11 
The probability of storm surge occurrence was highest in the 1970s (with at least one storm 
surge in every season; probability = 100%) and lowest in the 1960s (probability = 60%). 
From the beginning of the verified observation series, i.e. from 1950/51, the number of 
surges per decade increased from 13 in the 1950s through 15 and 22 in the two following 
decades to 34 in the 1980s. A scarcely smaller number of surges, namely 32, was recorded 
in the last decade of the 20 th century. This allows the conclusion that by the end of the 
century the threat of storm surge flooding had almost doubled as compared to the middle 
period. 
2.3 Monthly frequency distribution of storm surges 
Two factors are mainly responsible for the occurrence of a storm surge: a high daily 
reference sea level on a particular coast and strong, usually gale-force, onshore winds. Gale- 
force conditions are most frequent during the cold season from November (sometimes 
August) to February (sometimes April). 
Fig. 2.3. Mean monthly frequency distribution of storm surges in two periods, 1950 - 1975 
and 1976- 2000 
In the two periods considered, 1950 - 1975 and 1976 - 2000, the mean annual (seasonal) 
distributions differ only with respect to the month of maximum occurrence: in the first 25 
years, storm surges were most frequent in November (about 27% of events) while the 
maximum in the 1976 - 2000 period shifted to January, with about 29% of all storm surges. 
The storm surge seasons in the period from 1950 - 1975 were shorter. The first storms 
usually occurred as late as September (about 2%), and the last ones in March, with a 
frequency of 8%. Storm surge seasons in the period from 1976 - 2000 extended from the 
last days of August (with a frequency of only 1%) to April (about 3% of annual storm surges). 
The absence of these phenomena in May, June and July is characteristic of the monthly 
frequency distribution of storm surges.