Ice Winter Severity in the Western Baltic Sea in the Period 1301-1500 
51 
20-year low-pass filter 40-year low-pass filter 
Fig. 7. Long-term variations in the ice winter index numerals in the period 1401-1700. 
Fig. 8. Variations of the ice winter index since 1401. 
The variations of the ice winter index for the total period 1401-2005 are shown in Figure 8. The 
solid curve represents the smoothed ice winter numerals obtained by applying a Gaussian low- 
pass filter with a 40-year cut-off period. The dark areas denote the period of increased ice 
winter severity with regard to the arithmetic mean of 0.529 for the period 1401-2005 (horizontal 
solid line). The standard deviation a = ± 0.826. This diagram can be compared with that for 
1501-1995 period in Figure 3, by Koslowski and Glaser, 1999. 
The periods with increased or reduced ice winter severity related to the arithmetic mean of 
0.529 in the time from 1401-2005 have been compiled in Table 2. The results can be partly 
compared to the data in Table II shown by Koslowski and Glaser, 1999. The differences 
between the time periods in the two tables are attributable to the changed mean value for the 
total time period and to the inclusion of additional data for the period prior to 1520.