Ice winters are classified as “mild,” “normal,” and “severe”. The main factor in assessing the total degree of severity of an ice winter is the extension of sea ice.

Other conditions affecting shipping are taken into account including the duration of the ice period, the navigability due to wind and current conditions, etc. The degree of severity within limited areas may differ from the total degree of severity. During an ice winter classified as mild, for instance, the ice conditions in the Gulf of Bothnia may have been normal.

Diagram of ice distribution for the winters of 1900-2007

The diagram shows the maximum ice extension in the Baltic Sea, the Kattegat and the Skagerrak in different years.

The cut-off point between a mild and a normal ice winter is 98,000 km2. The cut-off point between a normal and a severe ice winter is 193,000 km2.

The 2008/09 ice winter may be designated mild for winter navigation in spite of the fact that the ice extension was almost normal.

 Large picture