Some Ideas on Sea Ice Climate and Navigation in the Baltic Sea 
75 
difference is 3 ranks. Some possible reasons have already been mentioned, but more detailed 
checks are necessary to ‘judge’ on the value of the one or other method. The diagram in 
Figure 4 shows at least for both time series the characteristic year to year variations, but of 
course for the mean ice extent in a smoothed version. 
Region of the northern Baltic Sea 
Maximum and mean ice extent by phases 
o o c 
° ° c 
1 
.L Lh 1 i ILIlh 
1960/61 1965/66 1970/71 19 
iu Ilk [ [ fi MifitiJ i 
75/76 1980/81 1985/86 1990/91 1S 
195/96 
ice winter 
maximum ice extent □ mean ice extent 
Fig. 4. Comparison of the mean ice extent by phases for the northern Baltic Sea and the 
maximum ice extent for the period 1960/61 to 1999/2000 
Concerning the above diagram as well as table 5 it has again to be considered that the values 
for the maximum ice extent are calculated for the Baltic Sea in total, i.e. with contributions from 
the southern regions. The comparison can be improved for the more severe winters, if only the 
ice cover for the northern region is calculated. 
Nevertheless, the use of the ice phases is a good help for the characterization of an ice winter. 
In Figurte 5 the columns show the ice extent for the phases 1-9 and 12-20 in comparison to the 
development of the ice extent in the winter 2002/03 calculated from the twice weekly ice charts 
(Mondays and Thursdays). The graph shows clearly the very rapid ice formation at the 
beginning of the winter - related mostly to the Gulf of Finland, which results already in early 
January in an ice extent of a nearly average winter. The considerably ice decay later on in 
January produces then an about normal ice extent in mid-winter, however, the ice thickness in 
the Gulf of Finland staid well above the mean values.