The ice in the Baltic Sea exists as fast ice and drift ice. Fast ice is situated in coastal and archipelago areas, where the depth is less than 15 metres. It develops during early ice season, and remains stationary to the melting period.
The drift ice has a dynamic nature being forced by winds and currents. Drift ice can be level, rafted or ridged, and its concentration could be 1-100%. In media, drift ice is occasionally called pack ice. Pack ice is drift ice with concentration more than 80%. The term has no dynamic meaning.
Drift ice movements are large: in stormy conditions thin drift ice field can move 20-30 km in a single day. The motion results in uneven and broken ice field with distinct floes up to several kilometres in diameter, leads, and cracks, slush and brash ice barriers, rafted ice and ridged ice.
The ridges and brash ice barriers are the most significant obstructions to navigation in the Baltic Sea. Powerful, ice-strengthened vessels can break through ice up to almost one metre thick, but they are not capable of navigate through ridges without icebreaker assistance. Ice dynamics affects navigation considerably – high pressure in the ice fields can be dangerous to the vessels, and it may at least cause time delays from hours up to days.
