The Baltic Sea is generally frozen each year from November to May. This presents special challenges in terms of ensuring safe navigation and keeping accurate timetables. The sea begins to freeze from the north and east at the end of November, and the last ice melts in the north around the end of May. During hard winters, nearly the entire surface of the Baltic Sea is frozen solid. These conditions, at their worst, may entirely prevent sea passage and may even crush or badly damage ice-bound vessels.
The ice conditions are affected primarily by the number of days below the freezing point and the winds. In the Baltic Sea, the south-south-western winds tend to form ice banks between and through which ships must travel. Freezing cold days add thickness to the ice and freeze the remaining open water. The western winds push the ice toward the bayhead.
At the bottom of the Gulf of Finland, the icy winter lasts an average of 120 days, while the average is only 30 at the mouth of the Gulf. The average ice thickness in the eastern parts of the Gulf is about half a metre during the winter, but the accumulated ridged ice often rises several metres high. At their maximum, these drifts form obstacles for winter navigation that grow several metres both underneath and above the sea surface.
The ability to select the best route is fundamentally affected by the properties of the ice. The ice may be ridged ice and may cause the ship to be ice-bound, or it may be a solid ice bank of up to a metre thick. The ice navigation is also affected by the size of an ice field, the thickness of the ice, and the direction and speed of the wind.
The abilities of ships to travel through icy waters vary greatly. The majority of the vessels sailing the Baltic Sea have been given a designated ice class.
Ice Advisors Ltd, c/o Finnpilot