85 
occur. Therefore, first ice in this area formed as late as the second half of January. To compute the 
cold sum, however, the negative daily means of air temperatures are used beginning as early as De 
cember. By contrast, the ice thicknesses measured in the winter of 2001/02 show the typical growth 
pattern of a mild ice winter. 
The newly developed equations for the computation of ice thicknesses in normal and strong ice 
winters also differ only negligibly from the base equations, see Figures 9.3 and 9.4. As a thin ice layer 
is to be described, measured ice thicknesses can be determined very well using Stefan’s equation (ice 
thickness is proportional the square root of the cold sum). In normal winters, the theoretical equation 
covers ice growth up to the maximum values reached in a winter. In strong ice winters, the square-root 
law is only valid for the description of ice thickness values that have been reached by the beginning of 
the first longer thawing period. Thawing periods, which are typical of winters in our latitudes and may 
last from a few days to several weeks, cause a stagnation in the ice growth or even a decrease in ice 
thickness. Ice development continues when the next freezing period sets it, but the increase in ice 
thickness can no longer be described by the base equation. 
♦ First ice Average first ice ♦ Last ice Average last ice 
o 
Csl 
CO 
m 
CO 
r- 
CO 
CD 
O 
t— 
Csl 
CO 
CT> 
CD 
o> 
cd 
CT) 
CD 
CD 
CD 
CD 
CD 
o 
o 
o 
o 
o 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
o 
o 
o 
o 
o 
T- 
T- 
T- 
I - 
T- 
T- 
T- 
T- 
T- 
T- 
Csl 
Csl 
Csl 
Csl 
Csl 
Ice winter (1991=1990/91) 
Figure 9.5. Beginning of freezing and end of ice season in Szczecin Lagoon in mild ice winters 
o 
Csl 
CO 
in 
CO 
r- 
CO 
CD 
O 
t— 
Csl 
CO 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
o 
o 
o 
o 
o 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
CD 
o 
o 
o 
o 
o 
T- 
1“ 
1“ 
1“ 
1“ 
1“ 
1“ 
1“ 
1“ 
1“ 
Csl 
Csl 
Csl 
Csl 
Csl 
Ice winter (1991 =1990/91) 
Figure 9.6. Number of days with ice in Szczecin Lagoon in mild ice winters 
In Figures 9.5 to 9.10, the beginning and end of the ice season as well as the number of days with 
ice in the past 13 winter seasons are compared with the mean values of three ice winter types. The 
mean values refer to the normal period 1960/61 - 1989/1990 and have been taken from the study „Ice 
conditions in the Szczecin Lagoon and Pomeranian Bay during the normal period 1961-1990“ (Szto- 
bryn, Stanistawczyk, Schmelzer, 1999).