Baltic Sea Ice Climate Workshop 2005 
72 
Table 3. Characteristical features of the Ice Winter Types in the German coastal and sea 
areas of the Baltic Sea (Number and percentage for the period 1961 to 2003) 
Ice Winter Type 
number / % 
Area 
Length of ice 
occurrence 
Ice thickness 
mostly 
Ice concentration 
mostly 
Very weak 
to weak 
20 / 46 % 
Inner coastal waters 
Outer coast 
1 - 4 weeks 
up to 3 days 
5 -10 cm 
up to 5 cm 
6/10-8/10 
1/10-3/10 
Moderate 
15 /35% 
Inner coastal waters 
Outer coast 
3-10 weeks 
up to 3 weeks 
10 - 30 cm 
up to 10 cm 
10/10 
6/10 - 8/10 
Strong 
2 1 5 % 
Inner coastal waters 
Outer coast, open sea 
6-12 weeks 
2-10 weeks 
20 - 30 cm 
15 - 25 cm 
10/10 
6/10 - 10/10 
Very strong 
5 /12% 
Inner coastal waters 
Outer coast, open sea 
2 - 3,5 months 
1,5-3 months 
30 - 50 cm 
30 - 40 cm 
10/10 
9/10 - 10/10 
Extreme strong 
1/2% 
Inner coastal waters 
Outer coast, open sea 
3 - 5 months 
2 - 3,5 months 
50-70 cm 
50-70 cm 
10/10 
9/10 - 10/10 
BSH, Ice Service - 09/2003 
The Ice Phases defined by Finnish scientists mainly for the northern region of the Baltic Sea 
consider “the fact that the Baltic ice winter does not develop in accordance with average 
conditions, but instead according to certain phases that recur year after year” (Lepparanta, M. 
et al. 1988, p. 19). The defined 20 phases - phases 1 to 10 for the ice formation period, phases 
12 to 20 for the period of ice decay; phase 11, which should represent a complete ice cover of 
the Baltic Sea, did not occur in the period of investigation - describe special stages of the ice 
development during the winter reflecting distinct geographical features of the ice distribution 
and extension as e.g. boundaries of the coastal fast ice and the seaward extension of the ice 
cover related to oceanographic boundaries and natural characteristics of the different sea 
areas. 
In the above cited ‘Phase Atlas’ the ice phases and relevant statistical data were calculated for 
the 17 years period 1963/64 to 1979/80. This period was extended with respect to the dates of 
the occurrence of the different phases to the 40 years period 1960/61 to 1999/2000 (Table 4). 
Furthermore the ice extent (km 2 ) was calculated for the single phases with the ICEMAP 
software (Berglund, R., 2003). As it has already been mentioned in the Atlas (p. 20), to fix the 
date of the beginning/ending of a special phase is a somewhat subjective decision, as there 
can be delays between the different sea areas (e. g. Gulf of Bothnia and Gulf of Finland) due to 
regional differences in the frost and wind regime. Nevertheless, the ice phases offer a good 
‘natural synthetic’ approach to calculate the severity/character of an ice season. 
This is done by using not just the ice extent of the single ice phases in a single winter, but 
consider their duration, too, in order to receive a ‘weighted’ measure for the season. The result 
is a mean ice extent - for each phase its duration (number of days) is multiplied with the ice 
extent (km 2 ); the sum is divided by the total duration of the ice season. 
The comparison of the mean ice extent and the maximum ice extent results in a sometimes 
quite different ranking of the single ice winters (Table 5).