Ice that appears on seas or oceans is called sea ice. Ice formed from seawater is saline, unlike lake or river water ice. As a “rule of thumb” it can be said that about a third of the salinity in the water from which it is formed remains bound in sea ice. Salt and other impurities are located between the ice crystals, and as a result of this, sea ice is weaker than freshwater ice.
In the Baltic Sea ice conditions vary greatly. On average, ice covers an area of some 218,000 square kilometres. The annual ice cover is at its greatest between January and March, usually around the end of February/beginning of March.