8 Recent Change—Sea Ice 151 
Table 8.2 Change in 
freezing date, break-up date, length of ice 
season and ice thickness 
in the Finnish coastal zone 
of the Baltic Sea 
Kemi, Bothnian Bay 
Uto, Archipelago Sea 
Loviisa, Gulf of Finland 
Probability of ice appearance 
1.00 
0.81 
1.00 
Trend 100 year -1 
0 
-0.19 
0 
Freezing date 
Number of observations 
112 
97 
106 
Mean date 
10 Nov 
27 Jan 
7 Dec 
Trend (days 100 year -1 ) 
7 
(24) 
20 
Trend pre-1950 (days 100 year -1 ) 
12 
(65) 
25 
Trend post-1950 (days 100 year -1 ) 
0 
(-15) 
24 
Break-up date 
Number of observations 
113 
97 
10 
Mean date 
20 May 
9 Apr 
24 Apr 
Trend (days 100 year -1 ) 
-11 
(-16) 
-20 
Trend pre-1950 (days 100 year -1 ) 
-5 
(-D 
-8 
Trend post-1950 (days 100 year -1 ) 
-17 
(-34) 
-38 
Length of ice season 
Number of observations 
113 
121 
102 
Mean (days) 
190 
50 
137 
Trend (days 100 year -1 ) 
-18 
-46 
-41 
Trend pre-1950 (days 100 year -1 ) 
-17 
-84 
-32 
Trend post-1950 (days 100 year -1 ) 
-16 
-36 
-62 
Ice thickness 
Number of observations 
94 
81 
Mean (cm) 
73 
46 
Trend (cm 100 year -1 ) 
9 
-25 
Trend pre-1950 (cm 100 year -1 ) 
13 
-29 
Trend post-1950 (cm 100 year -1 ) 
4 
-52 
Data for three sites: Kemi (1890-2010), Uto (1889-2010) and Loviisa (1894—2010) (Ronkainen 2013). The parentheses indicate values that are not 
based on all data because the probability of ice occurrence is less than one 
■ 1961-1990(max) ■ 1971-2000(max) ■ 1981-2010(max) 
• 1961-1990<with ice) • 1971 -2000{with ice) • 1981-2010(with ice) 
Fig. 8.7 Mean number of days with ice (all winters), mean number of days with ice (only winters with ice) and maximum number of days with ice 
on the southern Baltic Sea coast in the 30-year periods 1961-1990, 1971-2000 and 1981-2010 (Schmelzer et al. 2012) 
Kemi), ice thickness shows a slight increasing trend (+9 cm) 
over the past 100 years, while in the Gulf of Finland, a 
thinning trend is observed (-25 cm). 
In the southern Baltic Sea, ice thickness in the sheltered 
coastal waters reached 10-20 cm on average, and as much as 
40-62 cm in the extremely hard ice winter of 1962/63 
(Fig. 8.8). In most parts of the Baltic Sea offshore waters, ice 
thickness reaches 5-20 cm, but in particularly cold and long 
winters, the ice grows to 50 cm thickness in some areas. A 
maximum ice thickness of 70 cm was reached in the Kiel 
and Lübeck bays and in the waters of Rügen in the winter of 
1962/63. In Szczecin Lagoon, in the Bay of Puck and in 
Vistula Lagoon, the ice reached 62 cm thickness. Since 
1971, the maximum ice thickness has been about 50 cm in 
the southern Baltic Sea coast and up to 30 cm in the sea area 
north of Rügen.