The climate indicator for maximal 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.
Calculation of the ice extent is made from ice maps produced daily at SMHI. SMHI has digitised ice maps from 1957 until the present day, which is why the measurement series for maximal ice extent starts in 1957.
SMHI’s ice maps are made up of small boxes in different sizes, and for each box the percentage of ice cover is specified. The ice extent is defined as the sum of the areas of all the boxes that are at least 15% covered by ice. If a box only has 12% then it is not included, while the whole of a box with 18% ice cover is included in the total sum. It does not make any difference whether a box has 20% or 100% ice coverage, as long as the limit of 15% has been reached (or exceeded).
SMHI has digitised ice maps from 1957 until the present day. There is also a lot of information for earlier years, but comprehensive maps for the entire Baltic are difficult to find so far back in time. Nowadays there are satellite pictures, and previously a number of ice maps were drawn by pilots flying regular routes over the Baltic. What we do know is that just about the entire Baltic was covered with ice during the cold war winters of 1939/1940, 1940/1941 and 1941/1942. Another trio of cold winters with significant ice extent is 1984/1985, 1985/1986 and 1986/1987, again when almost the whole of the Baltic was covered with ice.