21 
weeks. There are only few moderate winters in which the ice thickness reaches values up to 30 
cm in the sea area, and up to 50 cm in Szczecin Lagoon. 
2.3. Meteorological and ice conditions in strong to extremely strong winters in the 
period 1961 -1990 
2.3.1. The meteorological conditions are described for the very strong ice winter of 1986/87. 
November 
Compared to long-term mean monthly temperatures, November 1986 was warm (about 6.7°C 
in the area of Szczecin Lagoon), and so was the beginning of December. 
December 
Until mid-December, westerly and south-westerly air flow prevailed over Europe. Over the 
Baltic Sea, numerous depressions moved east within a more or less intensive stream of 
maritime polar air. Between 14 and 21 December, a high tried to develop over Scandinavia. 
Due to its blocking impact, the air flow over the Baltic Sea Basin backed gradually to north 
easterly directions. The intrusion of cold air was accompanied by strong winds and snow fall. 
From 21 December, for six days, mean daily temperatures were negative in the area of 
Szczecin Lagoon (between -2.7 and -4°C, the minimum temperature in Trzebiez being as low 
as -6.6°C). On 22 December, a snow cover began to form in the region, with a depth of 1 to 4 
cm at the beginning and 36 cm on 25 December. Water temperatures in Trzebiez decreased 
from 2.8°C on 20 to 0.2°C on 25 December. Between 27 and 31 December, warm maritime 
polar air in a short series of low pressure troughs invaded the Baltic Sea again. Air 
temperatures rose to positive (daily maximum in Szczecin exceeded 5°C). The water 
temperature in Trzebiez was 1°C, but decreased during the following days. 
January 
In the first eight days of January, pressure fields of rather high intensity and predominantly 
mild atmospheric gradients allowed only marginal penetration of polar maritime air into the 
northern parts of the continent. Due to these rather stagnant conditions, the poor cyclonal 
activity over the North and the Baltic Seas involved air masses from the easternmost parts of 
Northern Europe, where the temperatures were very low (-20° to -40°). Mean daily air 
temperatures around Szczecin Lagoon varied from +1° to -8°C (Trzebiez). During this frosty 
spell, minimum temperatures in that area were as low as -2° to -18°C. On 4 January, a snow 
cover formed around Szczecin Lagoon. On 8 January, an anticyclone stabilised over 
Scandinavia from where it spread across the whole continent. A frosty easterly air flow 
established itself over the Southern Baltic Sea for 12 days. Very low temperatures were 
recorded over Central Europe. In the region of Szczecin Lagoon, e. g. in Trzebiez, mean daily 
temperatures ranged from -10° to -20°C, and the daily minima from -13° to -26°C. Farther 
inland (Szczecin), the minimum temperature on 14 January was as low as -30°C. The snow 
cover grew to 42 cm. From January 9, the water temperature in Trzebiez was 0°C. From 21 
January, a westerly air flow began to force its way east over the Baltic Sea; the anticyclone 
shifted east. Temperatures rose to slightly positive, and even the water temperature in Trzebiez 
rose to 0.2°C. Then, on 26-29 January, an intrusion of Arctic air took place. Strong and gusty 
northerly and north-easterly winds were followed by another rapid drop of air temperatures to 
about -10°C, with minima reaching -18°C. The snow cover grew again to 25 cm. 
February 
This frosty weather continued until 4 February, the snow cover lasted 2 days longer. Between 
5 and 20 February, various largely cyclonal atmospheric conditions passed across the 
southern coasts of the North and Baltic Seas, with prevailing mild maritime polar air flows. 
With overcast skies and rainy weather, the air did not cool below 0°C. Water temperatures in