Climate indicators – sea ice

The sea ice climate indicator shows the maximum ice extent for the Baltic Sea and Kattegat for each year since observations started in 1957. The value is based on daily analyses of the ice situation in the Baltic.

The climate indicator for maximum ice extent shows the size of the area covered by sea ice for the entire Baltic Sea up to and including the Kattegat – an area of about 420 000 square kilometres. Neither the Skagerrak nor lakes Vänern, Vättern and Mälaren are included.

Annual maximum ice extent in the Baltic Sea from 1957
Annual maximum ice extent in the Baltic Sea from 1957 calculated from ice charts. The black line shows a moving long-term average with a three year standard deviation. Enlarge Image

 

Calculate maximum ice extent

Calculation of the ice extent is made from ice charts produced daily during the winter season at SMHI. The ice in the ice charts are divided into different ice areas and assigned a concentration in whole tenths. The ice extent is defined as the sum of the areas of all the areas that have a concentration of at least one tenth.

Before 2019 the calculations was based on a courser division of the ice areas in a predefined grid and the limit for which areas to be included were drawn at a concentration of two tenths or more. The new method provides a greater accuracy but generally a slightly lower total ice extent.

Historical data

SMHI has digitised ice charts of maximum ice extent in the Baltic Sea from 1957 onwards, the year when overview ice charts were first produced based on coastal observations as well as ship reports. Therefore, the measurement series for the sea ice climate indicator starts with 1957. For previous years, a lot of information has been preserved, but few complete overview ice charts of the entire Baltic Sea area.

Satellite images have been available since the end of the 1960s, and before that a lot of ice charts were drawn for smaller limited areas by pilots on regular routes across the Baltic Sea. It is also known that during the cold war winters of 1939/1940, 1940/1941 and 1941/1942, virtually the entire Baltic Sea was covered in ice.